Columnar basalt is definitely beautiful and interesting rock formation but it is nothing new to a seasoned geologist. Columnar sandstone, however, is probably different story. I had never heard anything about such rocks when I accidentally just stumbled upon one.
There is a large “crater” named Makhtesh Ramon in the Negev Desert in Israel. It is described as the largest makhtesh in the world. That really sounds impressive until you learn that these landforms actually occur nowhere else than in the Negev desert.
So it is simply largest in the Negev but it is impressive anyway. What is makhtesh? It is neither a meteorite crater nor is it a volcanic caldera although it is frequently described as a crater. It is an erosional landform of a structural dome which has softer rocks (sandstone) below and harder (limestone) on top of it. Intermittent rivers called wadis erode the softer rocks faster and create sharp escarpments like crater walls. Makhtesh Ramon has pretty impressive measures. It is 40 km long, 2-10 km wide and 500 meters deep.
Makhtesh Ramon no doubt is worth a visit in its own right but I was most impressed by what I saw in the middle of the makhtesh. There was a hill made of sandstone. But what kind of sandstone! It was like a huge pile of columnar logs. No wonder that this hill is named Ha-Minsara (The Carpentry Shop).
I don’t know exactly how it was formed but I believe the mechanism had to be very similar to the formation of a columnar basalt. Something had to provide lots of heat. There are volcanic rocks nearby in the crater, so it probably was magma. What else could it be? After this baking episode the sandstone slowly cooled and cracks formed when the cooling rock mass contracted. I’d be glad to hear if anyone has a better explanation.
Here are some pictures of the Makhtesh Ramon and the Ha-Minsara in the middle of it.
http://picasaweb.google.com/107509377372007544953/Chert#5807632865374734066 The rim of the Makhtesh Ramon.
http://picasaweb.google.com/107509377372007544953/Chert#5807632703023946434
http://picasaweb.google.com/107509377372007544953/Chert#5807632708113129730
http://picasaweb.google.com/107509377372007544953/Chert#5807632760583574834
http://picasaweb.google.com/107509377372007544953/Chert#5807632772659826002
http://picasaweb.google.com/107509377372007544953/Chert#5807632770462746386 I can’t believe my eyes that this is indeed sandstone.
Have you seen a syncline in 3D? Here is a picture I took couple of months ago in the Spanish Pyrenees. It was an exhausting hike. About 20 km horizontally and 1 km vertically but I really enjoyed it.
An explanation for those of you who do not feel very comfortable in the company of awkward speaking geologists: A syncline is a U-shaped fold. It is a common rock formation in mountainous areas. This particular syncline is interesting because one can see it from different directions. In most cases only two dimensions are visible.
It seems that the most common foram test in Hawaii light-colored beach sand is Amphistegina. This foram (genus) lives in Indo-Pacific oceans and some species in Western Atlantic also.
The tests of Amphistegina found on the coastline of Hawaii islands are usually heavily wave battered and barely recognizable. This foram appeared in early Eocene (about 50 million years ago). It prefers warm and shallow water (less than 30 m) and inhabits mainly seafloor between the coastline and coral reefs. Amphistegina has quite hard test compared with many other much more fragile foram tests which allows them to survive in wave agitated water.
For more information and pictures of less battered tests, take a look at the page of Amphistegina on foraminifera.eu.
The wave battered tests of Amphistegina (foraminifera). Oahu, Hawaii. The width of the view is 9 mm.
Heavy minerals are minerals with a density greater than 2.8…2.9 g/cm3. Why these numbers and what for is this range needed? Why not just one number? The most common minerals in most sand samples are quartz and feldspar. Calcite and dolomite are common cementing minerals in sandstones. All of these minerals have densities below the range shown above. Heavy minerals are usually volumetrically insignificant. However, there are a large number of heavy mineral species, each of them having their own story to tell. Therefore geologists often need heavy minerals to get as much information out of the studied rock as possible.
Heavy minerals in beach sand from Sri Lanka. Most intensely colored grains are spinel crystals. Width of view 20 mm.
http://picasaweb.google.com/107509377372007544953/2015#6190951519258082962 Heavy minerals including gold on a gold pan. This concentrate is panned from river sediments in Lapland, Finland.
It is very uncomfortable to do if only one grain out of one hundred or less is what we are looking for. We therefore seek methods to somehow separate heavy minerals from the bulk of the sand. Obvious way to do that is to use some heavy liquids which have a density greater than that of quartz (2.65 g/cm3) but lighter than that of most minerals.
Several liquids with slightly different densities have been used. That’s the reason why there is range instead of fixed value. The liquid used to separate heavy minerals from the rest is usually bromoform (its density is 2.89 g/cm3). It is liquid at room temperature and feels abnormally heavy as one is usually not used to liquids as dense as bromoform. Its high density is a result of three bromine atoms in its chemical formula (CHBr3). This liquid has a nasty downside. It is poisonous and has a disgusting odor. I guess it is actually a good thing because you don’t want to be exposed to the vapors of it. There are some more recently developed alternatives though which do not have such an adverse health effects, most notably polytungstate liquids.
Heavy minerals are useful to study the provenance of a sand or sandstone. “Provenance” is a fancy term geologists love to use when they talk about the place where the sand grain broke out of its parent rock and began its journey as a sediment particle. How do we study provenance? We take a look at the heavy mineral fraction and make sure what is its composition. Let’s say it contains garnet, staurolite, and kyanite. What can we say about that? I think we can reasonably safely assume that this sand is a weathering product of a metamorphic terrane because this mineral assemblage is very typical to metamorphic rocks. If there are lots of augite, magnetite, and olivine, then it probably comes from an igneous source.
Heavy minerals are sorted out by a running water near the coastline at Pfeiffer Beach, California.A closer look at the Pfeiffer Beach sand. Main heavy mineral species are garnet, epidote, zircon, magnetite, spinel, staurolite, etc. Width of view 8 mm.
It is a very broad approach. Unfortunately more detailed studies into the provenance often give debatable results because there are so many factors that can alter the composition of the heavy mineral suite. These are mostly weathering, burial diagenesis, hydrodynamic sorting, and mechanical abrasion during the transport.
Many diamond-bearing kimberlite pipes are discovered by studying heavy mineral fraction of a sand. We need to look for pyrope (which is heavy mineral) for example. This is a rare Mg-bearing garnet that is associated with diamonds in kimberlite pipes. Its presence in the river sand may give us a hint that a kimberlite pipe may be nearby. To hunt down its location, we have to go upstream and take many samples until pyrope and some other index minerals found in abundance in kimberlite pipes suddenly disappear. Heavy minerals have other applications in forensics, oil and gas industry, etc.
Heavy minerals sometimes get naturally concentrated as a heavy mineral sand and there were, of course, no bromoform involved. It was moving water either in a stream or beach that did the job. Sometimes the sand is so concentrated in heavy minerals that it has a real economic value as an ore. Sand collectors also love these black sand deposits. Such heavy mineral concentrates are called placers.
Gold panning is an activity used to separate gold flakes and nuggets from these placers. However, gold is not the only mineral that is mined from placers. These minerals are also cassiterite (tin ore), ilmenite (titanium), magnetite (iron), rutile (titanium), monazite (rare earths), chromite (chromium), zircon (zirconium), etc. Australia is particularly well-known heavy mineral source, but heavy mineral deposits occur in many places.
Some common and not so common heavy minerals in sand and some of their properties:
Mineral
Density
Stability in weathering
Stability in diagenesis
Provenance
Anatase
3.82…3.97
High
High
Felsic igneous rocks, hydrothermal veins, alteration product of titanite or ilmenite.
Ole Nielsen wrote an interesting post about the world’s highest volcanoes. There are several candidates. Nevados Ojos del Salado and Llullaillaco being the most serious contenders depending on whether we count all of them or only the historically active ones. Which volcano claims the title is not of paramount importance to me. What is interesting is that they are all located in South America. This is nice to know that they are there but so what?
The Andes in Chile. It is considered to be the longest mountain range on Earth (7000 km). Image: Wikipedia.
This is just a mere fact that gives us no further understanding of the mechanisms that allow these volcanoes to become so high. I thought that maybe there are some easy answers in the web but as it usually happens, there are tons of descriptions and trivial facts but very little really useful insight.
So I just tried to collect my pieces of thoughts together and come up with some ideas.
I know that the highest point on the surface of the Earth is the tip of Chimborazo volcano in Ecuador. Some of you may protest that at least yesterday Mt. Everest was higher. Yes, it most likely still is but Chimborazo is the highest point if measured from the center of the Earth. This is so because the Earth’s equatorial radius is longer than its polar radius (the difference is 22 kilometers). Chimborazo is definitely not the highest mountain if measured from the mean sea level (even not in South America) but it is nicely located almost at the equator. Its height from the mean sea level is 6268 meters.
How does that help us answer the question? Well, I just thought that if the volcano is farther away from the center of the Earth, then gravity should be weaker there which makes it easier for the volcano to grow little bit higher. Highest volcano in the Solar System is Olympus Mons. Its height (measured from the base) is 25 kilometers and it is located on Mars where gravity is much weaker that it is here.
We also have to consider the effect of centrifugal force because our Earth is rotating. This force gets stronger as we move higher. So it also helps to counteract the gravitational pull.
The value of gravitational acceleration (which takes into account both factors described above) is 9.78 (m/s2) at the equator and 9.83 at the poles. The difference is there but only about 0.5%. It means that we (and mountains) would weigh about 0.5% more at the poles than at the Equator. That is clearly not enough to explain why the volcanoes in the Andes are so high.
Maybe we have to blame the climate? Atacama desert that is right there next to the highest volcanoes is very dry. Perhaps weathering and erosion are not keeping pace with the uplift as successfully as it is doing in other, more humid regions? I think it might be one part of the answer.
The Andes are on the one hand compressional orogene that is situated at the border of Nazca and South American plates but on the other hand it is also a volcanic range that sits on top of the subduction zone. The highest peaks there are volcanic edifices. Perhaps they are so high because they sit on top of the orogenic belt.
Perhaps the subduction and mountain building in this region is particularly intense which allows the volcanoes to become so high? I don’t have good data for that but the sea floor spreading is very fast in the Southern Pacific, so the subduction should be intense as well.
Sea floor subducting beneath the Andes is young and the spreading in the East Pacific Rise very fast. Image: NOAA.
Maybe the fact that the subduction angle there is very small helps to push the mountain range higher. It is small because the subducting slab is relatively young and therefore more buoyant than older slabs.
Last but not least, Earth’s surface is pushed higher in the regions where the crust is thicker. The crust beneath the Andes is very thick (even 60 kilometers) which is definitely one of the contributing factors.
Earth’s crust beneath the Andes is very thick. Image: USGS.
So I found seven possible contributing factors:
1. Equatorial bulge
2. Centrifugal force
3. Climate
4. Orogene + subduction
5. Intensity of subduction
6. Subduction angle
7. Thickness of the crust
Amphiboles are elongated and generally dark-colored silicate minerals. Amphiboles are both structurally and compositionally similar to pyroxenes (other large group of silicate minerals). Both amphiboles and pyroxenes are very important rock forming minerals. However, their presence in sand is generally much smaller than one would assume. Amphiboles weather pretty easily (although not as rapidly as pyroxenes).
This is amphibole arfvedsonite from Greenland (Ilimaussaq intrusion). Second grain in the upper row demonstrates strong luster and third grain shows typical cleavage striations. The width of the view is 15 mm.
The composition of amphiboles is quite complicated. It is complicated because of numerous possible replacements of ions in several different sites in the crystal structure.
Most common amphibole is hornblende. This mineral has no definite composition either. It is therefore also divided into several minerals. However, these subdivisions are not important here because the identification of them is impossible without pretty sophisticated and very expensive analytical tools.
Amphibolite is a very common rock where amphiboles as the name suggests are key ingredients. Amphiboles in amphibolite are mostly hornblende. White mineral is plagioclase. Senja, Norway. Width of sample 9 cm.
How to identify amphiboles? They are generally black or dark green. Sand grains made of amphibole are usually elongated. They may have vertical striation (sign of a cleavage). There are actually two cleavage planes but the chance to see both of them when dealing with sand grains is not too high. Amphiboles usually have strong luster which distincts them from pyroxenes which are usually duller black or green.
Mineral besides pyroxenes that could easily get misidentified as amphibole is black tourmaline (schorl) and black spinel. They are not nearly as common in rocks but as sand grains they are much more resistant. Tourmaline has very strong pleochroism and it is often brownish. Spinel grains very often have smooth spots with intense luster that resemble volcanic glass. Amphiboles have no such feature. Spinel and tourmaline are also often rounded because usually they have been sand components for a long time. Amphiboles, however, tend to be more often prismatic. They are generally younger because they simply won’t last as long.
Sint Maarten is a sothern part of Saint Martin Island in the Caribbean. It belongs to the Netherlands. Needless to say, weather is generally very nice there and beach sand light-colored. What is the composition of this sand? It is a coral sand. It contains fragments of corals, forams, gastropods, sea urchins, clams, etc.
Carbonate biogenic beach sand from Sint Maarten. The width of the view is 10 mm.
Selected biogenic sand grains picked from the sand. The width of the view is 6 mm.
Sea urchins spines are very common biogenic sand grains. Sea urchins are sometimes called echinoids although it is not precise because sand dollars and few other groups are echinoids as well. However, sea urchins tend to leave behind the most visible and numerous traces. I don’t know how many spines one animal may have but this number definitely exceeds 100. Hence it is no surprise that there are lots of spines in the sand but precious little sea urchin tests. These of course tend to break into fragments which makes finding and recognizing them even more complicated.
Elongated colorful grains are sea urchin spines. The width of the view is 10 mm. Muizenberg, South Africa.
Sea urchin spines are often colorful. Green, white, and purple are very common shades. They are easily recognizable even if the fragments are not elongated because they have very well developed and characteristic strucure. Sea urchins inhabit all oceans. Hence they may be present in both low and high latitude beach sands.
Alive sea urchin on the seabed near Sardinia. Photo: Marco Busdraghi/Wikimedia Commons.
Trash thrown into the sea is a global and unfortunately ever-increasing problem. Most of the garbage is plastic because the decay of this material takes enormous time. Another type of resistant trash dumped into the sea is glass.
http://picasaweb.google.com/107509377372007544953/Coll#5852307320112952114 Glass Beach, Kauai, Hawaii Islands. Transparent, green, and brown glass pebbles. Dark grains are fragments of olivine basalt (without olivine which is due to weathering replaced by iddingsite). There are also biogenic grains (forams, mollusks, echinoids, and corals). Although I am somewhat reluctant to say so, but I think we have to admit that even trash can sometimes become beautiful. The width of the view is 20 mm.
A very small part of it may indeed come from castaway sailors sending bottle messages but mostly it is the result of people using sea as a huge garbage dump. Glass is not as visible and annoying in the sea as plastic because it is heavier than water and sinks to the bottom. And rounded glass pebbles can easily be mistaken for some minerals. It may look very similar to quartz, especially if the glass is transparent.
In our particular case the transparent grains are indeed glass pebbles, not quartz. It is easy to distinguish between the two with the help of a polarizing microscope. Unlike quartz, glass has no crystal structure and is therefore dark (showing no interference colors) when placed between the two crossed polarizers of the microscope.
Sea glass has lost its sharp edges because of the tumbling action of waves. It is frequently washed ashore but the concentration of seaglass in the beach sand is usually very low. There are some notable exceptions such as the Glass Beach in Kauai Island or the Glass Beach of Fort Bragg in California. Glass has concentrated there because these places were used as garbage dumps in the past.
Glass Beach, Fort Bragg, California. Transparent, blue, and green glass pebbles. There are some biogenic grains (mollusk shells, echinoids) and rock fragments (mostly lithic sandstone). The width of the view is 28 mm.
The majority of trash on the coast is anything but beautiful. Plastic garbage is not only ugly sight but also a serious threat to marine life. The photo was taken in Iceland.
My last post about the mysteries of sand was successful. I got some useful insight. Here is another photo of seashells picked from a single sand sample.
This time it is from Majorca, Spain. If You know more precisely to which organisms these seashells belonged or think that some of them may be misidentified, don’t hesitate to contact me.
Seashells picked from a sand sample collected in Majorca, Spain. The width of the view is 10 mm.
, 2nd Edition. Prentice Hall.
