Trip around Uppsala

On Sunday the 22^nd of April, the day after the excursion to Väddö, we had another fieldtrip, this time in and about Uppsala, our hometown. Just as in Väddö, the vast majority of the rocks of Uppsala are about 1.9 billion years old igneous and metamorphic rocks, so there were no fossils to hope finding.

The weather was quite awful this day. The sky was about as grey as the rocks. Even light rain is enough to demoralise any poorly motivated student, and, not knowing how much water my camera can handle, I did not dare to grant me the fun of taking a lot of pictures either – I only have photos of the least uninteresting localities. Motivation was scratching the bottom this day.

The first notable exposure was by a thermal power station. This might sound silly, but what really was curious about this site was the pattern of cracks and fractures – called joints in geological terms.

Some fissures are cutting others, others are parallel to one another. Some are very parallel. 

And on a larger scale:

Joints can form in various ways, but these feel like the work of tectonic forces – i.e. related to the movement of crustal plates – perhaps triggering earthquakes that shake the rocks loose, making them break along planes of weakness.

Some cracks, however, seem to be of a completely different origin.

The smooth, rounded shape instantly got me thinking about water erosion. But, then we found more of them. 

And so we realised that they were actually man-made drilling holes. You can even see two large screws at and near the base of the middle hole.

A few of these joints (the real ones) have been filled with precipitated minerals, in the same fashion as the calcite example from Väddö. Basically, mineral is precipitated along the walls of the fissure, and, as the mineral accumulates, after some time, it fills the crack entirely.

The next site was a bit outside the actual town, close to my home. I live in a small community called Vänge. The locality is clearly exposed by the road I have taken back and forth to town at least five times a week for more than six years, and I have never noticed its strikingly obvious features until now.

The exposure is dominated by light pink granite, dotted with some darker minerals. But there are some prominent deviations:

I was shocked that I had never paid attention to these distinct dark bands, clearly contrasting the granite.

From the second picture it is clear that these are parallel intrusions – sheets of molten rock that penetrated through the already existing granite body. If you look really closely at the first picture, you can also see signs of contact metamorphism, a chemical reaction in the first rock (the granite) caused by contact with the magma. In this reaction, the granite lost much of its potassium, the element that gives the potassium feldspar in the granite its light red colour; the granite was bleached by the dark intruding material. This is even more visible in a thin section of granite that was sandwiched by two parallel intruding bodies that emerged very close to each other, forcing a reaction in essentially the entire granite block in between.

The final noteworthy destination was another granite-dominated exposure, this time behind a Lidl store in town. They had surveillance cameras there, so I wonder what they are up to.

Among the granite, we found a section with impressive pegmatite, an extremely coarse-grained rock type, otherwise fairly reminiscent of granite. This pegmatite was unlike any I had ever seen (not that I have seen many, but still). It had absolutely enormous quartz crystals (the light grey, translucent ones).

The presence of large crystals in igneous rocks is normally a sign of the rock having cooled slowly at great depths (with high temperature). Slow cooling means that crystals have long time to accumulate and grow. Pegmatites, however, are exceptions to this rule. Crystals of this size are among the last to form, when most of the rocks that make the magma viscous (slow-flowing) have cooled and the magma consist largely of gases and more fluent liquids, allowing mineral ions to migrate faster toward each other and quickly form large crystals.

There were also slabs of breccia conglomerates – sedimentary rocks with big clasts (rock fragments) and a fine matrix (the mass in between the clasts).

Hehe, no, really, that is just cement. 

But, we did find some living organisms as well. It might be weird, but the climax of most of my geology excursions has been finding something actually alive and moving. (There have been plants everywhere, of course, but they are not as fascinating.)

Here, we have a gastropod (snail) mollusc and two isopod arthropods, probably wood lice. Isopods are among the most primitive land animals (yes, arthropods were the first animals on land, and wood lice relatives were among them). They also remind me of a rather silly lab we did in biology class about a year ago, where we examined the one of the primitive movement patterns of wood lice: taxis, a directional movement in response to a stimulus. That is, if you stimulate them somehow, they will move either toward the stimulus if it is favourable, or move away if they dislike it. Based on this, we wanted to investigate what kinds of stimuli they prefer. So, we simply created a few different environments around a handful of wood lice and waited to see where they moved. It was quite fun, actually.

Hard rock geology at Väddö

Saturday the 21^st of April, I went to Väddö, a part of a near-shore island in the western coast of central Sweden, together with essentially the same group of students from Estonia, plus the rest of us who took the bedrock geology course on weekends, minus the PhDs and postdocs, and with other teachers (barring one, who was with us to Estonia too, but, being specialised in rocks, did not have much to say in Estonia – now, however, she could go wild).

This course had two main parts: the evolution of life, and the structure and properties of rocks and the Earth’s crust. The former was the focus of the Estonia trip, and the latter was the only thing we could study now in Väddö. This was a place with clear igneous and metamorphic features – that is, rock types and structures formed by igneous activity (melting and recrystallising rock) and metamorphism (deforming rocks into different rock types). In other words, we would look at magmatic and tectonic forces shaping this coastal cliff area.

The rocks were about 1.9 billion years old, so there was no point in hoping to find fossils. Besides, fossils are only found in sedimentary rocks.

Maybe I should mention that there are three rock groups: igneous (formed after molten, liquid rock cools and solidifies), sedimentary (formed as sediments – rock and mineral fragments, and possibly also organic matter that later forms fossils – are compacted and cemented – pressed and clustered together into solid rock), and metamorphic rock (which basically are either igneous or sedimentary rocks that have been altered somehow to the point where they become a different rock). Melting or metamorphism of sedimentary rocks destroys any fossils.

Since the trip was only for a day, and because the features were just scattered around, it will be impractical to divide this trip as with the Estonian excursion. Instead, I thought I could make this a sort of picture tour.

Coastal features

Currently, I am taking another course, which includes coastal processes – how waves shape the coasts. This is mainly done by weathering (breaking rock apart), erosion (transporting the weathered rock fragments) and deposition (dropping the carried rock load). In other words, the waves pick up rocks from one place and drop them somewhere else.

The shapes that are produced depend mostly on which type of rock weathers more easily. Water-soluble rocks tend to vanish very rapidly and can be carried far before being precipitated. Among the non-soluble rocks, the softer ones are usually weathered and eroded away faster than the harder material. As for deposition, larger rock fragments are harder for the water to carry, and are therefore dropped more readily. Harder rocks tend to break into larger pieces, and are therefore usually transported shorter distances – i.e. deposited more closely to their origin.

In practice, the rate of destruction also depends on how high up the rocks are – if they lie above the reach of most waves, there will be little weathering and erosion, but if they are located exactly where the waves strike as hardest, they will be more affected by their erosional power. Also, if it is only partly submerged, the portion that is mostly in contact with the water will be more destroyed, and, in this way, we can get some pretty impressive cliffs.

Climbing these cliffs was rather dangerous, but a lot of fun!

Also, if a lump of bedrock sticks out from the shore, the waves will attack it more vigorously. This can be seen below, where the end that sticks out to the right is much smoother than other rock parts. 

Water and wind act to smoothen rock, as it is the rough, sticking-out edges that get weathered first. This is why one can tell the degree of water or wind weathering by how smooth and rounded the rock is.

More chemical erosion and deposition

Insoluble rocks can be made soluble if they come in contact with some acid, possibly carried with the water, which can break it down into components that are water-soluble. In this way, we can get some quite peculiar, small holes in rocks.

Here we see entire lengths of such holes, lined up toward the shore.

And here is a large concentration of small acid-holes in one spot. The dark colour is because the rock is wet.

Acid may have had a hand in these eroded-out stripes.

But a closer look makes me doubt that. The stripes are not smooth, as would be expected.

They seem more like a result of rock layering, each layer slightly shorter than the underlying one, sort of like a staircase. I am totally bewildered.

When dissolved substances – or solutes – are “un-dissolved”, they are said to be precipitated. Solutes can be precipitated either as a result of some change in the water chemistry, or by a temperature change. The amount of solute a dissolving liquid – or solvent – can carry depends on the temperature of the solvent: the higher the temperature, the more solute it can maintain in solution, and the opposite for lower temperatures. Therefore, substances are also precipitated if the temperature is lowered to the point where the solvent is oversaturated and must precipitate the excess solute. (If you find this confusing, ask a chemist – he or she will have the solution.)

Some five metres from the shoreline, the rock was covered by precipitated mineral crystals.

This might have been caused by a third way of precipitation: evaporation of the solvent. Water could have reached this part of the shore during high tides, trapped and left to be evaporated away by the sun, leaving the dissolved substances behind, as the reduced amount of water becomes oversaturated with solute. In this case, the mineral would be an evaporite – a collective term for minerals formed through precipitation caused by evaporation – actually considered as sedimentary rocks.

Precipitated minerals can also fill cracks in a rock mass.

The white stripe here is calcite (calcium chloride) that has been precipitated on the walls of a narrow fracture. More and more of the mineral was deposited until it eventually filled up the entire space. In one part, rock on one side of this fracture has been removed, showing roughly what it looks like in transection.

Deformation

Deformation refers to any kind of change in shape or position of a rock mass. Note that it has nothing to do with metamorphism, something I usually forget. Deformation has to do with directed pressure acting on rock, either breaking or reshaping it – referred to as brittle versus ductile deformation. Brittle material breaks because the pressure breaks the chemical bonds holding the particles together, while ductile materials bend because, as the bonds are broken, new bonds are formed with nearby particles. A typical example of ductile materials are metals and modelling clay.

These different types of deformation can be seen and compared in this structure.

A familiar analogy would be when breaking a caramel-filled chocolate bar: the chocolate crust breaks brittlely, while the caramel behaves ductilely. Here, two rock masses have likely been pulled apart, resulting in a similar scenario.

Whether a rock deforms brittlely or ductilely depends, among many things, on its chemical properties, and its temperature. Softer rocks are more likely to deform ductilely, since their weak bonds offer little resistance to breaking, so there is generally little displacement after the pressure overcoming the resistance, and so new bonds can be formed rapidly. Rocks that are near their melting point will also probably deform ductilely, since they are close to a liquid, flowing state. Probably, the rock that deformed brittlely in the picture above has a much higher melting point than the other material, and the temperature at the time of the formation of that feature would have been close to the melting point of the ductilely deformed rock.

Rocks deform ductilely in two main ways: elongating or bending. The best way of explaining the difference is through a simple figure. The big, red arrows symbolise pressure in the direction of the arrow head; the thin, green arrows show the direction in which the rock deforms. Note that elongation can also occur if a rock is pulled apart by two oppositely directed forces.

Compression leads to lengthening of the rock along an axis perpendicular (right-angled) to the compressing forces, while bending simply… bends the rock.

Intrusion and contact metamorphism

What does this look like? Anyone wants to say it? Come on, I know we are thinking the same. Yes, exactly, it looks like vomit all over that block.

But it is not, really. (Surprise! Hehe) It is a messy stream of a light pink mineral called potassium feldspar, a member of the feldspar group – the most common minerals on Earth, actually.

How did it get there? Well, the pattern it forms gives a clue – only one type of substance would create such a pattern: a liquid. So the potassium feldspar was liquid at the time it flowed over that block of rock. And, liquid rock is magma, and magma spells igneous activity.

From this, we can tell two things: the underlying rock is older than the feldspar vein (otherwise, the feldspar could not have flowed over it), and it has a higher melting point than potassium feldspar (because this block must have been solid at the time the feldspar was liquid). Some process made a stream of molten potassium feldspar spew across this already solid block.

Can you also see that the otherwise dark grey or blue rock has been coloured pink on the surface around the feldspar vein? The hot potassium feldspar did not just spread over the block, it changed it a little too. The material was not warm enough to cause the other rock to melt, but it triggered a chemical reaction with the directly associated parts of the dark rock. This is called contact metamorphism – parts of the underlying rock were metamorphosed by the reaction triggered by contact, and are now a different type of rock.

Here is another example, with less extensive, but more distinct contact metamorphism.

The narrow, slightly darker green bands on the sides of the green stripe have been formed through contact metamorphism.

This is also a different type of igneous process than the one above. Here, the green mineral magma was not flowing on top of the other rock, but actually flowed in through it – either through a pre-existing fracture, or melting its way in. This process, termed intrusion, is fairly common, and there were many examples here at Väddö. In fact, in the above case, you can even see another mineral (perhaps the same as the substrate rock) having intruded across the first intrusion (retribution, maybe?), forming a criss-cross of intruding bodies.

If there are many parallel cracks, we can get beautiful things like these:

Or, if the cracks branch out, we can get a pattern more like the vomit-thingy before.

Since different minerals weather at different rates, intrusion can give rise to even more peculiar structures. If the intruding material is softer, it leaves long gaps.

If the intruder is more resistant, it will rise up as a narrow ridge as the surrounding material is eroded away.

Here is the same tongue of rock seen from below.

If the host rock on one side collapses, as has happened in the picture below, it show us what an intruding body can look like below the surface.

The other intruding bodies, where we only could see the top, look like narrow bars, but we forget that they may very well be long, flat sheets such as the one above.

Sometimes, the intrusion is not straight, and can form bizarre features like these:

And if there is deformation after the intrusion:

The intruding tongue does not have to be narrow. Here is a vast stream of the rock pegmatite, characterised by a high content of potassium feldspar, and by being very coarse-grained.

And it is pegmatite that leads us to the next section.

Minerals

Pure minerals are not easy to find, especially not when a lot of deformation has been around. But, in some of the pegmatite tongues, we found a large supply of the marvellous mineral muscovite. When absolutely pure, it is very light grey and shiny, but the most fascinating feature is that it is so flaky – easily breaking into incredibly thin slices – said to have a perfect cleavage. Yes, geologists get lonely sometimes… (Ahem, excuse that tasteless joke; geologists are really jolly people!) 

It is difficult to make this mesmerizing mineral justice trying to describe it with words and photos; the only way to truly understand it is to actually hold a piece in your own hands and feel a true wonder of geology.

More on the visual side, we also discovered a fair amount of garnets – a usually red, crystalline mineral with a quite distinct crystal structure: they look like balls flattened from various angles, slightly reminiscent of a football. I do not have much to say about these, but they sure look nice!

Life

In geological structures, there can be traces of life, most prominently fossils, but also other forms (not really, but I am trying to give this section some kind of introduction). Since there were no fossil-containing rocks here, we had to rely on these other clues.

Among some gravel and plants, we found clear shell structures of gastropods (slugs), indicating a shallow marine environment.

There were also some lower vertebrates, swimming around in small ponds.

On bare rock, we even discovered coprolites (fossil faeces), probably from a giant pterosaur, so fresh it had not even been fossilised yet. Could they still be among us??