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.