Siim

Golden lava flows

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I am pretty sure most of the readers of this blog live in more southerly latitudes than I do. I live in Estonia and my latitude is 58 degrees north. This is comparable to southernmost points of Alaska.

The days are the shortest now. The Sun is rising only 9 degrees above the horizon and sky is usually covered with thick and gray cover of clouds. Needless to say that there is not much colors outside.

You may live in more fortunate conditions but I bet the chances are pretty high that some additional sunny colors would do only good. Here they are. Beautiful yellow flowers growing on lava rocks in Tenerife, Canary Islands. These pictures were taken in April about 1500 meters above the sea level. If you know what flowers these are, let the rest of us know it as well.

http://picasaweb.google.com/107509377372007544953/Chert#5807632972153327906
http://picasaweb.google.com/107509377372007544953/Chert#5807632999552942626
http://picasaweb.google.com/107509377372007544953/Chert#5807633011024578498

Sand grain Christmas card

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Christmas is approaching fast. Here is an attempt to make a Christmas card composed of sand grains. You may say that it is artistically not very high standard but you see here really tiny objects that were painstakingly arranged in an attempt to make the scene reasonably recognizable.

Sand grain christmas card
White snow is quartz sand from Florida. Stars are forams (Baculogypsina) from Japan. The snowman is made of forams as well (Sorites from Cyprus). His eyes are amphibole grains (from Spain). There are some sea urchin spines (Baleares Islands) and bryozoans (The Caribbean) as snow-covered bushes. Red lights are almandine garnet crystals (California). The whole scene is about 15 mm in width.

With this card Sandatlas wishes you all Merry Christmas.

Volcanic mystery in Santorini

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While hiking in Santorini I stumbled upon an interesting rock. What first striked me was the size. It is slightly more than one meter in diameter and weighs several tons. This area is covered with pyroclastic rocks but most of it is ash mixed with fist-sized or smaller clasts of pumice and scoria.

This apparently andesitic block doesn’t seem to be aerodynamically shaped, so it probably isn’t a volcanic bomb. If it was ejected from the crater at all, it was likely already solid or mostly solid. What also puzzles me is the lack of deformation in the tephra beds below the rock. I try to imagine such a giant landing in high speed on a soft bed of ash and pumice and it doesn’t make any sense to me.

Is it possible that the current location of the rock is not where it originally landed? Maybe it rolled downhill to the current location? Or maybe signs of original deformation are somehow lost? I’d like to hear your interpretation.

http://picasaweb.google.com/107509377372007544953/Rocks#5871905464397689442

Update

Gareth Fabbro responded quickly to my question. You can see his answer in the comments thread. So this rock is neither volcanic bomb nor is it a block thrown out of a volcano. It is most likely fragment of an old lava flow that is removed from its original location by a volcanic eruption and associated pyroclastic flows 3600 years ago during the Minoan eruption which most likely also destroyed the mythical Atlantis.

Rounded star sand

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Tests or shells of Baculogypsina (foraminifera genus) look very spectacular. I wrote about them in an article of star sand. However, they are sand grains after the owner of the test dies and like all other sand grains as time goes by they become more and more rounded.

That’s why we usually do not sea Baculogypsina tests as beautifully preserved as are those collected in Hatoma Island, Japan. One often has to dive to collect the most beautiful specimens where waves can not break them.

Beach sand in Bali (Indonesia) contains Baculogypsina tests as well but at first sight they are hardly recognizable. It is really no “star sand” anymore. More appropriate name would probably be “sphere sand”.

Rounded Baculogypsina tests from Bali
Rounded tests of Baculogypsina from Bali, Indonesia. The width of the view is 6 mm.

Foram species Baculogypsina sphaerulata
Foram species Baculogypsina sphaerulata from Hatoma Island, Japan. The width of the view is 15 mm.

In Memory of Arnold Bouma

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This is my addition to Ron Schott’s post on the passing of Arnold Bouma.

Here are my pictures of a turbidity current sequence. Not a complete Bouma sequence and not very good quality pictures but a sequence of turbidity currents it is without doubt.

Bouma described the sequence, later named after him, in 1962. It was almost half a century ago! So he must have lived a really very long life? Actually not so long. He was young when he made his most famous discovery. The same applies to many famous scientists. Einstein and Darwin for example were not even 30 when they already knew things that made them very famous later.

Mudstone and siltstone
Turbidite sequence in the Spanish Pyrenees. Lighter and tougher layers are composed of sand and silt mostly. Darker and softer layers are mostly mudstone that were deposited on top of the sandy layer.

Turbidite sequence
The layers are tilted because of the orogeny that created the Pyrenees and pushed the former marine sediments into near vertical position.

Actinolite sand from Ontario

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Heavy mineral sand is probably my favorite sand type. They are natural concentrates of many interesting minerals. These sands are beautiful to look at and very educational as well. Here is a sample taken from the shore of Halls Lake, Ontario, Canada. Thanks to Frances Vandervoort for this sample.

My photo equipment unfortunately is at its limit here. This sand and many other good samples are too fine-grained. I hope I will soon be able to take higher magnification images.

Most important components of this sand are quartz (which is hardly surprising) and actinolite (it isn’t rare either but usually not as abundant as here). Quartz is transparent and actinolite is black. Actinolite is a member of amphibole group. Other notable minerals are almandine, orthoclase, epidote, augite, pumpellyite, and magnetite.

Some of you might ask that how can you possibly be so sure about these minerals? How can you say that this feldspar is orthoclase and amphibole is actinolite? Indeed, this is difficult, and I do not claim to have an ability to do this with optical microscope only. X-ray diffraction is the keyword here. This technique significantly helped me in the identification process.

What is the possible source rock of this sand? To say that for sure, one really needs to go out and familiarize him/herself with the local geology. I have never been there, so I can only theorize. Most minerals seem to point to the metamorphic source. This part of Canada is a shield. There is lots of very old (Proterozoic) metamorphic and igneous stuff there.

Actinolite could form in metamorphosed calcareous sediments but it could be a product of regional metamorphism also (glaucophane schist facies). Pumpellyite also favors regional metamorphism and glaucophane schist facies but unfortunately there is no trace of glaucophane itself. Augite points to igneous source. Almandine usually comes from schist (regional metamorphism). This sand most likely has several if not many source rocks contributing to its composition but some of them can not be far away. Actinolite for example looks fresh and is very abundant here but its resistance to weathering processes is not good.

Heavy mineral sand from Ontario
Sand sample from Ontario, Canada. The width of the view is 10 mm.

Update

Thanks to Howard I just discovered from where to download Canadian geological maps. Here is an excerpt from a geological map of Halls Lake area:

Halls Lake geomap
Geological map of Halls Lake area. The sample was probably collected at the NW corner of the lake. Photo courtesy: Department of Natural Resources Canada.

Pg — Irregular granitic gneiss
Pga — Regular granitic gneiss
Pp — Porphyroclastic gneiss
Pst — Granitic straight gneiss
Pta — Medium to fine-grained tonalitic straight gneiss
Pt — Coarse biotite-hornblende orthopyroxene tonalitic orthogneiss 1450-1300 Ma.

Actinolite probably comes from gneiss (Pp). Hopefully it is amphibolitic gneiss. It could be Pt also. Maybe this ‘hornblende’ there is a field term? Sometimes geologists have a habit to name it hornblende whenever they see black amphibole.

Goethite

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Goethite is a common weathering product of iron-bearing minerals like magnetite, pyrite and siderite. It is an oxyhydroxide or iron — FeO(OH). The most characteristic feature of this mineral is brown to yellowish color. It may resemble hematite (Fe2O3), but hematite is either gray or reddish. It changes to hematite on dehydration. Powdered hematite is reddish brown, but goethite is yellowish brown. Both of these minerals are good paint pigments. Goethite is the main constituent of yellow ochre.

http://picasaweb.google.com/107509377372007544953/Rocks#5790667964963227490
Oolite consisting of goethitic ooids from Germany. Width of sample 12 cm.

It is a common iron-bearing mineral in soil. It is usually formed under oxidizing conditions. It precipitates directly from marine and meteoric waters and is the main mineral phase containing precipitated iron in these environments. It may form mixtures with other similar minerals which may alter the color. Limonite is a mixture of several iron oxides and hydroxides, goethite usually being one of the most important constituents. It may be mined as a mineral resource. Bog iron is mostly composed of it. Goethite is the principal component in sedimentary iron ore in the Lorraine Basin of France1.

The mineral is named after the German poet Johann Wolfgang von Goethe who was also interested in mineralogy.

http://picasaweb.google.com/107509377372007544953/Coll#5850315377439812994
Sand sample from Martha’s Vineyard, Massachusetts, USA. Brown rounded pellets are goethite grains. Width of view 10 mm.
http://picasaweb.google.com/107509377372007544953/Rocks#5854471542847046882
Bog iron is mostly composed of it. Width of sample from Estonia is 14 cm.

Hematitic iron ore (SEDEX-type) with goethite (yellow). Svinsås, Norway. Width of sample 13 cm.

References

1. Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.

Volcanic sand taken apart

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Photos of sand may be beautiful but their educational value is probably pretty limited. We tend to overlook details if thousands of grains are visible at the same time. So I decided to try another approach with a volcanic sand sample from São Miguel Island in the Azores Archipelago.

This sand contains olivine, plagioclase, and two pyroxenes (diopside and aegirine). This clearly indicates that the sand is the disintegration product of mafic volcanic rocks. There are also lots of volcanic glass and analcime grains. I don’t know whether the latter is a primary mineral or an alteration product of volcanic glass. Analcime is the only zeolite that may directly crystallize from magma.

The presence of both analcime and aegirine give a strong hint that the volcanism in the Azores archipelago is probably alkaline. This term may easily create confusion. Here it means that the magma is enriched in alkaline chemical elements (sodium, Potassium). It has not much to do with brines or bases dissolved in water. Analcime and aegirine both contain sodium.

Most of the components this sand is made of are unstable in the weathering environment. Hence, it has to be pretty immature sand. Some olivine grains are clearly weathered. They are dull yellowish green while fresh olivines are bright green.

Pay attention to the fact that there is absolutely no quartz. Oceanic islands with mafic volcanism are one of the few places where you can see sand without this mineral. Quartz and olivine are often mutually exclusive. Sure, everything may be mixed up in sand but these minerals in normal circumstances do not crystallize from the same body of magma. Magnesium rich olivine forsterite (olivine in this sample is also forsterite) will react with free silica and form pyroxene: Mg2SiO4 (forsterite) + SiO2 (quartz) → 2MgSiO3 (enstatite, one of pyroxenes). This equation means that we may have olivine and pyroxene or pyroxene and quartz but no olivine and quartz together.

Common constituents of volcanic sand.
These are the major constituents of a sand sample collected on the São Miguel Island, The Azores Archipelago, Portugal. The width of the view is 19 mm.
Azores sand
Same grains together as a sand sample. Try to find the minerals shown on the previous photo. The width of the view is 14 mm.

Almandine

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Almandine is one of the garnets. The most common of them. Its ideal composition is Fe3Al2(SiO4)3. Fe can be replaced with Mg or Mn. If there is Mg more than Fe, we call this garnet pyrope. If there is more Mn than Fe, then it is spessartine. There are more garnet varieties but they do not form continuous solid solutions with the three mentioned above.

Almandine is a common mineral in metamorphic rocks, mostly schists and gneiss. But it is a usual constituent of felsic igneous rocks too. Almandine is very common in sands heavy mineral fraction. It is so widespread because it is a major component of several rock types and moderately resistant to the weathering. It is among the most beautiful minerals one can see in sand. If you notice pink grain, it most likely is almandine. However, it may also have darker red color. The composition is not fixed and therefore there is no single identifiable color.

It is not too hard to distinguish garnet from other minerals because it is isotropic. You still need a polarizing microscope though. But there is really no sure and easy method to say what type of garnet it is. Color might be a good indicator. Spessartine tends to have more yellowish colors and pyrope is dark red or even purplish. However, color is often misleading indicator. So it probably is better to call these reddish grains garnets without attempting to specify. Things might get more complicated because there are some insiduous minerals that may be misidentified as garnet. These are staurolite and red chrome-bearing spinel.

http://picasaweb.google.com/107509377372007544953/Chert#5808422097271089698
Almandine is the most common of garnets. These crystals contain some magnesium which puts them part way into the pyrope group. That might also explain darker red color. Thanks to Bill Beiriger for these beautiful crystals. Width of the view is 10 mm.

You can find lots of pictures of almandine-bearing rocks in an overview article of the garnet group.

Glacial arch

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Glacial arch is a natural bridge made of ice. Here is a picture of a glacial arch formed at the base of the Briksdalsbreen glacier which is a small arm of Jostedalsbreen — the largest glacier in continental Europe.

These archs form because something preferentially melts the ice where the arch will form. It is running water in most cases. Here you can see that there is a debris slope behind the arch. So there is lots of running water in rainy days. And it rains a lot in Norway.

Glacial arch
Glacial arch at the edge of the Briksdalsbreen glacier.

Briksdalsbreen
Briksdalsbreen glacier. An arm of Jostedalsbreen – the largest glacier in continental Europe.