Field Geology

I haven’t written a post for several days. I actually have an excuse. I am geologizing in Cyprus. I have gathered some nice material for future posts and hope to see much more in the remaining seven days.

I’d surely like to write a blog as well but internet connections aren’t so easy to find here. My hotel room, for example, doesn’t have such a luxury.

I came here mostly to see the world-famous (among geologists) Troodos ophiolite complex. Unfortunately, the most special part of it (rocks from the mantle) are still snow-covered. They are exposed in the mountains just around the highest peak of Cyprus.

This is a major disappointment to come here from cold and snowy Estonia just to discover that people here are skiing as well. But this means that I have more time for other parts of the island. There is so much more to see. Geology in Cyprus is varied and well exposed.

I won’t promise but I will try to post a photo with short geological explanations every remaining day of my trip. These photos are taken during the past few days here in Cyprus.

Take a look at the beautiful swallowtail gypsum exposure as well that is located only about 100 meters away.

Pothole is a vertical, cylindrical erosion feature in a riverbed (most commonly).

Potholes have variable sizes in the range of several dm to a few meters in extreme cases and they are commonly more deep than wide. I spotted several potholes in Norway in a dry part of the riverbed, one of them is shown above.

How do they form? We need rapidly flowing water that is capable of carrying coarse sand, pebbles, or boulders that are whirled around in the depressions of the rocky riverbed which slowly do their grinding job. You can see many of them resting on the bottom of the pothole. You can even see small grooves on the sides of the pothole which show how the pebbles move once they are trapped inside the pothole.

Potholes aren’t rare but we need several things to happen in one place to see them. The river has to flow rapidly, the river should preferably flow on the bedrock, and perhaps it would be better to have a relatively easily erodable type of rock (limestone should do well). Finally, potholes have to be dry sometimes. Otherwise, we won’t spot them.

http://picasaweb.google.com/107509377372007544953/Rocks#5805071036033437666
Pothole in a riverbed (more than half a meter across). Photo taken in Norway.

Here is an interesting pair of photos taken in the Spanish Pyrenees which illustrate several hugely important geological concepts.

So, what is going on here and how is it important? It is obvious that there are two distict rock formations on the first image. The gray layer is composed of limestone and on top of it is siltstone. We know that usually younger rocks are on top of the older ones but this isn’t true in this particular case. The limestone formation is younger than the siltstone formation. Note how it looks like the lower limestone formation is cutting the bedding of the siltstone formation. It shouldn’t be that way if the silt was deposited on top of the eroded surface of the limestone formation. There is a fault plane between them which means that the upper formation is pushed on top of the lower one. Such faults are called reverse or thrust faults and they are very common in mountainous areas.

Now if we move on to the second image (to do that I had to climb few hundred meters up in the mountains) we see that the order of rocks is reversed — limestone is on top of the siltstone formation. This is normal succession because younger rocks lay on the older ones. There is no fault plane between these formations but these rocks are not where they originally were — geologists say that they are allochthonous. It means that they have been moved from their original location but in this case they did so together as one block.

It is important to understand that the limestone formations on both photos are the same (more or less). We just have several copies of one crustal layer, one pushed on top of the other. The whole thing is hugely important in geology because that’s how mountains are mostly made and this particular set of outcrops in my opinion is especially good real-life illustration. In most cases it isn’t so easy to understand what is going on because rocks in mountainous areas are often severely folded and metamorphosed. Here you see only little folding on the second image and the rocks are not metamorphosed. Another very important aspect is that the blocks that were moved are relatively small. This is different in the Alps, for example, where it is much harder to to see and understand what is going on.

Cap de Creus is a small peninsula in NE Spain north of Barcelona. It is a geological wonderland — a metamorphic terrane that is penetrated by pegmatitic magma which contains huge tourmaline crystals and blue K-feldspar. Right now we see there a jigsaw of different lithologies with interesting honeycomb or tafoni weathering patterns. I thought to post these pictures in several different posts but maybe it is better if they are in one place for you to enjoy.

Dark-colored schist and light-colored pegmatite.

Schist and pegmatite.

Here you can see how crazily is pegmatite mixed with schist.

My fellow geology students climbing on a pegmatite.

It may be hard to believe but this black stuff in the middle (about 1 meter in length) is tourmaline.

Tourmaline in pegmatite.

Small tourmaline crystals (here they seemed to be small). My boot for scale.

Tafoni or honeycomb weathering benefits from the proximity of The Mediterranean that constantly provides salty water that helps to create this type of weathering pattern.

Tafoni in schist.

Blue microcline (probably) in pegmatite.

The Mediterranean…

…was warm and inviting. We did several swimming breaks this day.

Oceanic islands are like sanctuaries where beach sand is not dominated by quartz.

However, there are exceptions. Here is a photo of Playa de Las Teresitas in Tenerife:

Playa de Las Teresitas is a beach in the eastern part of Tenerife. The sand there is unusually light-colored for a volcanic island.

Tenerife isn’t a continental island. There are no outcrops of granite, gneiss or similar rocks. How can it be that there is a long beach (about 1 km) with sand that is composed of almost pure quartz?

The answer is simple. This beach is not natural, it is yet another sign that we seem to be living in the Anthropocene. The sand itself is of course natural but it has been brought in from the Sahara.

Even Wikipedia covers this beach with a separate article. I found one sentence in that article especially amusing: It is one of the most popular beaches of the Canary Islands, and the only one on Tenerife that does not have the black, vulcanic sand that most of the rest of the Canary Islands suffer from.

Wow, I didn’t know that volcanic islands suffer from the black sand. I think they actually don’t care. But obviously humans seem to dislike black sand. Otherwise, I’d see no point bringing huge amounts of sand from the Sahara. There is one understandable reason — black sand gets very unpleasantly hot in the middle of a sunny day. But probably people dislike it also because of its color. Black sand is dirty, isn’t it? I don’t know of any research projects that have investigated the issue but I find it hard to believe. Black sand is composed mostly of pyroxene, magnetite, plagioclase, and olivine. There are no good reasons to think that these minerals are somehow dirtier than quartz, K-feldspar, and biotite. I even believe that black sands are cleaner because they are mostly farther away from main pollution sources.

I love black sand but I don’t enjoy sunbathing. So, my relations with it are somewhat different from the majority. I love it because it is refreshingly different and contains interesting minerals. I calmly tolerate white beaches on oceanic islands as long as they are interesting curiosities but I’d definitely change my mind if such projects become the norm. Luckily, that will not happen anytime soon because these projects consume lots of money. Lack of money is often very good thing. Take a look at the southern shore of the Persian Gulf. There are people who have lots of money not because they are smart but because they have lots of crude oil. The result is a series of stupid projects like artificial sand islands that are really needed only for those that have huge amount of money but have no bright ideas what they should do with it.

Playa de Las Teresitas with breakwater visible on the left. It is needed to prevent storm waves from carrying the sand to the sea.

We all have a general understanding what valleys are because they are so common landforms. They start forming wherever there is a downward surface irregularity which attracts running water (in most cases). Valleys tend to be exceptionally durable landforms — once they form, they tend to grow and grow.

U-shaped valley has been typically carved by a glacier. Photo taken in Scotland.

But not every hole is a valley. Valleys are long and they are sloping in one direction. There is typically river or stream running in the bottom of the valley but not always, some valleys are dry. Geologists often talk about U-shaped and V-shaped valleys. U-shaped valleys are typically carved by a glacier. Such landforms are very common in previously glaciated mountainous areas. V-shaped valleys are even more common because they are usually eroded by running water. This is not an exhaustive classification. There are more valley types. Tectonic valleys, for example, are formed by entirely different forces.

V-shaped valley has been typically eroded by running water. Photo taken in Scotland.

There is a nice exposure of pillow basalt just by the road that connects the Teide NP in Tenerife to the eastern part of the island. I didn’t know its existence before but when I saw these rocks while driving through the misty cloud forest that surrounds the Teide I knew I have to stop and take a closer look.

I had no doubt that these are pillows. However, there is one annoying aspect that troubled me. This outcrop is approximately 1200…1400 meters above sea level. Why is this a problem? Because pillow basalt forms underwater.

I think there is rather small possibility that these pillows formed in a lake. If we assume that they formed in seawater then there really can not be another explanation that the whole island has been pushed up by more than a kilometer. This is rather remarkable. I thought that oceanic islands grow larger mainly by the addition of new lava and pyroclastic layers. They of course swell also if the volcano is active which Tenerife is, although the most active regions of the Canary Islands at the moment seem to be the western part of the archipelago (La Palma, El Hierro).

But now I know a better explanation. These are not pillows at all. This is just a classic and beautiful example of spheroidal weathering. There are no radial fractures visible which should develop in pillow basalt because of contraction while cooling rapidly and there seem to be no flash-frozen crusts.

However, it is a really nice example of spheroidal weathering. What is the reason that we find it there? First of all, volcanic rocks often weather to smectite clay which expands when wet. Repeated diurnal cycles of taking on and losing water help to form such a weathering pattern. The particular location is especially suitable for that type of weathering because there are lots of moisture — its a cloud forest surrounding the volcano. You will even see fog in the first picture.

Dikes are sheetlike igneous intrusions. We usually see them in two dimensions but sometimes they form prominent threedimensional landforms.

Here is a photo of a dike I saw while spending a vacation in Tenerife. I didn’t have an opportunity to stop there and inspect it more closely. Even the photo is taken while driving but it should be understandable that this dike forms really impressive wall. The photo is taken inside the Las Cañadas caldera which surrounds the Mount Teide which is a highest volcano in Europe and second overall only after the volcanoes of Hawaii (Mauna Loa and Mauna Kea). I mean measured from the base of the mountain which is kilometers beneath the waves.

How is it possible that such thing survived the caldera collapse? Well, the caldera there is a bit unusual. It is not a depression of a vertical collapse. It was more like an enormous landslide. I don’t know for sure but probably the dike which is almost perpendicular to the caldera wall right behind it (and therefore more or less parallel to the direction of landslide) just survived the event and was later preserved better than the surrounding rocks because it is made of hard rock, not loose pyroclastic material.

I have also written about syncline in 3D and another interesting landform in the Las Cañadas caldera which is claimed by some to be the most photographed rock in the world — The Cinchado.

http://picasaweb.google.com/107509377372007544953/Chert#5807632643396376626
Dike in the Las Cañadas caldera in Tenerife, The Canary Islands.

My home country Estonia has been geologically quiet place for a loooooong time.

I usually think its a bad thing. No volcanoes, no mountains, little mineralogical and lithological versatility, no real structures — everything is more or less parallel and layered. The sedimentary layers that cover the crystalline basement have never been buried very deep. Maximum perhaps 1 km or so. Therefore, these sedimentary rocks are only weakly altered by diagenetic processes. Because of that we have something rather remarkable. We have a layer (up to 70 meters in thickness) of bluish gray clay in the bedrock that is half a billion years old (from the Cambrian). I really mean clay, not claystone. This clay is still quite soft and becomes muddy when wet.

Here is a photo of this clay with mud cracks. The mud cracks are fresh but the clay itself is really old. The photo is taken in a clay quarry. The clay is used to make cement.

Recent mud cracks in Cambrian clay.

http://picasaweb.google.com/107509377372007544953/Rocks#5786990837301660770
The same clay before getting wet. The width of the sample from Kunda is 11 cm.

Several years ago I visited the Isle of Mull in Scotland. It is geologically interesting island (largely composed of igneous rocks) but I mostly remember the weather. There seemed to be water everywhere. Clouds reaching to the ground, slight rain almost all the time, and soil saturated with water.

It was windy as well, so it was not the best possible day for hiking. Another difficulty that I faced there were the signs of “private property” or something similar with the same meaning. The population of the island is small — less than 3000 but these people seem to enjoy solitude. I didn’t want to disturb them but wanted to see geology. These two wishes went against one another, so I had to make compromises here and there.

Here is one picture from that island. It describes pretty well the weather conditions and general look of the island — green wet mountainsides and sheep. There is one additional interesting geological element. The stone wall in the middle. Is it some sort of ruin of a manmade structure? No, it seems to be dike and it was not the only one. There were several near the western coast running roughly perpendicular to the coastline.

Dike is a tabular sheet of igneous rock that cuts through the surrounding rocks. They typically radially surround larger igneous intrusions which feeds them with fresh material and creates extensional cracks when the magma body rises. These cracks or weaknesses in the rocks are used by molten dike-forming magma which is usually basaltic in composition.

Dike in the Mull Island (near the western coast), Scotland.

Why is this dike standing out? Most likely because it is made of tougher rocks than the rocks surrounding them. Can’t say for sure what type of rocks surrounded these dikes but rocks that cropped out at the coast were some sort of hard sandstones. I am writing this based on my memory. I had no field notebook then. So take these memory pictures with a grain of salt.

I spent a night in tent and left the next day. This day weather was better and everything seemed more pleasant. I hope one day I will go back because the geology there is variable and deserves much closer look. I saw less than I had planned but this happens almost all the time. It is true that geology is not a rabbit, it will not run away but unfortunately it is often hidden. Weathering, soil, vegetation, private property, and foul weather are perhaps the most powerful enemies we have to face in the field. Geology on a geological map and geology accessible in the field are two very different things.