Siim

Rocks from Fennoscandia

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The Fennoscandian shield is located in Finland, Sweden, and NW Russia. Part of it is in Norway also. Shields are extensive areas of exposed basement rocks. Basement rocks are hard crystalline rocks that usually have formed during orogenies. The lithology of shield areas is generally versatile and very interesting.

Shield is tectonically stable area, just like platforms which are covered by sedimentary rocks. Crystalline basement is exposed in shield areas but that doesn’t mean that these rocks are outcropping everywhere. They are still mostly covered with loose sediments, soil, and vegetation but there are no sedimentary rocks on top of the basement. Well known shield areas are located in Canada, Fennoscandia, Australia, and several other places.

Here I am focusing on the Fennoscandian shield. It is close to where I live — Estonia. My home country is located on the sedimentary platform but the Scandinavian shield is very near. Continental glaciers that reigned here mere 12,000 years ago brought lots of boulders with versatile origin and composition.

Whenever I write a post about the rock from the Fennoscandia, I also add it here. These posts may be about rocks that were brought to Estonia by the glacier but often they are about the Fennoscandian rocks in their natural birthplace.

Mylonite from Estonia Metamorphic rock with ‘eyes’ or augens.
Most famous gneiss in the Internet The story of a gneiss hand sample. I am not sure where this rock is from but there is a high probability that it is from the Fennoscandian shield, most likely from Karelia where I have seen similar gray gneisses.
Rapakivi granite Rapakivi is a special type of granite with an interesting texture.
Ruskeala marble Marble from Ruskeala quarry in Karelia.
Anorthosite Anorthosite is famous for its play of colors. Find out what causes it.
Jotnian sandstone This feldspar-rich rock is usually named either Jotnian sandstone, quartzite, or arkose.

Jotnian sandstone

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Overview and images of sandstone as a rock type are here: Sandstone

This rock type is usually named Jotnian sandstone arkose or quartzite. The bulk of these stones are probably from the bottom of the Gulf of Bothnia (between Sweden and Finland) but some are from the Finnish and Swedish mainland as well.

This sandstone contains lots of feldspar. That’s why it is often described as arkose – sandstone that contains at least 25% feldspar. The abundance of feldspar indicates that arkose is quite immature rock type – its travel distance from the source rock to the deposition location have been relatively short and the journey rapid.

In this particular instance it seems to be the case as well because in Western Finland our arkose is deposited in the fault bounded structural basin (Satakunta Formation). The maximum thickness of the sandstone formation is not known but it is probably well over one kilometer. The source material is the Svecofennian basement (1.9 billion years old). The arkose itself is approximately half a billion years younger. It is probably deposited as a fluvial sand (deposited by a running water).


Coarse-grained arkose. Pink grains are feldspar and gray ones quartz crystals. This rock sample was collected in the NW corner of Estonian mainland. The width of the view is 2 cm.

Arkose
Jotnian sandstone from Estonia. Width of sample 15 cm.

This post is a part of the Rocks from Fennoscandia series.

Desert sand

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Deserts cover huge areas, mostly between 10-30 degrees N and S of the equator. Many of these deserts are sandy, at least partly. Good example is Sahara — the largest desert in the world. To describe a desert sand, we first have to make clear what we are talking about. When we imagine desert sand, we probably think of sand dunes. That is what I am writing about here although dune fields cover only about 20 percent of modern desert areas.

Sand dune in Moroccan Sahara
Sand dune in Sahara (Morocco). Note how wind is blowing sand grains off the crest of the dune.

Dune sand is generally very well-sorted. It means that all the sand grains are roughly the same size. There is almost no dust. It is blown away and deposited elsewhere, possibly as a loess deposit far away from the source area. There are also no gravel or boulders because wind is not capable of carrying such a heavy load.

, Libya
Desert sand composed almost exclusively of rounded quartz grains. Sand sample is from the Sahara Desert (Erg Murzuk), Libya. Width of view 1 cm.


Rounded quartz grains (covered with rust-colored hematitic pigment) plus biogenic and lithic fragments from the Dubai Desert, United Arab Emirates. Biogenic grains clearly indicate that the sea can’t be very far away.


Dune sand from the Gobi Desert, Mongolia. Width of view 1 cm.

Roses from fire

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

Lava roses or more correctly rosettes really do exist. I stumbled upon one when visiting Tenerife. Such formation is a special type of columnar basalt.

When basalt sheet cools slowly it contracts and vertical cracks emerge and propagate through the cooling lava, leaving behind prismatic columns. These columns are quite well-known phenomena of volcanic regions. However, sometimes the cooling lava mass doesn’t have a sheet-like morphology. What if it is a lava tunnel with a spherical cross section? In this case such lava rosettes can form. Bedrock is surrounding hot lava mass and cools it equally from all sides.

Lava rosette
Lava rosette in Tenerife. The width of the view is more than 10 meters.
Columnar basalt
Columnar basalt in the Massif Central, France. The width of the view is approximately 20 meters.

As it should be done

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You won’t understand until you see. That’s especially true in geology. That’s why I am sceptical about books which should teach us how to identify minerals and rocks. You need to see, touch, and feel the material to really start to understand it. Sure, it isn’t always possible. We have to take the best of what schemes and photos can provide but if possible we should try to do better. While visiting Scotland I noticed one especially good example of how things should be done. Sedimentary sequence (more precisely small model of it) composed of various rock types laying above the Lewisian gneisses is shown on the photo below.

The dark gray layer on the upper part of the sequence is called the Moine Schist. It is a metamorphic rock. Something definitely seems to be wrong here: how is it possible that a metamorphic rock is on top of a sedimentary sequence? Since rocks metamorphose deep below the ground where temperature and pressure are extremely high, it is not possible that only the uppermost layer of a sedimentary sequence is metamorphic.

The solution to this geological puzzle is actually rather simple but it took a very long time to figure it out. The Moine Schist was originally marine mud but it was not deposited on top of the Durness limestone where it lies now. These rocks are pressed one on top of the other later during mountain building and the Moine schist was already formed as a solid metamorphic rock by that time. Such planes along which one rock complex is sliding past the other are called faults. The whole structure shown below is called the Moine Thrust. It is world-famous among geologists because of its very long history of research.


Moine Schist was metamorphosed during the Silurian period.


Durness Limestone is a dolomitic limestone of the Cambrian period.


Salterella Grit is a fossiliferous sandstone of the Cambrian period.


Fucoid Beds is a dolomitic siltstone and Pipe Rock a sandstone, both Cambrian.


Basal Quartzite is actually not a quartzite (metamorphic rock). It is a sandstone of Cambrian age. Such pure quartz-rich sandstones are sometimes called orthoquartzites. Torridonian Sandstone (arkose) is by far the oldest rock of the sequence. It is approximately 1,000 million years old.

Pumice and scoria from Atlantis

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Volcanic material on the Island of Santorini, Greece. Dark layer is composed of scoria or cinders. Light-colored layer is made of pumice.

Both pumice and scoria are made pretty much the same way. They are basically pieces of lava froth which have been thrown out of a volcano and landed as very porous volcanic rocks. It is mostly chemical composition that makes the difference between these two. Pumice contains more silica which makes its parent magma very viscous and resistant to release trapped gases. Scoria contains bigger bubbles which are easily visible to the naked eye. Scoria is much heavier than light-weight pumice that easily floats on water.

But what is the connection between all this and Atlantis? The Island of Santorini (or Thira) is the most probable location of the ancient Atlantis. This legendary island, if it really existed, was destroyed by the same forces which produced the scoria-pumice sequence shown below. Hence, these rocks may have taken part in destroying the mysterious Atlantis.

Take a look at a close-up photo of scoria from Etna volcano.

I also recommend to check out a gigapan of a pyroclastic sequence in Santorini.

Santorini tephra

Conglomerate from the Pyrenees

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Overview and images of conglomerate as a rock type are here: Conglomerate

These beautiful stones from the Spanish Pyrenees may have formed in several different ways. One possibility is an ancient riverbed. However, the angularity of individual stones (clasts) makes this assumption rather unlikely.

It is easy to notice that riverbeds of mountain streams are usually composed of rounded stones. Another major way how these rocks form are gravity driven debris flows on flanks of mountain ranges where alluvial fans are formed.

The conglomerate contains two major components – gray limestone and reddish sandstone. Therefore we need both of these rocks to be exposed nearby to feed an alluvial fan with a coarse grained material. Nearly vertical mountain walls of versatile composition are pretty good candidates for the provenance for such a rock type.

The rock is few hundred million years old. So it was formed well before the Pyrenees mountain range emerged. Pyrenees are a relatively recent phenomenon (geologically speaking of course) which were formed approximately 50…25 million years ago. These rocks have had an opportunity to witness and take part in the slow formation of the mountain range.

http://picasaweb.google.com/107509377372007544953/Chert#5807633132794889714
http://picasaweb.google.com/107509377372007544953/Chert#5807633133466686530

Ghana sand

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This sand sample is from the Western coast of Ghana. Sand sample is coarse-grained and is composed of several distinct and interesting components. Here is a short illustrated description of this sand sample.

This sand is pretty unusual because it is largely composed of lithic fragments. Most sands are composed of mineral fragments, majority of them being quartz of course. Quartz is present in this sample also but it is a relatively minor component. Lithic sands are immature sediments. It means that their source rocks are not far away. Volcanic rocks of bimodal character (basalt-rhyolite) are present on the coast in this particular case. Typically to ocean coasts at low latitudes, this sand contains many different biogenic grains. Most of them are clam shells and sea urchin spines.

Sand from Ghana
Sand sample itself. Most grains are about 1 mm in diameter.

Sea urchin spines
Sea urchin spines.

Clam shells
Clam shells.

Altered basalt
Altered basalt (green color is a result of chloritization and epidotization).

Rhyolite fragments
Rhyolite fragments.

Lithic sand

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Lithic sand is predominantly composed of small fragments of rocks. Such sands and sandstones are said to be lithic. Lithic sand is unusually immature sand type. It means that the source rocks must be not very far and the weathering agents have not yet had time to break these rocks down to their constituents (individual mineral grains).

http://picasaweb.google.com/107509377372007544953/Rocks#5789446247782283426
Lithic sand from the West Beach, Whidbey Island, Washington, USA.

There are very many rock types and consequently almost an endless array of possibilities to categorize lithic sands into different types. Rock fragments that often occur in sand are amphibolite, sandstone, quartzite, chert, schist, phyllite, granite, and basalt. Most lithic sands are dark in color.

Basalt pebbles
Sand formation in progress. Pebbles and granules of basalt near the southern tip of La Palma slowly transforming into black sand.
Caption
Lithic fragments of mica schist in the sand. Width of view 20 mm.

Mixed carbonate-silicate sand

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Sand is usually composed of mineral grains but especially marine samples contain often biogenic fragments as well. There are many occurrences of either pure or almost pure mineral and biogenic sands but sometimes these two are mixed up in all proportions to form an interesting hybrid carbonate-silicate sand type.

http://picasaweb.google.com/107509377372007544953/Rocks#5789430272034997042
Mixture of yellow and white biogenic and black volcanic sand grains. Jungmun Beach, Jeju-do island, South Korea.