Category Archives: demonstration (difficult)

Mid-Atlantic Ridge

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The Mid-Atlantic Ridge above sea level in Iceland.

On my recent trip to Iceland, I had to seize the opportunity to take pictures of plate tectonics at work. Imagining oceanic plates drifting apart is quite difficult, and Iceland is one of the few places in the world where a mid-oceanic ridge reaches above the water level.

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A (in red) on the European plate side, A (in blue) on the American plate side, both walking down a rift valley.

We first went to Thingvellir, but since I wanted pictures with no wandering tourists on them except for us, we had to continue the search for the perfect rift valley.

I don’t really know all that much about plate tectonics, but from what I read in various places, the plates here don’t converge continually, but in episodes. Apparently, the last big event and subsidence of the valley floor occurred with an earthquake in 1789.

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E investigating a rift valley

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E and A in a rift valley for scale

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Just beautiful

Geysers

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Photos and movies from geysers on Iceland.

I already talked about hydrothermal activity in this post. But then after writing it, we went and did the golden circle tour and now I have even more images and movies!

First, this is what the field looks like in which all the hot springs and geysers and what have you are located.

In this field, there are different tastes of hot springs. Some are just hot, others clearly boil.

Some are small geysers.

And then there is the big geyser. As you will notice, I got a bit movie-happy. But – in contrast to most other tourists – we actually figured out how you can see whether it is likely to erupt soon or not. And then we found the sign explaining it…

So this is what it looks like from up close.

And there will be more when I’ve figured out how to rotate a movie.

Hydrothermal springs

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Hydrothermal springs that you can visit without a deep-sea submersible.

When teaching about hydrothermal springs, I usually use a video a friend of mine took of hydrothermal vents on the mid-Atlantic ridge on the WHOI submersible Alvin. But being on Iceland now, there is much better material available which students can even go and experience themselves.

In the Blue Lagoon close to Reykjavik.

I am too chicken to take my camera under water in the Blue Lagoon to film the hot springs, but there are other hot springs all over Iceland that are less scary, for example this one that my friend Astrid found in the middle of a meadow.

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View from the top into the hot spring – do you see the bubbles breaking the surface?

And here I even dared take my camera under water.

View of the hot spring under water – that’s where the bubbles come from!

Granted, this is not quite as impressive as a black smoker or the Blue Lagoon. But the water in the whole little lake was warmer than about 40 degrees Celsius, and the hot spring is sitting randomly in a field. That’s hand-on geothermal heating for you!

Interference of waves.

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Movie on wave interference – two wave fields arriving perpendicular to each other, interacting and leaving.

When talking about waves, it is often difficult to explain that wave heights of different components of a wave field can be added to each other to give a resulting wave field, but that each of those components continues to travel with its own direction and speed and comes out of the wave field basically unaltered. Students learn about constructive, destructive and complex interference (see image below), but it is hard to realize that those interactions are only momentary.

Constructive, destructive and complex interference of waves.

When I was on my way up to Isafjördur to teach CMM31, my friend Astrid and I happened to find the perfect example for the phenomenon described above. We were in Gardur in southwest Iceland and took a sunset walk to the lighthouse.

Old lighthouse in Gardur, southwest Iceland.

The lighthouse is located at the end of a pier and we observed a spectacular wave field. Two distinct fields were meeting each other at an almost 90 degree angle, interacted and left on the other side still clearly recognizable.

Two wave crests meeting at approximately 90 degree angle.

The waves met, interacted, and left the area of interaction. Watch the movie below to get an impression!

How a CTD works

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Movie on how the most important instrument in oceanography works.

On our cruise on the WHOI research vessel Knorr in 2011, Sindre Skrede (find him on twitter or vimeo for many more exciting pictures and movies!) and I made a movie for his blog, describing the most important oceanographic instrument. We recently translated the movie from Norwegian to English and here it is. Enjoy!

Internal waves in the atmosphere

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A photo of internal waves in the atmosphere.

Internal waves exist on the interface between fluids of different densities. In the ocean they are mostly observed through their surface imprint. In the tank, we could also observe them by looking in from the side, but this is hardly feasible in the ocean. But luckily vision is easier in the atmosphere than in the ocean.

On our research cruise on the RRS James Clark Ross in August 2012, we were lucky enough to observe atmospheric internal waves, and even breaking ones (see image above). This is quite a rare sight, and a very spectacular one, especially since, due to the low density contrast between the two layers, the waves break extremely slowly.

It is really hard to imagine what it looked like for real. This movie shows the view of Jan Mayen – the volcano, the rest of the island and then the atmospheric waves. Please excuse the wobbly camera – we were after all on a ship and I was too excited to stabilize properly.

Filling the tank

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A movie of patterns observed in the flow when filling the tank for this experiment.

Even though there are tons of scientific things to discuss with this movie, like the different refraction of light in the two layers of different densities, or the filaments, or the restratification processes, I am mainly posting this because I think it is beautiful. Enjoy!

Details of lee waves in the tank.

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A movie focusing on details of the lee waves in the tank.

In this post, we investigated lee waves in a tank in a general way. Here, I want to show a detail of those lee waves:

In this movie, the concept of hydraulic control becomes visible. On the upstream side of the mountain, the dense water layer forms a reservoir which is slightly higher than the mountain. On top of the mountain and towards its lee side, the layer of denser water is stretched thin and has a smooth surface until about half way down the mountain, where waves start to form. In this thin, smooth layer, flow speeds are higher than the wave speeds, hence disturbances of the interface are flushed downstream and cannot deform the interface. Only about halfway down the mountain, the phase speed becomes equal to the flow speed, hence waves can both form and stay locked in place relative to the mountain.

For more information on internal waves, check out these posts [which are scheduled to go online over the next couple of days]:

Surface imprints of internal waves

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How internal waves in the ocean can be spotted on the surface.

Under certain conditions, internal waves in the ocean can be spotted at the ocean’s surface due to changes in surface roughness or to the movement of floating foam or debris. They can be spotted if half their wavelength is longer than the distance between the interface on which the internal wave is traveling and the water surface, so that the orbital movement caused by the internal waves reaches the water surface. In the tank, they can also be seen – for example by adding small floating particles to the water surface.

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Internal wave in a tank. Seen from the side due to different coloring of the two layers, and on the surface in the distribution of floating tracer.

In the movie below, you can see the interface between water layers of different densities and the water surface with particles on it. The particles make it easy to spot how the water surface is being stretched and squeezed as internal waves travel through underneath.

For more information on internal waves, check out these posts [which are scheduled to go online over the next couple of days]:

Internal (lee) waves in a tank.

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Lee wave experiment in a large tank with a moving mountain.

In this previous post, we talked about internal waves in a very simple experiment. But Geophysical Institute has a great tank to do lee wave experiments with that I want to present here (although it doesn’t seem to be clear what will happen to the tank when the remodeling of the main building starts in November – I hope we’ll be able to save the tank!). I think it has originally been used for real research, but these days the GEOF130 lab is the only time this tank gets used.

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Tank for internal lee wave experiments – a “mountain” is moved through the tank and generates internal waves.

In this tank, a “mountain” can be moved all the length of the tank through more or less stagnant water, thereby simulating a current going over a non-moving mountain (which might be a slightly more realistic setup). At the lee of the mountain, lee waves form on the interface between two water layers of different density.

For more information on internal waves, check out these posts [which are scheduled to go online over the next couple of days]: