Ignimbrite

June 11, 2025January 7, 2013 by Siim

Ignimbrite is a pyroclastic rock formed by very hot ground-hugging cloud of volcanic ash, blocks, and gases known as pyroclastic flow or pyroclastic density current. Ignimbrite is synonymous with flood tuff, welded tuff, ash-flow tuff and pyroclastic flow deposit¹.

Ignimbrite rock sample — Rheomorphic welded ignimbrite from Gran Canaria. “Rheomorphic” means that the rock mass was able to flow in a ductile manner after the deposition. Light-colored elongated clasts are pumice fragments that were flattened and stretched by the mass of the overlying hot rocks, high temperature, and ductile flow. Such elongated pumice fragments are common in this rock. They are known as fiamme (Italian for “flames”). Width of sample 9 cm.

The term “ignimbrite” was coined by the New Zealand geologist Patrick Marshall in 1935. This term was originally used only to refer to welded tuffs. These are pyroclastic rocks that were so hot right after the deposition from the pyroclastic cloud that individual clasts adhered to each other. However, this restriction no longer applies. This term includes all pyroclastic flow deposits, no matter whether they are welded or not¹.

http://picasaweb.google.com/107509377372007544953/Tenerife#5830766299941990930
Pyroclastic flow running down the flank of Mayon in the Philippines during the eruption of 1984. Photo courtesy of USGS.

Pyroclastic flows are the most deadly expressions of volcanism². They are associated with explosive volcanism. Ignimbrite formations are highly variable in bulk volume (0.1 to over 1000 km³) and runout distance (1 to over 100 km)³. Their appearance is variable too. Many colors are possible and they may be unwelded which makes it hard to distinguish some ignimbrites from tuff. Its deposits are frequently hydrothermally altered. The water needed for that may come from former lakes and rivers that got buried beneath the fiery cloud. Hydrothermally altered ignimbrite is often beautifully colorful. Smaller deposits are associated with former river channels because pyroclastic flows are gravity-controlled and therefore tend to follow the valleys. Larger pyroclastic flows do not care much about topography. They just cover former valleys with thicker and highlands with thinner deposits.

Whether ignimbrite is welded or not depends mostly on the temperature in the deposit right after the deposition. Most ignimbrites tend to be felsic, although basaltic varieties are known as well. Felsic ignimbrites tend to weld if the temperature is at least 500 to 650°C. Temperature within pyroclastic flows may reach even 1000 degrees. It is no wonder that they are so deadly given also the speed at which they move (up to 700 km/h).

Well-known huge ignimbrite deposits are Bishop Tuff in western USA and Taupo ignimbrite in New Zealand. Large part of the post-erosional rocks in Gran Canaria are also ignimbrites.

Ignimbrites are composed of pumice and scoria (highly vesicular and often glassy volcanic rocks). Sometimes larger volcanic blocks are included, but most of the material ignimbrite is composed of are lapilli and ash.

http://picasaweb.google.com/107509377372007544953/Tenerife#5830757878227475474
Ignimbritic coastal cliff in Gran Canaria. Southern coast of Gran Canaria.
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Close-up of the same outcrop reveals that it contains strongly flattened pumice fragments (fiamme). One reddish clast with a more mafic composition is also somewhat deformed but noticeably less than pumice. It is so because pumice is highly vesicular and therefore compressible. Welding temperature for felsic pumice is also lower than it is for mafic rocks which have higher melting temperatures. Up is to the left. Width of view 30 cm.
http://picasaweb.google.com/107509377372007544953/Tenerife#5829978158983537218
Block-and-ash flow deposit is a small volume pyroclastic flow deposit that contains large amounts of juvenile volcanic blocks. Block-and-ash flows are generally created by the collapse and fragmentation of volcanic domes². Northern coast of Gran Canaria. Width of view 2 m.
http://picasaweb.google.com/107509377372007544953/Tenerife#5830757815158545458
Ignimbrite can be very beautiful. Width of view 12 cm. Western part of Gran Canaria.
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Hydrothermally altered sequence of ignimbrite in the western part of Gran Canaria. Chlorite is probably responsible for the green and hematite for the reddish coloration.
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Ignimbrite from Gran Canaria. Width of view 20 cm.
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Ignimbrite from the NE coast of Gran Canaria. Width of view 20 cm.
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Boulder of ignimbrite on the western coast of Gran Canaria.

*Ignimbrite is used as a building stone in Gran Canaria.*

References

1. Tilling, Robert I. (2007). Ignimbrite. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 9. 20-21.
2. Schmincke, Hans-Ulrich (2005). Volcanism. Springer.
3. Freundt, A. & Wilson, C. J. N. & Carey, S. N. (1999). Ignimbrites and Block-And-Ash Flow Deposits. In: Encyclopedia of Volcanoes (Ed. Sigurdsson, H.). Academic Press. 581-599.

Basalt

June 13, 2025December 14, 2012 by Siim

Basalt is a very common dark-colored volcanic rock composed of calcic plagioclase (usually labradorite), clinopyroxene (augite) and iron ore (titaniferous magnetite). Basalt may also contain olivine, quartz, hornblende, nepheline, orthopyroxene, etc. Basalt is a volcanic equivalent of gabbro.

Basalt rock sample — Basalt is a fine-grained and dark-colored rock. Black color is given to basalt by pyroxene group mineral augite. Width of the sample is 12 cm.

Basalt is usually black or dark gray and relatively featureless. It is composed of mineral grains which are mostly indistinguishable to the naked eye. Basalt may also contain volcanic glass. Basalt may contain phenocrysts (larger crystals within fine-grained groundmass) and vesicules (holes that were filled by volcanic gases).

Black color is given to basalt by pyroxene and magnetite. Both of them contain iron and this is the reason why they are black. So this is iron again which is responsible for the coloration of basalt. Plagioclase, volumetrically usually the most important constituent, is mostly pale gray in color.

Glowing pahoehoe lava flow — Basalt lava flowing in Hawai’i (Kilauea volcano, Pu’u O’o vent).

Basalt is a major rock type that occurs in virtually every tectonic setting. Basalt is clearly the most common volcanic rock on Earth and basaltic rocks (including gabbro, diabase and their metamorphosed equivalents) are the most common rocks in the crust². Basalt is also common on the Moon and other rocky planets of the Solar System.

What makes basalt so common? Basalt is the original constituent of the crust from which almost all other rock types have evolved. Basalt forms when mantle rocks (peridotite) start to melt. Rocks melt incongruently. It basically means that melt that forms has a different composition from the source rocks. Of course, it can only happen if rocks melt only partially, but this is exactly what happens in the upper mantle. It melts partially to yield basaltic magma which is less dense and rises upward to form new oceanic crust in mid-ocean ridges or volcanoes and intrusives (dikes, sills) in many other tectonic regimes. Basalt is the source rock of other more evolved volcanic rocks like dacite, rhyolite, etc.

Basalt pebbles near the southern tip of La Palma slowly transforming into black sand typical to volcanic oceanic islands.

Basalt sample collected near the Giant’s Causeway, Northern Ireland. Width of sample 8 cm.

La Palma gabbro — Gabbro is a coarse-grained (intrusive) equivalent of basalt. This sample of gabbro comes from La Plama. La Palma is an oceanic island, but some parts of it are uplifted and there are deep ravines like Caldera de Taburiente that cuts deep into the interior of the island and allows intrusive rocks like gabbro to be exposed. Width of sample 5 cm.

Basaltic rocks may carry xenoliths from the mantle. Here is a bright green dunite xenolith inside basalt from Hawai’i. Width of sample 8 cm.

Classification

Basalt has a strict chemical definition. It is defined in the TAS diagram shown above. Basalt is an igneous rock that contains more than 45 and less than 52% of SiO₂ and less than five percent of total alkalies (K₂O + Na₂O)³.

Neighboring rock types like basaltic andesite, basanite, picrite (picrobasalt), trachybasalt and even more distant rocks like phonotephrite or andesite may have very similar look and can be easily mistaken for basalt in many cases.

Basalt is widespread in many tectonic regimes, but there are slight variations in chemical composition which allow more precise classification. MORB is an acronym for “mid-ocean ridge basalt” and OIB for “oceanic island basalt”. MORB is a result of partial melting of the upper mantle which is already recycled many times while OIB is at least partly from more deeper part of the mantle (deep-sourced mantle plumes that feed hot spots like Hawai’i or the Canary Islands) and is therefore less depleted in incompatible chemical elements.

Andesite is similar to basalt, but it contains more silica and is generally lighter in color. White crystals are plagioclase phenocrysts, but they contain less Ca and more Na than plagioclase in basalt does. Andesite is very common product of subduction zone volcanism. Santorini, Greece. Width of sample 7 cm.

Composition

Average chemical composition of basalt determined by 3594 chemical analyses of basaltic rocks² (numbers are mass percents, recalculated volatile-free to total 100%):

SiO₂ — 49.97
TiO₂ — 1.87
Al₂O₃ — 15.99
Fe₂O₃ — 3.85
FeO — 7.24
MnO — 0.20
MgO — 6.84
CaO — 9.62
Na₂O — 2.96
K₂O — 1.12
P₂O₅ — 0.35

Minerals that host these chemical elements (chemical composition of igneous rocks is traditionally expressed in oxides) are augite, plagioclase and titaniferous magnetite. These minerals are difficult to demonstrate because they are too small to be seen in typical basalt, but some basaltic rocks are porphyritic (lots of porphyritic rocks can be seen here: porphyry) and show some of these minerals nicely (unfortunately not magnetite, though).

*Basalt porphyrite from the Isle of Mull, Scotland with many plagioclase phenocrysts. The rock is 8 cm in length.*

Porphyritic basaltic rock from Tenerife. Phenocrysts are plagioclase (white) and augite (black). Width of sample 14 cm.

Heavy minerals in beach sand — Magnetite crystals are always microscopic in basalt, but sometimes they form black stripes in light-colored sand. Here are heavy minerals (mostly magnetite) as a residue of weathering of basaltic rocks. White Park Bay, Northern Ireland.

Olivine augite basanite — Basaltic rock (most likely basanite) from Caldera de Taburiente, La Palma. Black is pyroxene augite, orange is olivine or more precisely what is left of it. Orange patches are former olivine crystals that are now composed of a mixture of silicates and iron oxides which is known as iddingsite. Olivine is a common mineral in many basaltic rocks. Width of view 9 cm.

Basalt in the field

Subaerial basalt forms lava flows or pyroclastic fields and cones. Two main types of basaltic lava flows are aa lava and pahoehoe lava.

Aa lava has rough rubbly irregular crust while pahoehoe is smooth. Lava crust of aa type is broken into pieces while pahoehoe retains its continuity. Both lava flow types are massive beneath the crust and this massive interior may be columnar. Columns are separated from each other by narrow cracks which form because cooling basaltic magma contracts. Cracks start to form at the surface and propagate deeper as lava cools. Submarine basalt usually forms pillows. Pillow basalt forms as a result of very rapid cooling. Outer part of forming pillow cools very quickly in contact with cold seawater while the interior still fills with molten lava.

Basalt mostly forms lava flows because it is among the least viscous magma types and therefore does not generate explosive volcanic eruptions, but sometimes pyroclastic material is formed when magma contains more volcanic gases. Basaltic rocks can be thrown out of volcanic vents as lapilli (singular: lapillus) and volcanic bombs. Basaltic volcanoes are fed by dikes (planar intrusive rock bodies when solidified that cut through other rocks) and sills (similar to dike but generally parallel to preexisting bedding planes).

Basaltic lava flow of Kilauea Volcano in Hawai'i
Basaltic lava flows of Kilauea volcano in Hawai’i.

*Aa lava in the foreground. La Palma, Canary Islands.*

*Pahoehoe lava (ropy lava). La Palma, Canary Islands.*

Columnar basalt at Giant's Causeway — Basalt columns. Giant’s Causeway, Northern Ireland.

Pillow lava in Cyprus — Pillow lava near Fasoula, Troodos ophiolite, Cyprus. Pillow lava is very common on Earth, but difficult to find because almost all of it is on the ocean floor. Examples can be found on land usually where former ocean floor is tectonically squeezed between two blocks of continental crust.

Scoriaceous lapillus from Etna, Italy. Despite being 5 cm in width it weighs only 15 grams because it is filled with gas bubbles (vesicules). Similar rock type with a felsic composition is pumice.

Sometimes dikes are so close to each other that the whole outcrop is composed of them. These sheeted dikes in Cyprus once fed volcanoes on the ocean floor.

Dikes are composed of basalt and diabase. Diabase is nothing more than coarse-grained basalt. Here is a contact between basalt (on the left) and diabase in Cyprus. The basaltic dike is fine-grained because it is younger and was chilled (it lost heat rapidly to the diabase dike on the right).

Lava rosette — Columns in basalt are perpendicular to the cooling front. In this case it is evident that basalt formed a tube (filled lava tunnel). Such conduits are common phenomena in volcanic islands and provide a way for the volcano to enlarge itself because magma can flow great distances inside such thermally insulated tubes before solidifying. Tenerife, Canary Islands.

Dikes and sills are often visible on the ground and may become notable landforms. Salisbury Crags in Edinburgh is basaltic sill.

Metamorphism and weathering

Basalt is largely composed of minerals with little resistance to weathering. Hence, basalt as a whole also tends to disintegrate faster than granite and other felsic rock types. Magnetite is one of the most resistant common minerals in basalt and forms the bulk of heavy mineral sands. Other minerals disintegrate and release their components to water as ions or form clay minerals. Iron and aluminum are among the least mobile ions and therefore tend to form laterite deposits enriched in these elements.

Basalt metamorphoses to a number of different rock types, depending on pressure, temperature, and the nature of volatile compounds that react with minerals in basalt. Most common metamorphic rocks with basaltic protolith are chlorite schist, amphibolite, blueschist, and eclogite.

*Black sand forms in volcanic islands when quartz and biogenic grains are not available. Here is a basaltic cliff and black sand on La Palma, Canary Islands.*

Greenschist — Chlorite schist is a low-grade metamorphosed mafic igneous rock, often with a basaltic protolith. Iron-bearing green sheet silicate mineral chlorite gives slaty cleavage to the rock. Width of sample 14 cm.

Etymology

The term “basanite” was already used in antiquity and “basalt” is probably a faulty transcription of basanite. It was German scholar Agricola (Georg Bauer) who first mentioned “basalt” in 1546. He referred to black columnar rocks from Stolpen (near Dresden in Germany) which is indeed basalt even according to modern classification principles¹.

References

1. Tomkeieff, S. I. (1983). Dictionary of Petrology. John Wiley & Sons.
2. Best, Myron G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.
3. Le Maitre, R. W. (2005). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, 2nd Edition. Cambridge University Press.

Aplite

June 11, 2025November 7, 2012 by Siim

Aplite is a fine-grained igneous rock. Most aplites are granitic in composition — chief constituents are feldspars and quartz. Aplite is associated with granitic rocks that are similar in composition and they may be associated with pegmatites.

Aplite rock sample — Granite (upper left) and aplitic vein (lower right) cutting through it. Aplite and granite are similar in composition but differ greatly in grain size. Granite formed before and its crystals as it seems had ample time to grow. The vein intruded the already solidified granitic rock body and solidified quickly due to rapid heat loss. The width of the sample from Switzerland is 12 cm.

Quartz and feldspar are often intergrown in aplite to form a graphic texture but this is not seen with the naked eye due to very small grain-size of aplitic rocks. Aplites form narrow dikes and veins which are generally less than one meter in thickness and often much less than that. Aplitic veins are sometimes only centimeters in thickness. Aplites occur either within granitic intrusions or within countryrock surrounding the granitic intrusion. In either case it forms a narrow intrusive rock body within other rocks. Aplite is fine-grained because it solidified relatively quickly due to rapid heat loss to the surrounding cooler countryrock.

Aplites are often associated with pegmatites. Pegmatitic magma is rich in volatile constituents which makes it significantly less viscous. Chemical elements are relatively free to move in such magma which means that once formed crystals can keep growing because necessary building blocks are right there. When such magma loses its volatiles because they migrate higher, it gets much thicker and solidifies as normal granite does. However, because it loses heat rapidly, very fine-grained texture forms instead of normal appearance of granitic rock.

Porphyry

September 24, 2019November 5, 2012 by Siim

Porphyry is an igneous rock characterized by porphyritic texture. Porphyritic texture is a very common texture in igneous rocks in which larger crystals (phenocrysts) are embedded in a fine-grained groundmass.

Porphyry rock sample
Porphyry is an igneous rock that contains larger crystals (phenocrysts) in a fine-grained groundmass. K-feldspar phenocrysts in this sample. Width of view 7 cm. TUG 1608-2807.

It seems simple enough but unfortunately there are many different interpretations. Sometimes it is assumed that porphyry is granitic in composition¹ while other sources claim that composition plays no role whatsoever². Some authors make a difference between porphyry and porphyritic rock. This is based on field relations. True porphyry according to this interpretation is an intrusive rock. Extrusive (lava) rock may have a porphyritic texture but it should be named porphyritic rock, not porphyry³.

Another system which I am familiar with because it seems to be widespread in continental Europe is to contrast between “porphyry” (feldspar phenocrysts are alkaline) and “porphyrite” (feldspar phenocrysts are plagioclase)⁴. According to this system, we can talk about “rhyolite porphyry” and “basalt porphyrite”, for example, but never “basalt porphyry” and “rhyolite porphyrite”. Nowadays it seems to be common to use appropriate rock type name in addition to “porphyry” (two examples in the previous sentence), but it used to be very common to use the name of the minerals that form the porphyritic texture. “Plagioclase porphyrite” and “quartz porphyry” have been used instead of “basalt porphyrite” and “rhyolite porphyry”.

Yet another source of confusion are the terms “porphyritic” and “porphyraceous”. Rocks are said to be porphyritic if their groundmass is fine-grained ar aphanitic and porphyraceous if their groundmass in visible to the naked eye. Hence, rhyolite and basalt as fine-grained volcanic rocks are porphyritic and granite, syenite, etc. as coarse-grained plutonic rocks are porphyraceous. Phenocrysts that make up porphyry should be felsic (quartz, feldspar).

Well, as you can see, mankind is very good when a need arises to make simple concepts difficult to understand. What is in my opinion important to undestand and what is agreed upon by all is that porphyritic rocks are always igneous rocks and they contain crystals that are noticeably larger than the crystals surrounding them.

How do porphyritic rocks form? This is in most cases fairly simple concept to grasp. Crystals need time to grow. In porphyritic rocks some large crystals had this time while others (groundmass) solidified quickly. Hence, porphyritic rocks started to solidify as normal intrusive rocks but something happened that resulted in quick loss of heat and rapid crystallization. This something may have been the emplacement of magma into narrow cracks near the surface or maybe volcanic eruption that brought magma to the surface.

This interpretation, however, is unable to give an adequate explanation to the question why are deep-seated plutonic rocks sometime porphyritic (or porphyraceous). They definitely cooled slowly and it is highly unlikely that there were noticeable changes in the cooling rate. These rock probably exhibit porphyritic texture because some crystals started to form before others and had therefore more time and room to grow. The role of volatile components in magma is probably important as well. Hence, we do have a reason to believe that there are several different mechanisms involved and in many cases it may be a complicated task to unravel the cooling history of a particular igneous rock.

Pictures

Porphyry contains large crystals in the fine-grained matrix. Rhomb porphyry (latite) from Norway is a rock type associated with continental rifts. Intrusive equivalent of rhomb porphyry is larvikite (monzonite). Width of sample 13 cm.

Rhomb porphyry rock sample
Rhomb porphyry sample from the Oslo Rift.

Another rhomb porphyry sample from the Oslo Rift.

Diabase
Basalt porphyrite, plagioclase porphyrite or diabase? Probably all of them, it is mostly a matter of preference and depends on local traditions. White phenocrysts are plagioclase crystals. The Isle of Mull, Scotland. The rock is 8 cm in length.

Diabase with augite and plagioclase phenocrysts from Tenerife.
Basaltic rock from Tenerife. Phenocrysts are plagioclase (white) and pyroxene (black). Width of sample 14 cm.

Olivine basalt
This one from Oahu is clearly not the most classic version of porphyry because it is mafic, it is extrusive, and the phenocrysts are mafic. It is often assumed that phenocrysts that form the porphyritic texture must be felsic (feldspars (preferably alkali feldspars) or quartz). The green mineral here is olivine. But it does meet the most important and universal characteristics of porphyritic rocks — it is igneous and some minerals are clearly larger than the groundmass. Width of sample 6 cm.

Rhyolite porphyry
Porphyry from Scotland with K-feldspar and quartz phenocrysts. Width of sample 8 cm.

Andesite porphyrite with plagioclase phenocrysts from Santorini. Andesite is an extrusive equivalent of diorite. Width of sample 7 cm.

rhyolite porphyry
Porphyritic rhyolite from Estonia. Rocks like that crop out in the Baltic Sea. They were brought to Saaremaa by the advancing glacier during the ice age. It is locally known as quartz porphyry. Width of sample 9 cm.

References

1. Best, Myron G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.
2. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.
3. Rose, W. I. (2007). Porphyry. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 14. 254-255.
4. Spry, A. (1990). Porphyry. In: The Encyclopedia of Igneous and Metamorphic Petrology (Ed. Bowes, D. R.). Springer. 479–480.

Composition of the crust

June 13, 2025October 27, 2012 by Siim

The crust is compositionally distinct outermost rocky layer of the Earth. What is the crust made of? The answer to this question depends on whether we want to know which chemical elements, minerals or rock types it is made of. It may be surprising but about a dozen chemical elements, minerals, or rock types is all that it takes to describe approximately 99% of the crust. This article is about these really common and maybe some a little less common but noteworthy building blocks of the ground beneath our feet.

Common rocks in the crust. Igneous rocks in the first row: granite, gabbro, basalt. Metamorphic rocks in the second row: gneiss, schist, amphibolite. Sedimentary rocks in the third row: sandstone, shale, limestone.

Chemical elements in the crust

These figures vary among different studies because we really have no way to know for sure.

This is an estimation of the chemical composition of the crust based on our understanding of the relative proportions of different rock types in the crust and their average composition.

Our understanding is limited for sure because the average continental crust is more than 40 km thick but we have no way to sample it directly. Deepest mines reach only 4 kilometers and deepest drillhole is 12 km deep.

Element	Mass percent	Common minerals	Common rocks
Oxygen	46.6	Silicates, oxides, etc. Oxygen is extremely widespread in the crust and very reactive as well. Volumetrically insignificant part of all the minerals contain no oxygen.	Almost every common rock type contains oxygen. Only sulfide ore bodies and evaporite beds are almost free of oxygen, but they are volumetrically relatively insignificant.
Silicon	27.7	Silicon has its very own large group of minerals known as silicates. More than 90% of the crust is composed of silicate minerals. Silicon and oxygen are the two most common chemical elements in the crust that also happen to like each others company very much. Pure silicon oxide is known as mineral quartz which makes up 12% of the crust. There is not a single common non-silicate mineral that contains silicon — silicon always combines with oxygen.	Silicate minerals are the building blocks of most common rock types (basalt, granite, schist, gneiss, sandstone, etc.). Carbonate rocks (limestone, dolomite rock) and evaporites (gypsum rock, rock salt) are notable exceptions. They do not contain silicon if they are pure. Opaque ore minerals (oxides and sulfides) are frequent minor components of most rock types. They are also free of silicon.
Aluminum	8.1	Very widespread in silicate minerals (feldspar, clay minerals, mica). Aluminum hydroxides (boehmite, diaspore, gibbsite) are economically important as aluminum ore minerals.	Feldspars are very common minerals in the crust, more than half (51%) of the crust is made up of this mineral group. Mica and clay minerals are also common, both comprise about 5% of the crust. Hence, aluminum is extremely widespread as well. It is usually not very concentrated in silicate minerals, though. Aluminum has been extracted from silicate rocks very rarely. Bauxite which is aluminum-rich laterite formed in humid hot areas contains aluminum hydroxides and is primarily mined for aluminum. Aluminum in bauxite is a residue of chemical weathering of silicate rocks.
Iron	5.0	Iron is a widespread element in minerals. Notable iron-rich silicate minerals are pyroxenes, amphiboles, olivine, black mica biotite, garnet, etc. Iron is also an important element in sedimentary rocks. It is just like aluminum hard to dissolve and carry away with water. Iron is common in lateritic soil and forms rust-colored iron oxide mineral hematite. Hematite is responsible for the red coloration of many minerals and rock types. Iron oxide magnetite is common as an accessory mineral in metamorphic and igneous rocks. Iron sulfide pyrite is the most common sulfide mineral. Iron also occurs in carbonates (siderite, ankerite), clay minerals (glauconite, chlorite). Iron is a strong chromophore element, it gives dark coloration to its host minerals. This is why most pyroxenes and amphiboles are black.	Iron is actually the single most abundant chemical element in the whole of Earth, but most of it is in the core. Basalt, gabbro, amphibolite, greenschist, etc. are the most notable crustal rocks that contain lots of iron. There is a large number of rock types that contain significant amount of iron, but most of the iron mined comes from metamorphosed sedimentary rocks known as BIF (banded iron formation).
Calcium	3.6	Calcium is also very widespread. It is always present in plagioclase feldspars (39% of the crust), but the amount of calcium varies there. The most important pyroxenes and amphiboles (augite and hornblende) contain calcium. Calcium occurs in many other silicate minerals like garnet, epidote, wollastonite, titanite, etc. Calcium is a constituent of calcite which is very important mineral chiefly in sedimentary environments. Calcium phosphate apatite is a common mineral as well. Gypsum is a major evaporite mineral that is chemically hydrated calcium sulfate. Calcium fluoride is known as mineral fluorite.	Occurs equally successfully in igneous, sedimentary, and metamorphic rocks. Especially well-known calcium-bearing rock type is limestone. Its metamorphosed equivalent is marble. Marble is composed of calcite just like limestone. Calcite is a remarkable mineral. Even igneous rock composed of pure calcite exists. It is known as carbonatite, but it is very rare when compared with limestone and marble. Calcium tends to be part of minerals like plagioclase, pyroxenes and amphiboles in igneous rocks. Major calcium-bearing metamorphic rock is amphibolite (metamorphosed basalt, calcium is hosted by hornblende and plagioclase). Phosphorite is another important calcium-bearing sedimentary rock (calcium is hosted by phosphate mineral apatite). Calcium also occurs in evaporites as a mineral gypsum.
Sodium	2.8	Sodium is widespread in silicate minerals. It is an important constituent of both alkali feldspar and plagioclase. Sodium-bearing pyroxenes are relatively rare. Sodium is somewhat more widespread in amphiboles but not as much as calcium. Well-known sodium-bearing silicate mineral is tourmaline. Sodium is an important component of feldspathoids, but both feldspathoids and tourmaline group minerals are relatively rare. Major sodium-bearing mineral in sedimentary environments is halite (NaCl).	Igneous and metamorphic rocks that contain feldspar. Much of sodium from weathered igneous and metamorphic rocks is dissolved in seawater. Rock salt is the most important sodium-bearing sedimentary rock.
Potassium	2.6	Potassium and sodium are similar chemical elements both chemically and geologically. Potassium is an important constituent of alkali feldspars. Most alkali feldspars contain much more potassium than sodium and are therefore frequently referred to as K-feldspars. Important potassium-bearing silicate minerals are micas (5% of the crust). Biotite and muscovite are the most important micas and they both contain potassium. Most important potassium-bearing sedimentary mineral is sylvite (KCl).	Alkali feldspars and micas are common rocks in silicate igneous and metamorphic rocks (granite, gneiss, schist, etc.). Much of potassium from weathered igneous and metamorphic rocks is dissolved in seawater. Sylvite is not as common evaporite as halite (rock salt) because it takes much higher evaporation rate to precipitate sylvite.
Magnesium	2.1	Magnesium is very widespread in the mantle beneath the crust. Olivine and pyroxene are the most important Mg-bearing minerals there and these minerals are also constituents of some crustal rocks, especially dark-colored igneous rocks. Amphiboles also contain magnesium but less than pyroxenes. Magnesium ion is similar to iron in size and can therefore easily replace iron in the lattice of minerals. This is the case in olivine, pyroxenes, amphiboles and even micas (phlogopite is a Mg-rich variety of biotite). Important Mg-rich minerals in metamorphic rocks are talc and serpentine. Magnesium in the sedimentary environment occurs chiefly in carbonates dolomite and magnesite. Lots of magnesium is dissolved in seawater. Magnesium is extracted from seawater.	Important Mg-bearing igneous rocks are ultramafic rocks (peridotite, pyroxenite). Rocks that contain lots of pyroxenes like basalt and gabbro contain Mg also but to a lesser extent. Metamorphic Mg-rich rocks are serpentinite and talc schist. Most important Mg-bearing sedimentary rock is dolomite rock which is former limestone converted to dolomite by Mg-rich meteoric water percolating limestone.
Others	1.5	Other common elements in the crust are titanium, hydrogen, phosphorous, manganese, fluorine, etc. Their occurrence is somewhat more restricted, but they are all important elements in minerals and rocks. Hydrogen is actually extremely widespread constituent of a wide array of minerals, but it is the lightest chemical element and therefore does not form a significant part of the crust by mass.

The most abundant minerals in the crust

Most important minerals — More than 90% on the crust is composed of silicate minerals. Most abundant silicates are feldspars (plagioclase (39%) and alkali feldspar (12%)). Other common silicate minerals are quartz (12%) pyroxenes (11%), amphiboles (5%), micas (5%), and clay minerals (5%). The rest of the silicate family comprises 3% of the crust. Only 8% of the crust is composed of non-silicates — carbonates, oxides, sulfides, etc.

If these minerals are really that common, we should all be more than familiar with them. Yes, I believe we are. Even if we don’t know how to name them, we have surely seen them. For the most people the silicates mentioned above are so drab and commonplace that we probably fail to notice or pay any attention to them. Below is a selection of photos showing these minerals in their natural environments (outcrops and hand samples). I am intentionally showing minerals within rocks because this is how they occur in the crust. Beautiful samples with perfect crystal faces may be nice to look at, but they are rare in the crust. I do not value such crystals as a teaching material. You are extremely unlikely to find them on your own and therefore they teach us very little.

Plagioclase is the most important mineral in the crust. It is common in mafic igneous rock like the diabase sample above. White elongated phenocrysts in finer basaltic groundmass are plagioclase crystals. Black crystals belong to pyroxene (mineral augite). Both augite and plagioclase occur also in the fine-grained groundmass. Large crystals formed slowly before the magma erupted and the rest solidified rapidly. Plagioclase is so common because basaltic rocks and their metamorphic equivalents are very widespread. Most of the oceanic crust is composed of basaltic rocks. The sample is from Tenerife, Canary Islands. Width of sample 14 cm.

Another sample of basaltic rock but this time with lots of olivine. Olivine (green) is denser than plagioclase and pyroxene (both are present in the groundmass) and therefore sinks to the bottom of lava flows where olivine cumulate rocks form. This olivine basalt sample is from Oahu, Hawaii. Width of sample 6 cm.

Clay minerals are too small to be shown individually. Even with a light microscope you will see only mud or dust depending on whether these minerals are wet or dry. Clay minerals are silicates that are the products of weathering of other silicate minerals, mostly feldspars. The picture was taken in a clay quarry in Estonia.

Biotite is one of two major mica minerals. The other is light-colored muscovite. The sample is from Evje, Norway. Width of sample 11 cm.

The most abundant rock types in the crust

Rocks are divided into three broad groups: igneous, metamorphic, and sedimentary rocks. The oceanic crust is largely composed of basaltic igneous rocks which are covered by a thin veneer of sediments which are thickest near the margins of the continental landmasses. The continental crust is much thicker and older. The continental crust is also much more variable and structurally very complex. Virtually all the rock types known to man occur in the continental crust. Even meteorites, xenoliths from the mantle, and ophiolites (fragment of former oceanic crust) are constituents of the continental crust because that’s where we found them.

Roughly three fourths of the continental crust is covered by sedimentary rocks and almost all of it is covered by loose sediments (soil, sand, dirt, etc.). We are most likely to encounter these materials, but it is important to understand that despite being so ubiquitous on the surface, they make up only about 8% of the whole mass of the crust. Sediments consolidate to sedimentary rocks after burial. Sand turns to sandstone, limy mud to limestone, clay to claystone. Sedimentary rocks are stable only in the upper parts of the crust. High pressure and temperature in the deeper parts metamorphoses them (minerals recrystallize) to various metamorphic rocks. The bulk of the continental crust is made of metamorphic rocks. Igneous rocks are also common on the surface in volcanically active regions, but they also occur deeper in the crust as granitic (mostly) intrusions.

Important sediments are sand, clay, mud (wet mixture of clay and fine sand), and limy mud. Widespread sedimentary rocks are limestone (2% of the crust by volume), sandstone (1.7%), claystone (4.2%) which are lithified versions of the loose sediments mentioned before. Chemical sediments like halite and gypsum are important as well, but their overall volume is clearly less than 1% of the crust. Important igneous rocks are granite, granodiorite, gabbro, basalt, diorite, andesite, etc. It is very difficult to say what is the percentage of these rocks. Important metamorphic rocks are metamorphosed equivalents of widespread sedimentary and igneous rocks. Common metamorphic rocks are slate (metamorphosed claystone), schist (met. claystone, higher grade than slate) quartzite (met. sandstone), marble (met. limestone), gneiss (met. igneous rock or sedimentary rocks), amphibolite (met. basaltic rocks).

Sediments and sedimentary rocks

Majority of carbonate rocks were once carbonate mud on the seafloor. This mud is made of tiny carbonate shells of foraminifera, coccolithophores, gastropods, etc. This sample is a coral sand from Bermuda which is composed of bits and pieces of coral reefs and foram tests. Width of view 32 mm.

Sandstone is a lithified sand. This sand sample is a dune sand from the Gobi Desert, Mongolia. Width of view 10 mm.

Limestone is usually composed of calcareous remains of marine living forms. Sometimes they are large enough to be seen with a naked eye. Here is a fossiliferous limestone from Estonia (Ordovician) with trilobite, brachiopod, bryozoan, etc. fossils. Width of sample 16 cm.

Gypsum is an evaporite mineral. Evaporites are water-soluble chemical sediments that crystallize out of concentrated (high salinity) seawater in lagoons. Picture taken in Cyprus.

Sandstone outcrop in Estonia — Sandstone is a lithified sand. Reddish color is due to fine hematite (iron oxide) powder that covers quartz grains forming the majority of the sediment. An outcrop of Devonian sandstone in Estonia.

Limestone is a lithified limy mud in most cases. An outcrop of Silurian limestone in Saaremaa, Estonia.

Mudstone (alternative names are shale, claystone, and argillite) is a lithified mud. An outcrop in Scotland. Hammer for scale. Mudstones are the most common sedimentary rocks.

Mudstones form in most cases as rapidly moving subaqueous avalanche of muddy water moves down the continental slope. Such a sediment flow is known as turbidity current. Turbidity sequence is typically composed of many alternating layers of siltstone (very fine sandstone) and mud. Silt settles faster than clay minerals do and therefore every current is composed of two distinct layers (there may be even more). Here is a picture of dark-colored mudstone and light-colored siltstone underneath it. These samples are from a turbidity current from Spain. The samples are from a single outcrop, but they were not next to each other there. The width of the samples is about 20 cm.

Alternating layers of muddy and sandy sediments in a turbidity sequence. Photo is taken in Morocco. An outcrop of turbidite in Morocco. Sedimentary sequences like that were previously known as flysch. This term is nowadays rarely used because the explanation how flysch forms is clearly outdated now. It used to be an integral part of the geosynclinal theory which attempted to explain the mountain building process before we understood that much better explanation known as plate tectonics exists.

Igneous rocks

Igneous rocks are classified according to their silica content. Rocks that contain lots of silica are usually light-colored. Most important minerals are feldspars and quartz. These rocks are referred to as felsic rocks (feldspar + silica). Common felsic rocks are granite and rhyolite. Mafic rocks are low in silica but relatively high in magnesium and iron. They are dark-colored and named mafic rocks (magnesium + ferric). However, no matter whether they are felsic or mafic, these rocks always contain much more silicon than magnesium or iron. Important minerals in mafic rocks are pyroxene, plagioclase and sometimes also olivine or amphibole. There are also rocks intermediate in composition (diorite and andesite).

Igneous rocks are further classified as intrusive (plutonic) and extrusive (volcanic) rocks. Intrusive rocks are coarse- and extrusive rocks fine-grained. Granite, diorite, and gabbro are intrusive rocks. Rhyolite, andesite, and basalt are volcanic rocks. Felsic rocks are much more viscous and therefore relatively rarely break out to the surface. They usually solidify as intrusive rocks. Hence, granite is a very common rock type while rhyolite is not rare but nowhere as widespread as granite. It is different with mafic rocks. Basaltic magma is less viscous and relatively easily flows to the surface. Basalt is very common rock type, especially in the upper part of the oceanic crust. Andesite is somewhere in the middle. It is pretty common rock type associated with subduction zone volcanism but not as widespread as basalt.

By the way, the average composition of the continental crust is that of andesite. Hence, we believe that it gives us a hint how the continental crust formed. It is subduction zone volcanism that creates lava intermediate in composition that is less dense than basaltic rocks of the oceanic crust and is therefore not able to dive back into the mantle. So, the continental crust is not recycled by the conveyor belt of the oceanic crust and can only grow bigger and bigger as time goes by.

Rhyolite is a volcanic equivalent of granite. The sample from Scotland is 8 cm in width.

Gabbro is a mafic intrusive rock. The sample from Cyprus (from the Troodos ophiolite which represents former oceanic crust) is 7 cm in width.

Basalt is a volcanic equivalent of gabbro. The sample from Northern Ireland is 8 cm in width.

*Andesite is a common volcanic rock which is intermediate in composition between mafic and felsic rocks. White mineral is plagioclase. The width of the sample from Santorini is 7 cm.*

Metamorphic rocks

Quartzite — This rock type was once sandstone, but it got buried so deep that quartz grains fused together to form a tough metamorphic rock known as quartzite. Sample from Norway. Width of sample 9 cm.

Marble is a metamorphosed limestone. It is composed of calcite. The outcrop is located in Karelia.

Caption — Schist is a strongly foliated metamorphic rock, most likely metamorphosed mudstone. Photo taken in Scotland.

A hand sample of schist from Spain. Width of sample 9 cm.

Gneiss sample — Gneiss is a very common metamorphic rock. Perhaps up to one fifth of the crust is composed of gneissic rocks. This specimen from Karelia has a composition of an ordinary granite: pink K-feldspar, gray quartz, and black biotite. Width of sample 11 cm.

More pictures

http://picasaweb.google.com/107509377372007544953/2015#6190951257660732594
Width of view 20 cm. Verpeneset, Norway.
http://picasaweb.google.com/107509377372007544953/2015#6190953225560791170
Width of sample 14 cm. Nordfjord, Norway.
http://picasaweb.google.com/107509377372007544953/2015#6191004671906123490
Selje, Norway.
http://picasaweb.google.com/107509377372007544953/2015#6191004675414948226
Width of view 20 cm. Selje, Norway.
http://picasaweb.google.com/107509377372007544953/2015#6191004786537859586
Nordfjord, Norway.
http://picasaweb.google.com/107509377372007544953/2015#6191004786391669266
Nordfjord, Norway. Width of view 30 cm.
http://picasaweb.google.com/107509377372007544953/2015#6191004804924257138
Nordfjord, Norway.

References

1. Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.
2. Tomkeieff, S. I. (1983). Dictionary of Petrology. John Wiley & Sons.
3. Best, Myron G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.
4. Smulikowski, K. (1990). Eclogite. In: The Encyclopedia of Igneous and Metamorphic Petrology (Encyclopedia of Earth Sciences Series) (Ed. Bowes, Donald). Springer. 684.
5. Holland, Timothy J. B. (2007). Eclogite. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 6. 41-42.
6. Rapp, R. P., Watson, E. B., & Miller, C. F. (1991). Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites Precambrian Research, 51 (1-4), 1-25 DOI: 10.1016/0301-9268(91)90092-O

Limestone

June 13, 2025October 3, 2012 by Siim

Limestone is a very common sedimentary rock consisting of calcium carbonate (more than 50%). It is the most common non-siliciclastic (sandstone and shale are common siliciclastic rocks) sedimentary rock. Limestones are rocks that are composed of mostly calcium carbonate (minerals calcite or aragonite). Carbonate rocks where the dominant carbonate is dolomite (calcium magnesium carbonate) are named dolomite rock. Carbonate rocks together make up one-fifth of all sedimentary rocks in the stratigraphic record³.

Limestone varieties — Varieties of limestone. Chalk (upper left) is a marine limestone consisting of tests of microscopic algae and foraminifera. Tufa (upper right) is a chemical precipitate of calcium carbonate. Fossils are very common in marine calcitic sedimentary rocks. Rocks such as coquina are wholly composed of fossils but so-called normal limestones may be also highly fossiliferous. The sample (lower left) is from the Ordovician. Grainstone is a coarse-grained grain-supported variety that contains almost no limy mud (micrite).

Limestone is defined by these two criteria: it is a sedimentary rock (1) and it is composed of calcium carbonate (2). There are other rocks that are composed of calcium carbonate. Carbonatite is a rare type of igneous rock and marble is a common metamorphic rock. Both are chemically composed of calcium carbonate, but they are not limestones because they are not sedimentary rocks.

Other than that, there are no more restrictions. Hence, limestones can be, and indeed are, formed by very different mechanisms. Some limestones are clastic (or detrital) just like sandstone. However, there is an important difference. Most detrital limestones are made of biogenic grains rather than weathered bits and pieces of preexisting rocks. Calcium carbonate in the form of aragonite and calcite are extremely common biominerals. They are used to build tests, shells, exoskeletons, etc. of millions of marine (mostly) species. These tests sink to the bottom of oceans or other waterbodies and form a limy sediment (named ooze if the tests are microscopic) which becomes limestone after burial and lithification. However, some detrital limestones (oolites) are composed of abiogenic particles like ooids. Biogenic limestone may grow in situ by the growth of carbonate skeletons (coral reefs) or by trapping of sediments in bacterial mats (stromatolites).

Limestone formations are usually clearly layered. Pure rock is light-colored. An outcrop of Silurian limestone in Saaremaa, Estonia.

Stevns Klint in Denmark is a world-famous K-T boundary. It is composed of bryozoan limestone (upper part) and chalk (lower) which are both types of limestone. These layers are separated by a narrow layer of clayish material which formed 65 million years ago. The clay is very rich in iridium – a metal abundant in iron meteorites but rare on Earth. This layer formed as a result of meteorite explosion on the Yucatán Peninsula in Mexico. The event marks the end of the Mesozoic Era and is associated with the mass extinction that also ended the reign of dinosaurs. Dark gray spots are chert nodules and the boulders on the coast are also made of this rock type.

Limy mud is often purely chemical precipitate. Tufa and travertine are varieties of chemical limestones. Tufa is soft and porous variety that is associated with springs. Ground water may contain thousand times as much carbon dioxide as would be in equilibrium with air⁴. Such groundwater is acidic and dissolves limestone. Limestone (or more precisely limy mud) precipitates out of ground water when it emerges above ground as a spring and loses much of its CO₂. Travertine is a hard variety of chemical limestone. Travertine is the material speleothems (stalactites, stalagmites, and other dripstones) of karst caves are made of. Travertine formed in these caves is simply the result of a complete evaporation of water containing mainly calcium carbonate. Travertine deposited from hot springs may be a result of the loss of carbon dioxide from the water as pressure is released upon emerging at the surface⁵.

Limestones are strongly affected by diagenetic processes that accompany lithification. Aragonite will be replaced by calcite (if the original carbonate mineral was aragonite), dissolution may be significant (there is a picture of stylolite below), and calcite may be replaced with dolomite. However, calcite is resistant to metamorphism. Marble is a metamorphosed limestone, but it is still composed of carbonate minerals. The crystals are just bigger, they are recrystallized, and original structures (like fossils) are obliterated.

Limestone is often impure. Calcium carbonate may be mixed with siliciclastic material (clay, sand) in all proportions, but calcium carbonate must form the majority. Otherwise, we have calcareous sandstone or marl instead of limestone. The term “marl” is mostly used in the field and often somewhat muddy limestones, even if they contain more calcium carbonate than mud, are called that way.

Limestone has many industrial uses. It is used as a building material, as a raw material in the manufacture of portland cement, quicklime, etc. Calcium is an important micronutrient. Several commercial drugs (antacids and calcium supplements) are nothing but pulverized limestone (calcite)⁶. It is an important reservoir rock of crude oil and gas.

Limestone, crinoid, Ordovician, Estonia — Limestone is often rich in fossils. Sea lilies (attached crinoids) are echinoderms that are not extinct, but their best days have passed. They were so abundant during the Paleozoic that many limestones are full of their fossils. Bits of crinoids in a limestone slab from the Ordovician of Estonia.

Limestone from the Ordovician of Morocco containing lots of nautiloid fossils.

Limestone drill cores from Hiiumaa, Estonia.

Limestone variety grainstone — Coarse-grained limestones are known as grainstones. The sample is from Estonia (Ordovician). Width of sample 10 cm.

Tufa is a chemically precipitated soft and porous limestone usually associated with springs. The hard variety of tufa is travertine. The sample from Estonia is 13 cm in width. Green lithic fragments are pieces of glauconitic sandstone.

Tufa towers in Mono Lake, California. These limestone formations grew in the lake when the water level was higher. Mono Lake is a soda lake. Its water is also rich in carbonate ions. Springwater that enters the lake water carries dissolved calcium which reacts with carbonates and precipitates as tufa.

Despite being composed of calcium carbonate, this rock sample is not limestone because it crystallized from magma. This is rare igneous rock known as carbonatite. The width of the sample from Germany is 8 cm.

Limestone with abundance of Ordovician fossils. Trilobites, brachiopods, bryozoans, etc. Width of sample from Estonia is 16 cm.

Foraminifera are microscopic marine amoeboid protists with calcareous tests. These tests provide material for the formation process of limestone. Genera Calcarina, Sorites, and Baculogypsina from left to right. Width of view 10 mm.

Globigerina ooze is a type of foraminifra ooze (globigerina is a genus of planktonic forams) that covers large areas of seabottom. These forams are collected from the Weddell Sea near Antarctica. They covered seafloor at the depth of 3500 meters. Red circles are around *Orbulina universa*, green could be *Rotaliida*, and yellow ones are *Globigerina* but some of them may be *Neopachyderma* also. The width of the view is 5.1 mm.

This coarse-grained limestone from Cyprus shares both sandstone and limestone properties. It is clearly clastic like sandstone, but it is composed of carbonate grains of biogenic origin. Such rocks are known as calcarenite, they are considered to be a subtype of limestone. The width of the sample from Cyprus is 7 cm.

Calcarenite outcrop in Cyprus — An outcrop of calcarenite from Cyprus.

Layer of coquina in a calcarenitic limestone in Morocco (coastal cliff between Essaouira and Agadir). Coquina is a type of porous limestone that is chiefly composed of fossil debris. Width of view 40 cm.

Ooid sand from Abu Dhabi, The United Arab Emirates. The width of the view is 5.5 mm. Oolite is a lithified ooid sand.

Glauconitic limestone from Estonia (Ordovician). Limestone with a significant amount of clay minerals is known as marl. Width of sample 13 cm.

Tempestite is a storm deposit. Limestone bed was severely disturbed by sea waves in shallow water, but the broken off pieces redeposited quickly and lithified again as a brecciated limestone. The sample from Estonia is 13 cm in width.

An outcrop of limestone with a tempestite layer in Estonia from the Silurian.

Marly chalk from Cyprus — Marly (impure clayey limestone) chalk from Cyprus. Width of sample is 7 cm.

Coral sand from Bermuda. Pink grains are tests of foram *Homotrema rubrum* which give pink color to the sand of Bermuda.

Coral sand consisting of pieces of corals and foraminifera from Hawaii. The width of the view is 16 mm.

Biogenic grainsZakynthos biogenous sand grains — Miscellaneous biogenic grains (forams, gastropods, sea urchins, ostracods, clams, coralline algae) from the beach sand of Zakynthos, Greece. The width of the view is 7 mm. Want to know who is who on this picture, then check this post: Sand full of mysteries.

Boulders of chalk laying on the White Park Bay beach in Northern Ireland. Chalk is a type of limestone consisting of tiny marine microorganisms coccolithophores which just like foraminifera have a calcareous tests. Coccoliths (tests of coccolithophores) are significantly smaller than foram tests.

Chert with chalk — Chert nodules in chalk. Holes in chalk were probably created by boring molluscs which prefer limestone substrate and bore into the rock with the aid of an acid secretion. White Park Bay, Northern Ireland.

Travertine — Brecciated travertine in Death Valley. Width of view 30 cm.

Limestone with mudstone layers — This is a muddy limestone in which light-colored carbonate-rich layers are alternating with siliciclastic (muddy) layers. Biri, Oppland, Norway. Width of sample 9 cm.

A reef knoll (microbial biolithite) in Cyprus. Biolithite is a limestone constructed by organisms that grew and remained in place¹ (coral reefs, for example).

Stromatolite in Saaremaa, Estonia. Stromatolite is a sedimentary structure produced by sediment trapping and binding by microorganisms (mostly cyanobacteria). Width of sample 18 cm.

Large boulder or crystalline rock (glacial erratic) is about to fall into former karst cave with a collapsed roof in Ordovician limestone. Kostivere, Estonia.

A limestone pavement in the Pyrenees (Spain) showing evidence of karst dissolution.

Thrust fault — An outcrop in the Pyrenees, Spain. Older siltstone formation is pushed on top of younger limestone formation. Description of this outcrop is here: How the Pyrenees were made.

Durness Limestone is a dolomitic limestone of the Cambrian period from Scotland.

Stromatoporoids (not to be confused with stromatolites) from the Silurian of Estonian bedrock. Stromatoporoids were important reef-builders of the Paleozoic. They were probably related to sponges and not to modern reef-builders corals. Width of sample 10 cm.

Tabulates are an extinct order of corals that were widespread reef-builders during the Paleozoic. The width of the sample from Estonia is 8 cm.

Nautiloid fossil from Estonia — Nautiloids were once very common in oceans, but today they are represented by few species of family Nautilidae. They are cephalopods, related to octopuses. This fossilized nautiloid from Estonia (width 17 cm) lived during the Ordovician period.

Nautiloid Ordovician — *Nautiloid fossils are very common in certain Estonian limestones.*

References

1. Jackson, J. A. (1997). Glossary of Geology , 4th Edition. American Geological Institute.
2. Trenhaile, A. (2004). Notch, coastal. In: Encyclopedia of Geomorphology (Goudie, A. S.). Routledge. Volume 2. 725.
3. Stow, D. A. V. (2005). Sedimentary Rocks in the Field: A Color Guide . Academic Press.
4. Barnes, I. (2007). Spring (hydrology). In: McGraw Hill Encyclopedia of Science & Technology , 10th Edition. McGraw-Hill. Volume 17. 303-304.
5. Siever, R. (2007). Travertine. In: McGraw Hill Encyclopedia of Science & Technology , 10th Edition. McGraw-Hill. Volume 18. 606.
6. Nesse, William D. (2011). Introduction to Mineralogy , 2nd Edition. Oxford University Press.

Coquina

December 21, 2016October 1, 2012 by Siim

Coquina is a detrital limestone consisting of shells or shell fragments. The constituents are mechanically sorted (usually by sea waves), transported and often abraded because of transport and sorting. It is a porous and soft weakly to moderately cemented rock.

Coquina
Sample of coquina from Germany. Width of view 14 cm. TUG 1608-4738.

Hard and dense firmly cemented equivalent is coquinite. Coquina could be considered to be a subtype of calcarenite — a detrital limestone of sand-sized clasts (carbonate sandstone) but most examples are composed of clasts that exceed the upper limit of sand-grains size (2 mm). This is not an absolute requirement but generally this rock is imagined to be composed of shells larger than 2 mm (at least partly).

Most samples are composed of invertebrate seashells, usually mollusks (bivalvia, gastropoda). Most coquinas are composed of shells of saltwater organisms but freshwater versions exist as well. Fresh rock is mineralogically composed of aragonite because this is the carbonate mineral mollusks use to build their shells. Coquinite, because it is generally much older, is usually composed of calcite. Some coquinas may be phosphatic if this was the material to build the shells. Coquina does not need to be pure limestone. Silicate minerals, especially quartz, may form part of the rock. However, carbonate grains need to form the majority of the rock. If not, the rock should be named a carbonate sandstone or conglomerate.

Shells forming coquina accumulate in high-energy (shallow water where waves break) environments like beaches, bars, raised banks, etc. The term “coquina” comes from the Spanish and means cockle (edible clams). Coquina occurs in many places all over the world but perhaps the most famous occurrences are in Florida, USA.

Coquina
Sample from Germany consisting of gastropod shells. Width of view 13 cm. TUG 1608-2537.

Layer of coquina in a calcarenitic limestone in Morocco (coastal cliff between Essaouira and Agadir). Width of view 40 cm.

Coquinite from Estonia consisting of Ordovician brachiopod shells (Borealis borealis).

Dendritic growth in crystals

December 2, 2015September 25, 2012 by Siim

Dendritic growth is a very common phenomenon in nature. We are all familiar with the way how trees grow by spreading branches and roots from the main trunk (that’s why we call this mode of growth “dendritic”). The term “dendrite” itself is used to describe branched projections of neurons.

Dendritic manganese oxide minerals in Morocco. Width of view 7 cm.

The same applies to inorganic world as well. Window frost is a beautiful dendritic phenomenon, albeit somewhat annoying. Rivers often form a dendritic drainage pattern as well although in this case we can not talk about dendritic growth in the narrow sense of the term.

Manganese oxides are well-known to form nice dendritic patterns on the surface of rocks in veins. Manganese oxides (there are several manganese-bearing minerals that grow this way) precipitate out of hydrous solutions in veins separating rocks. This (or sometimes also branchingly grown inclusions within other crystals) is what the term “dendrite” means in geology. Dendrites are common on the surface of sedimentary rocks, especially limestone.

http://picasaweb.google.com/107509377372007544953/Rocks#5792043665051804370
Dendrites of manganese oxide precipitated on the surface of limestone. Width of view 10 cm.

Such dendrites are sometimes mistaken for fossils and are therefore often used as an example of a pseudofossil. Pseudofossils are natural objects that may be mistaken for fossils. Not to be confused with fake fossils which are man-made rubbish produced to cheat us.

Dendritic growth commences when the material is well below its crystallization temperature. In this case, regular growth which forms crystals with well-developed crystal faces, is replaced with a crystallization mode which favors the formation of protrusions near the corners of crystals. That way new branches develop instead of regular crystal faces. This happens to snow flakes which form out of water vapor in air which is usually much colder than the normal crystallization temperature of ice. New branches do not occur all the time. This process goes on in an orderly fashion because we are talking about crystals here. These protrusions grow larger until they reach a point when the formation of new protrusions becomes favorable again and new branches start to develop. Such a branching network forms a natural fractal-like pattern. Patterns like that repeat themselves in a smaller scale, they look similar no matter what is the zoom or scale of view.

Oolite

June 12, 2025September 21, 2012 by Siim

Oolite is a sedimentary rock made up of ooids (ooliths) that are cemented together. Most oolites are limestones — ooids are made of calcium carbonate (minerals aragonite or calcite). Ooids are spheroidal grains with a nucleus and mineral cortex accreted around it which increases in sphericity with distance from the nucleus. Nucleus is usually either mineral grain or biogenic fragment. The term “ooid” is applied to grains less than 2 mm in diameter. Larger grains with similar genesis are pisoids (pisoliths). Rocks made up of pisoids is pisolite.

http://picasaweb.google.com/107509377372007544953/Rocks#5790668050221394914
Ooid sand from Antelope Island, The Great Salt Lake. Oolite forms when ooids like this get cemented together. The width of the view is 5 mm.

The terms “oolite” and “ooid” are derived from the Greek word for fish roe (oon) which ooids resemble⁴.

Ooids usually possess a clearly developed growth banding. Ooids may be spherical but some are elongated, depending on the shape of nucleus. Most ooids are marine, forming in shallow (less than 10 m, preferably even less than 2 meters), warm, and wave-agitated water such as the Persian Gulf and the Bahama Platform. Ooids in these places form a distinct type of sand — ooid sand. Ooids are kept moving by waves which enables accretion to occur on all sides. This is also the reason why ooids are so well-polished. Warm water is needed to lower the carbon dioxide content in water (higher temperature reduces the ability of water to keep gases dissolved) and thereby enhance the precipitation of calcium carbonate. It is believed that ooid formation is generally abiogenic process. However, the exact formation mechanisms are still unresolved⁴.

Most modern ooids are composed of mineral aragonite. Some ooids form in non-marine environments, the Great Salt Lake is probably the best known example of ooid formation in saline lake. Some ooids form in fresh-water lakes, caves, caliche soils, hot springs, and rivers. Even ooids made of evaporite minerals gypsum and halite have been reported¹. Sometimes ooids form even in human-constructed features such as drainage pipes and water treatment plants⁴.

Some ooids are made of silica (chert), dolomite or fine-grained phosphatic material (collophane). Such ooids are formed by replacement of original calcium carbonate, but they may be also primary. Especially phosphatic and iron-bearing ooids composed of hematite and goethite seem to have been formed as such.

Iron-bearing goethitic (limonitic) ooids are probably formed out of volcanic pyroclastic (volcanic ash) material deposited in sea. Concentric layering in iron-bearing ooids is thought to result from constant agitation of ooids associated with currents and expulsion of gas from the sediment². Such ooids may form oolites which contain nothing but brown iron-rich ooids, but more commonly they occur within other sedimentary rocks. Ooids occurring in Ordovician limestone in Estonia are strikingly similar in chemical composition, internal structures and REE (rare earth elements) distribution to modern iron-bearing ooids described from a vicinity of volcanically active island in Indonesia³.

Oolitic limestones form prolific oil reservoirs. Jurassic Arab sequence in the Middle East, Smackover reservoir of the Gulf of Mexico, and several formations in the Anadarko and Appalachian basins among others are examples of oil reservoirs in oolitic limestones⁴.

http://picasaweb.google.com/107509377372007544953/Rocks#5790667964963227490
Oolite consisting of goethite (limonite) ooids from Germany. Width of sample 12 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5790667990568337970
Close-up of goethitic oolite from Germany. Width of view 18 mm.
http://picasaweb.google.com/107509377372007544953/Rocks#5790667959600838626
Close-up of oolite from Germany. Width of view 3 cm.

Ooid sand from Abu Dhabi, The United Arab Emirates. The width of the view is 5.5 mm.

http://picasaweb.google.com/107509377372007544953/Rocks#5790668012394173538
Ooid sand from Stansbury Island, The Great Salt Lake. The width of the view is 5.5 mm.
http://picasaweb.google.com/107509377372007544953/Rocks#5790668029504177410
Ooid sand from Cancún, Yucatán, Mexico. The width of the view is 5 mm.

References

1. Barr, Donald J S. (2007). Oolite. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 12. 372-373.
2. Heikoop, J. M., Tsujita, C. J., Risk, M. J., Tomascik, T, & Mah, A. J. (1996). Modern iron ooids from a shallow-marine volcanic setting: Mahengetang, Indonesia Geology
3. Sturesson, U., Dronov, A., & Saadre, T. (1999). Lower Ordovician iron ooids and associated oolitic clays in Russia and Estonia: a clue to the origin of iron oolites? Sedimentary Geology
4. Siewers, Fredrick D. (2003). Oolite and coated grains. In: Encyclopedia of Sediments & Sedimentary Rocks (Ed. Middleton, V.). Springer. 502-505.

References

Classification

Composition

Basalt in the field

Metamorphism and weathering

Etymology

References

Pictures

References

Chemical elements in the crust

The most abundant minerals in the crust

The most abundant rock types in the crust

Sediments and sedimentary rocks

Igneous rocks

Metamorphic rocks

Further reading

More pictures

References

References

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