Rocks

Migmatites are mysterious rocks. We still do not know how they formed. Maybe they represent half-completed melting process? But maybe these lighter-colored bands are just molten material (magma) that intruded semi-molten and darker metamorphic rocks but didn’t form where we see them now?

I have seen many migmatites. Estonian coastline is littered with this material that were brought here by the continental glacier in the Pleistocene. Bedrock in Southern Finland is largely composed of high-grade metamorphic rocks. Thus, migmatite is a really common rock type here. Even the term “migmatite” was brought into the geological terminology by a Finnish petrologist Jakob Sederholm in 1907.

I tend to support the view that both formation processes were probably involved. Not necessarily always simultaneously but one here and another there. Therefore this dispute may somewhat resemble the famous arguments between blind men who all had very different opinions how elephant might look like.

Whatever the truth, I think we all agree that these rocks are pretty. Here is one nicely folded example, this time from Norway near Geirangerfjord. This rock is part of a local bedrock. I don’t remember exactly but the width of the view might be one meter or slightly more. This rock face is horizontal, I just took a picture of the rocks I was standing on.

Migmatite near Geirangerfjord in Norway.

The rock I am presenting today is clearly metamorphic but I am not sure whether I should call it mylonite or augen gneiss or mylonitic gneiss or something else. Perhaps it isn’t so important because these terms are often pretty vaguely defined and their meanings are overlapping. I encourage you to comment if you happen to know more about the subject.

This rock is a small glacial erratic about 30 cm in diameter near the coastline of NW Estonia. This rock was part of the Finnish bedrock and was carried south to Estonia by the advancing continental glacier during the Pleistocene. There is no doubt about that because such rocks are not exposed in Estonia. Estonia is a sedimentary platform composed of carbonate rocks and sandstone from the Paleozoic. The gray and yellowish stones surrounding the mylonite on the picture are limestone shingles from the local bedrock. Estonia is a paradise for fossil hunters (there are lots of trilobites, crinoids, bryozoans, cephalopods, etc.) but pretty boring for a hard rock geologists. However, there is lots of glacial debris which is mostly either high-grade metamorphic stuff (Svecofennian orogeny about 1.9 Ga) or rapakivi granites (anorogenic intrusions about 1.6 Ga).

The ovoidal reddish grains (porphyroclasts) are K-feldspar crystals or augens (“augen” is a German word that means “eye”) which have been subjected to some serious deformation. All this happened in a high temperature and high pressure conditions deep in the crust beneath a forming mountain belt where rocks behave like a very viscous liquid although they are still solid.

This rock is an example of a porphyroclastic texture. This is a texture where ovoidal and seriously strained but still largely survived crystals (porphyroclasts) are surrounded by finer foliated matrix. Rocks with a fabric like that are usually called mylonite or augen gneiss.

This article is a part of Rocks from Fennoscandia series.

Overview and images of gneiss as a rock type are here: Gneiss

Years ago as a first year geology student I photographed some of the hand samples we studied during the introductory geology courses. I also wrote articles to Estonian Wikipedia and uploaded some of my images.

It really was years ago. I was an undergraduate student then and now I’m finishing my masters studies. For three years I was away from active studies and worked as a science journalist. Yes, indeed, sensation-driven sexy-headline-writing journalist. This may explain why writing a blog post almost every day is not that difficult task for me. I am really used to it. Only thing that makes it harder now is that I write in a foreign language. But journalism I hope is history for me and I am back in geology trying to defend my masters thesis this spring.

I mentioned that I photographed some common rocks. I did so with a cheap point-and-shoot camera without a tripod but despite that I have seen one particular image in very many places. Perhaps you have seen it as well but don’t pay attention to it because there is no story connected with the image.

You can find this image illustrating the article about gneiss in Encyclopedia Britannica and Wikipedia among really many other places. I am pretty sure that this is the most famous gneiss in the Internet. Couple of days ago I was looking for information about the geology of Sri Lanka. I found one slideshow which was supposed to contain some images of rocks from Sri Lanka. The bedrock of this country is mostly composed of old metamorphic rocks, including gneiss. However, I did not believe what I saw there anymore since the moment I noticed my gneiss again. I don’t know from which country this particular hand sample is from but it is extremely unlikely that it is from Sri Lanka.

This particular image is not very sharp. It lacks location information. It even lacks scale. It was made by a first year geology student. What this story should teach us is that internet was then and still is quite poor of good geological information. We as geobloggers shouldn’t think that the Internet is already full of good information and we have nothing new to add. This is not true. Just take some pictures or write about something you know and show it to the world. Chances are pretty good that your modest contribution is the best Internet has to offer on this subject. And maybe one day you accidentally stumble upon your own image in Britannica.

This article is a part of Rocks from Fennoscandia series. I believe it could be from Karelia (part of a Fennoscanian shield) where I have seen similar gray and very old Archean gneisses.

Overview and images of basalt as a rock type are here: Basalt

Olivine basalt is basalt with olivine phenocrysts. Phenocrysts are mineral grains that are substantially larger than the groundmass surrounding them.

If the olivine grains are large enough they may be used as gemstones. Olivine as a gemstone is often named peridot(e). Olivine in basalt usually contains more Mg than Fe. Its chemical composition is (Mg,Fe)₂SiO₄

Olivine is denser than most other minerals in basalt and it is also one of the first to start crystallizing. Hence, it often sinks, if it has room to do so, and forms basalts that are abnormally enriched in olivine in the lower part of lava flows for example. Olivine crystals in this case were already formed before the lava poured out of the volcano because phenocrysts need time to grow. Rapid cooling in subaerial conditions would not allow it to grow as large as the crystals shown on the photo below which is a piece of olivine basalt from Oahu, Hawaii.

Olivine sand is a disintegration product of rocks with similar composition.

Scoria is a highly vesicular and glassy volcanic rock with a mafic composition.

Scoria is very common rock type in volcanic areas. Sometimes it is not considered to be a distinct rock type. In this case it is a structural variety of basalt, andesite, etc. The problem is that volcanic rocks are officially categorized according to their chemistry. Scoria, however, is defined mostly by its vesicular nature.

Upper parts of basaltic lava flows may be scoriaceous for example. So the rock can easily be scoria and basalt at the same time. It may feel awkward and confusing to some but geology is full of such peculiarities which may seem strange at first but is actually necessary and really makes sense if you think about it.

Scoria often occurs as a single piece of rock that was thrown out of a volcano. Sometimes entire volcanic cones are made of scoria which in this case is also called cinder. ‘Scoria’ tends to be preferred term nowadays but ‘cinder cone’ is often used.

The composition of scoria is usually mafic. Similar rock with felsic composition is called pumice. Scoria, unlike pumice, does not float on water although it feels unusually lightweight for a dark-colored volcanic rock. Scoria contains smaller number of vesicles than pumice and these vesicles, because of less viscous magma, tend to be much larger. Here is an outcrop in Santorini that contains both pumice and scoria in different layers. Not every vesicular dark-colored volcanic rock is scoria. Occasional vesicles here and there are normal in basalt. It has to contain really many vesicles to be named scoria.

Scoria is sometimes reddish in color. This is iron, more precisely its oxide, that gives it such a rust-colored appearance. Scoria contains appreciable amount of iron, that is why it is called to be mafic (magnesium+ferric).

Limburgite is a volcanic rock. This rock name is nowadays seldom used. It is one out of more than thousand rock names that were given to all kinds of igneous rocks during 19th and 20th century. Such a proliferation of names is understandable because at the time there were no uniformly accepted classification principles to use and everyone would be pleased to name a new rock type. But it definitely did not serve the best interests of geology. I am afraid no one fully understood what was going on at the time.

http://picasaweb.google.com/107509377372007544953/Rocks#5805071006844076034
Limburgite from Kaiserstuhl (the volcanism took place in the Miocene). The width of the rock is 11 cm.

Then came a man named Albert Streckeisen who just didn’t propose yet another scheme for the classification of igneous rocks. Instead he analysed all the work that was done so far, consulted with many other geologists, and became the first head of Subcommission of the Systematics of Igneous Rocks (under IUGS Commission on Petrology) in 1970.

Under his guidance were created QAPF diagram for the classification of plutonic rocks and TAS diagram for volcanic rocks which are both now widely used and considered to be a standard in igneous petrology. ‘Limburgite’ and many others were just dropped.

How to call this rock type now? Volcanic rocks are classified according to their chemical composition. Limburgite is similar to basalt but it contains less silica and more alkali metals. In TAS diagram it would fall in the field of basanite.

‘Limburgite’ may be dropped as an official rock name but geologists often stubbornly keep traditions alive and continue to use historical names in areas where these names were used before. So do I. This rock what I am presenting here is limburgite as much as one rock can be. It is from Kaiserstuhl (low volcanic range in Germany) where it was first defined by a German petrographer Harry Rosenbusch in 1872.

Typical limburgite contains lots of vesicules that are partly filled with secondary minerals (amygdaloidal texture). Limburgite contains phenocrysts of augite (black slender prisms) and weathered olivine (yellow spots) in a glassy groundmass. What makes this rock type special is the lack of feldspars.

Diabase (dolerite) is a dark-colored igneous rock. It is compositionally equivalent to gabbro and basalt but texturally between them. Diabase is a common rock type. It occurs mostly in shallow intrusions (dikes and sills) of basaltic composition. It grades to basalt when it solidifies rapidly and to gabbro when more time is given to the crystals to grow. The term “microgabbro” is sometimes used to refer to such rocks.

Diabase is composed of plagioclase feldspar (mostly labradorite) and pyroxene (augite). The crystals that make up dolerite are usually visible to the naked eye, but sometimes porphyritic rocks of basaltic composition with pyroxene and especially plagioclase phenocrysts are also named that way. Ophitic texture (randomly oriented plagioclase laths enclosed by pyroxene grains) is a characteristic feature of diabase. Minerals of lesser importance are magnetite, olivine, ilmenite, hornblende, biotite, chlorite, etc.

The term “diabase” has a long and complicated history. It was first defined by a French geologist Alexandre Brongniart in 1807. Brongniart named so rocks that were composed of feldspar and hornblende. Another famous French geologist René Just Haüy preferred to use the term “diorite” for such a mineral assemblage and this concurs with the contemporary use. It was Brongniart himself who decided to abandon the term in 1827. However, it somehow managed to survive, mostly thanks to German scholars who continued to use it but now to refer to the rocks which were composed of pyroxene and plagioclase.

It was a German geologist Harry Rosenbusch in 1877 who decided to introduce the age criterion and possibly created a problem that persists to this day. According to him, only rocks of pre-Tertiary (pre-Cenozoic) age were named diabase. In Great Britain the term “dolerite” was used (although it too originates from France, this term was first used by Haüy in 1822) which has a meaning of a fresh-looking diabase and the term “diabase” itself was left to altered and old dolerites only. The age criterion is not in use anymore, but both “dolerite” and “diabase” continue to be used. These two terms are synonymous.

Diabase is a popular ornamental stone. Crushed stone is often made of it and it seems to be a popular choice to use in sauna heaters.

http://picasaweb.google.com/107509377372007544953/Rocks#5787194233322780322
Diabase is a popular ornamental stone. This sample of dolerite is from a tombstone workshop. Unknown locality. The width of the view is 25 cm.

http://picasaweb.google.com/107509377372007544953/Rocks#5787194278199935586
Dolerite from Portrush, Northern Ireland. The width of the view is 9 cm.
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Diabase (microgabbro) from Estonia. This sample is taken from a drillcore. The width of the view is 4 cm.

http://picasaweb.google.com/107509377372007544953/Rocks#5787194326462518530
A piece of a dolerite dike of the Troodos sheeted dike complex in Cyprus.
http://picasaweb.google.com/107509377372007544953/Rocks#5787194349045543586
Sheeted dikes in Cyprus are composed of dolerite.

Obsidian is a massive volcanic glass. The term ‘massive’ has several (although related) meanings in geology but here it means that the rock (obsidian is a rock type, not a mineral) is homogenous. It lacks layering, cleavage, foliation, phenocrysts, etc. It is just a piece of volcanic glass without further conditions. In the majority of cases obsidian solidified subaerially (on land). Volcanic glass that formed underwater has alternative names like tachylite and hyaloclastite.

http://picasaweb.google.com/107509377372007544953/Rocks#5848238745712314674
Typical obsidian is either black or slightly reddish and often demonstrates beautiful conchoidal fracture. Width of sample 11 cm.

So the volcanic glass and obsidian are not synonyms although in many cases you can freely use both terms. It is definitely not wrong to use ‘volcanic glass’ instead of ‘obsidian’ but you should be careful the other way around — volcanic glass is not always obsidian.

Volcanic glass is an igneous rock that is composed of largely uncrystallized magmatic material. Most of it is not crystallized because the crystals had two difficult problems which restricted their growth. The first one is time. Large crystals need a lot of time to grow. There is very little of it when viscous magma is pushed out of a volcano and cools rapidly. I already gave a subtle hint what the second problem might be. It is the viscosity of magma/lava. If the magmatic body is very thick or viscous, the crystals have a really hard time forming because they simply don’t have new material coming in if almost nothing is able to move inside the magma body. The result is that everything solidifies just randomly as glass.

So obsidian forms from viscous magma only? Usually yes, but not always. Most obsidians are rhyolitic in composition. This lava is the thickest because it has the highest silica content. Why is that important? Because silica causes magma to polymerize. There are countless bridges (chemical bonds) between oxygen anions of silica (SiO₂) which is the reason why this magma is so hard to move. If the magma contains lots of metals (cations) then it is less viscous because these cations break the framework structure of silica. I think this point is important and worth paying attention to because lots of people seem to think that rhyolitic magma is more viscous than basaltic magma only because its temperature is usually lower.

http://picasaweb.google.com/107509377372007544953/California02#5876808909850640834
Obsidian with gray layers of rhyolitic pumice in Long Valley Caldera in California. Pumice and rhyolite have very similar composition but they differ in texture — pumice is highly vesicular while obsidian is massive.

Is some volcanic glass still basaltic in composition? Yes, but in this case the cooling has been really rapid. This is the case if basaltic lava is flowing into the water. There are some nice black sand beaches in Hawai’i that are composed of fragments of volcanic glass with basaltic composition.

How to identify small obsidian fragments like sand grains? In most cases it is not too hard to do by optical examination only. Obsidian is usually black although reddish varieties are pretty common also. Obsidian has a strong luster and conchoidal fracture. This means that fracture surface is smoothly curving (like a seashell).

Obsidian is usually black. This color is caused by minute inclusions and tiny crystals in the glass. Red color is caused by the same stuff that gives red color to weathered basalt, desert sand and K-feldspar. It is mineral hematite (iron oxide).

http://picasaweb.google.com/107509377372007544953/Rocks#5848481665432908098
Obsidian is usually dark and its surface is shiny.

Obsidian is not stable in the weathering environment but it does not mean that it can not last millions of years. Obsidian on the Moon may be billions of years old because the Moon is dry. The same applies here on the Earth as well. In dry areas obsidian can last pretty long. However, obsidian formations older than the Cenozoic (it began 65 million years ago) are unknown.

Here are two pictures of obsidian grains picked from coarse-grained sand samples.

Obsidian from the Yellowstone National Park. Width of view 35 mm.

Obsidian from NE California (The Cascades volcanic range). Width of view 22 mm.

Rapakivi is a type of granite. Its type locality is the Vyborg batholith which is located in SE Finland and Karelia, Russia. “Rapakivi” is a term that comes from the Finnish language (rotten or crumbled stone) but this rock type is not restricted to this particular area. The magmatic plutons showing rapakivi texture are found in many places all over the world, including USA, Brazil, China, etc.

Large orthoclase ovoid (5 cm) mantled by the oligoclase (one of plagioclase feldspar minerals) rim. Not all of the orthoclase phenocrysts are mantled by the plagioclase rims, though. The large ovoid on the lower right lacks plagioclase rim.

This rock is well known for its interesting texture — large ovoidal orthoclase phenocrysts are surrounded by plagioclase mantles. However, not all the rocks that are described as rapakivi granites show this texture. To avoid confusion, rapakivi is now defined as a granite which comes from a pluton that shows rapakivi texture at least partly. The rock with a rapakivi texture from the Vyborg batholith has a special regional name — wyborgite.

Rapakivi from the type locality and its surroundings is approximately 1.6 billion years old. But they are not necessarily restricted to the Proterozoic eon. Rapakivis from the Archaean and the Phanerozoic eons are known as well. The Vyborg batholith is one of the many rapakivi plutons in Northern Europe but most of them are covered by younger layers of sedimentary rocks.

Rapakivi is clearly plutonic rock but the volcanic rocks associated with it most likely existed as well. We have even some examples of very old rhyolitic lavas but most of it is long gone. This volcanic episode must have been very violent and explosive but the evidence is very scarce. The chance for a survival is not very high for a supracrustal rocks of that age. The emplacement of rapakivi plutons and associated rocks (diabase, anorthosite, rhyolite) was probably caused by a rifting episode which subsequently failed. These rocks are therefore anorogenic, their emplacement was not caused by the mountain building episode.

Why is this rock “rotten”? Because it is quite susceptible to weathering. Plagioclase feldspar weathers easily and the inequigranular nature of the rock makes it easier to disintegrate by the temperature changes. The boulders are often so weathered that it takes only bare hands to crumble it into smaller pieces.

Here are some glacial erratics of this type now on the coast of the Baltic Sea near the Vyborg batholith in Karelia.

Some orthoclase ovoids are really big (the largest in the middle is 8 cm in diameter). Karelia, Russia (The Gulf of Finland).

Weathered boulder on the coast of Karelia, Russia (The Gulf of Finland).

One more crumbled boulder on the Karelian coastline.

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Fresh plagioclase is greenish gray and orthoclase is more close to orange than pink tone. Luumäki, Finland. Width of view 35 cm.
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Weathered surface looks different. Plagioclase is light gray which makes the texture more easily noticeable amd orthoclase is also clearly paler pink. Luumäki, Finland. Width of view 30 cm.

Anorthosite is a fascinating rock and sparks interest even among those who usually don’t care about rocks. The reason is simple. Anorthosite is often composed of mineral labradorite which is famous for an iridescent effect called labradorescence. You’ll find more in this article: anorthosite and labradorescence.

The Moon highlands seem to be composed of anorthosite. We have both indirect and direct evidence for that. Measurements made recently by the Japanese lunar orbiter SELENE suggest that the lunar anorthosite may in many cases be almost totally monomineralic — composed entirely of plagioclase with very high calcium content. We have direct evidence also — American astronauts who visited Moon in the early 1970s brought back 61 rock samples that were found to be anorthosites.

It is wonderful to think that large portion of the Moons surface (highlands surrounding the basalt lowlands or marias) is schillering like anorthosites here on Earth often do. However, it is likely not the case. There are several differences between terrestrial and lunar anorthosites. Terrestrial anorthosites contain more sodium (sodium and calcium can replace each other in all proportions in the crystal structure of plagioclase). Plagioclase must have the composition of labradorite — one of plagioclase minerals. It means that 50-70% of the sites in the crystal structure which are occupied either by calcium or sodium ions are occupied by calcium. In the lunar anorthosites Ca-content is close to 100%. In order to have a labradorescence, the percent of calcium needs to be in the range of 48-58%. The effect of labradorescence is the result of a breakup of plagioclase crystals into many alternating lamellae of different (calcium and sodium rich) composition. If there is very little sodium present, such exsolution simply can not take place.

There are more differences between terrestrial and lunar anorthosites. Lunar anorthosites are light-colored, while some terrestrial anorthosites are dark. Here on Earth the cooling of anorthositic magma bodies took very long time. The crystals which show labradorescence are often very large, even pegmatitic (more than an inch in length). Lunar anorthosites, however, are quite fine-grained. Only very few crystals are larger than 1 cm.

Lunar anorthosite is very old. It is believed that it formed when the lunar magma ocean solidified which probably took place in the first 100 million years of the existence of the Moon. Lunar anorthosite is believed to be the result of a gravitational differentiation. Plagioclase is lighter than most other minerals found there and therefore rose to the uppermost part of the magma ocean. However, the details of this process are still hotly debated.

Take a look at the NASA Lunar Sample Catalog if you want to see more images and general overview of the rocks collected by the astronauts of the Apollo program.