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Imaggeo on Mondays: Exploring the East African Rift

10 Mar

This week’s Imaggeo on Mondays is brought to you by Alexis Merlaud, an atmospheric scientist from the Belgian Institute for Space Aeronomy. While the wonders of the African atmosphere feature in his photography, the East African Rift has a much bigger tale to tell. Drawing from all aspects of geoscience Alexis shares its story…

Kilimanjaro from Mount Meru. (Credit: Alexis Merlaud, distributed via imaggeo.egu.eu)

Kilimanjaro from Mount Meru. (Credit: Alexis Merlaud, distributed via imaggeo.egu.eu)

This picture shows Kilimanjaro, Africa’s highest mountain, at sunrise. It was taken from Socialist Peak, which marks the top of Mount Meru, some 70 km to the southwest. Both mountains are located in Tanzania and are among the largest stratovolcanoes of the East African Rift Zone. Unlike Kilimanjaro, Meru is active and its most recent eruption occurred in 1910.

Stratovolcanoes, also called composite cones, are built-up by alternating layers of lava flows, pyroclastic rocks, and volcanic ash. During a large eruption, huge quantities of ash and sulphur dioxide can reach the stratosphere, where they can affect the climate for several years, as did the eruptions of Krakatau in 1883 and Pinatubo in 1991. Sulphur dioxide is converted to sulphuric acid droplets, which spread with the ashes throughout the stratosphere. These aerosols screen some of the sunlight, decreasing the average surface temperature by about one degree. The temperature in the stratosphere simultaneously rises by a few degrees, due to the enhanced absorption of sunlight by aerosols.

There is a difference in the tectonic processes associated with these South East Asian volcanoes and the East African Rift: the former are located above a subduction zone while the rift is a divergent boundary.  An example of large volcanic eruption in a divergent zone is the Laki (Iceland) eruption in 1783, which yielded severe meteorological conditions and reduced harvests for several years in Europe. This eruption may have also helped trigger the French Revolution in 1789.

Plate tectonics in East Africa created Kilimajaro and have also played a role in early human evolution, by shaping the local landscape and the long-term climate, thus modifying the environment of our ancestors. East Africa is the area in the world where most of the hominid fossils have been discovered, including Homo sapiens – the oldest fossil record is 200,000 years old and started to move out from Africa 100,000 years ago!

A final thanks: thanks Cristina Brailescu for help climbing Meru and Emmanuel Dekemper for support on editing the picture. 

By Alexis Merlaud, Belgian Institute for Space Aeronomy

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

Imaggeo on Mondays: Scope for science and art

17 Feb

Great geoscience photographs aren’t always shots of beautiful landscapes. Sometimes there are stunning things to see at a much smaller scale. This week’s Imaggeo on Mondays showcases one such curiosity and highlights how research images can reveal a lot about the natural world when exhibited as a form of art.

Thin sections are a fantastic way of finding out more about rocks, soils and tissue structure. At 30 micrometres thick they are the most refined Carpaccio you could find on a geoscientist’s plate, and illuminating them under a microscope only makes the sections more splendid.

A sandstone sample viewed in plane polarized light (top) and cross polarised light (bottom). (Credit: Wikimedia Commons user Michael C. Rygel)

A sandstone sample viewed in plane polarised light (top) and cross polarised light (bottom). (Credit: Wikimedia Commons user Michael C. Rygel)

Under plane polarised light, you spot the fine details that make up each slice, the crystal grains, pore spaces and shell fragments that, together, make up your rock sample. And in cross polarised light there’s even more to be seen. Crystals, at first barely perceptible, shine out amid dark masses and appear as an array of bright and beautiful colours. Different properties like the refractive index and extinction of a crystal can let you work out what mineral you’re looking at, and the relationships between mineral grains offer clues to the rock’s history.

Under the microscope, where mineral and biological worlds meet. (Credit: Laura Gargiulo via imaggeo.egu.eu)

Under the microscope, where mineral and biological worlds meet. (Credit: Laura Gargiulo via imaggeo.egu.eu)

This image, by Laura Gargiulo, shows the surface of a sandy soil under cross polarised light. Sand grains – a veritable pick a mix of rock fragments and merged minerals – make up the majority and a slice of cellular plant material sits just south of the centre. Each of the colours and the way they change under polarised light reveal what each sand grain is made of and how these tiny fragments combine to make up the soil. While its purpose may be a scientific one, the image certainly has aesthetic appeal.

By Sara Mynott, EGU Communications Officer

If you are pre-registered for the 2014 General Assembly (Vienna, 27 April – 2 May), you can take part in our annual photo competition! Up until 1 March, every participant pre-registered for the General Assembly can submit up three original photos and one moving image related to the Earth, planetary, and space sciences in competition for free registration to next year’s General Assembly!  These can include fantastic field photos, a stunning shot of your favourite thin section, what you’ve captured out on holiday or under the electron microscope – if it’s geoscientific, it fits the bill. Find out more about how to take part at http://imaggeo.egu.eu/photo-contest/information/.

For the love of rocks

14 Feb

We often have a way of seeing patterns in otherwise random features, and rock outcrops are no exception.

Pegmatite intrusion in mylonitc granite. (Credit: Lucy Chernale via imaggeo.egu.eu)

Pegmatite intrusion in mylonitc granite. (Credit: Lucy Chernale via imaggeo.egu.eu)

Do you see the heart on its side? To the untrained eye an outcrop like this is simply a heart-shaped feature in an otherwise grey rock. But to the geologist, the layers, swirls, shapes and colours tell a story. The ribbons in the granite show that the rock has been subjected to sheer forces and have slowly deformed in response to them, a process known as ductile deformation. Metamorphic rocks that have deformed in this way are known as mylonites. This sort of deformation only occurs below 4 km depth, meaning that this rock used to be buried deep below the Earth’s crust and has slowly been brought to the surface.

The lighter, heart-shaped area is an intrusion of pegmatite, an igneous rock composed of large crystals (over 1 centimetre long) that would have filled a space in the country rock. Big crystals form when melt cools slowly, and the pegmatite’s centimetre-long ones show the intruding melt was well-insulated by its surroundings, allowing the crystals to grow.

It’s not only the larger patterns within an outcrop that tell a story, but the arrangement of minerals within the rock.  Garnets are relatively common rock-forming minerals, and they’re not always the glossy red gemstones jewellers are after. In fact, the colour of garnet can vary widely depending on its chemical composition.

Heart-shaped garnet in a mafic granulite. (Credit: Barbara Kunz via imaggeo.egu.eu)

Heart-shaped garnet in a mafic granulite. (Credit: Barbara Kunz via imaggeo.egu.eu)

This brown garnet heart sits amid an igneous rock rich in iron and magnesium – a mafic rock. Mafic lava flows relatively easily because it is low in silicates – the minerals that control lava’s viscosity. Its slow-moving counterpart, felsic lava, is filled with feldspar and quartz, and a lot more silicate. Looking at the minerals in an igneous outcrop lets Geologists work out whether the lava flowed rapidly, or was more slow-moving. Volcanologists can make models of the two in the lab using golden syrup, which, like lava, is a Newtonian fluid (a liquid that doesn’t change viscosity when forces act on it).  Adding water to the syrup makes for a more mafic lava and adding lentils, rice and sugar simulates one that is rich in crystals!

Back to the rocks. Microscopes make it possible to view the wonders of rocks on a much smaller scale. You can see the relationship between mineral grains, the condition of the crystals, and piece together the last parts of the rock history puzzle.

Thin section heart

Photo-micrograph of exsolved garnet in lamellar pyroxene. (Credit: Fatima El Atrassi via imaggeo.egu.eu)

A searching through Imaggeo reveals yet another garnet heart! And this one is tiny, a mere 2-3 millimetres across! The garnet is nested within a mineral known as pyroxene. Pyroxene forms at high temperatures and can become unstable as the temperature decreases, causing it to form this striped pattern. The patterns  each mineral forms, and whether they sit as small inclusions or jointed grains can tell us a little more about what happened to the lava as it cooled and crystallised.

Rocks are incredible windows into the history of the Earth, and indeed our solar system. Take a closer look next time you’re on a beach, beside a cliff, or standing on a set of stone steps – you might find out more about the Earth than you were expecting!

By Sara Mynott, EGU Communications Officer

Imaggeo on Mondays: Pitter-patter of little paws in Patomsky crater

10 Feb

This week’s Imaggeo on Mondays is brought to you by Dmitry Demezhko, who describes how Patomsky crater may have formed and why it keeps scientists puzzling…

Patomsky crater, also known as Patomskiy crater or the Patom cone, sits in the Irkutsk Region of Eastern Siberia. The site is a curious cone with a crater at the top and a small mound in the center. The cone totals some 39 metres in height and stretches more than 100 metres in diameter (at the base of the cone).

Patomsky crater – view from a helicopter. (Credit: Dmitry Semenov)

Patomsky crater – view from a helicopter. (Credit: Dmitry Semenov)

The crater was discovered in 1949 by Russian geologist Vadim Kolpakov and for a long time it was considered to be an impact structure with a meteoric origin. Later, Viktor Antipin suggested it could be a nascent volcano. But neither meteoritic nor volcanic matter was found there. The crater consists of proterozoic limestone and sandstone debris and, to date, there is no consensus among scientists regarding the crater’s origin.

View from the crater. (Credit: Dmitry Demezhko, distributed via imaggeo.egu.eu)

View from the crater. (Credit: Dmitry Demezhko, distributed via imaggeo.egu.eu)

During a short expedition in August 2010 we conducted a gravimetric survey at the crater and surrounding area, aiming to evaluate its internal structure. The gravity field shows that surface negative anomalies, where the gravity is unusually low, have deep “roots” and a joint source at depth. But the crater’s gravity field differs greatly from the fields of other well-known impact structures, suggesting that it may not have formed during a meteoric impact.

Downward continuation of the Patomsky crater (left) and Popigay impact structure gravity fields (right). (Credit: Demezhko et al., 2011)

Downward continuation of the Patomsky crater (left) and Popigay impact structure gravity fields (right), (click for larger). (Credit: Demezhko et al., 2011)

We suggest this structure formed in two stages. During the first stage tectonic processes similar to mud volcanism created a porous vertical channel. In the second stage, cryogenic processes would have played an important role in breaking apart the rocks to form the cone and crater.

There is a lot of mysticism and superstition surrounding Patomsky. Local residents call the crater “a fabulous Eagle’s Nest” and say that both people and animals bypass it. We didn’t sense anything mystical while working in the crater though – and this cute little animal lives quite comfortably there.

Downward continuation of the Patomsky crater (left) and Popigay impact structure gravity fields (right). (Credit: Demezhko et al., 2011)

“Inside Patomsky crater: a chipmunk” by Dmitry Demezhko. This image is distributed via imaggeo.egu.eu.

By Dmitry Demezhko, Institute of Geophysics UB RAS, Yekaterinburg

References:

Alekseyev, V. R.  Cryovolcanism and the mystery of the Patom Cone, Geodynamics and Tectonophysics, 3, 289-307, 2012 (in Russian)

Demezhko D.Y., Ugryumov I.A., Bychkov S.G.: Gravimetric studies of Patom Crater. In: Patom Crater. Research in the 21st Century. Publishing House of the Irkutsk State University, Irkutsk, p. 42–50, 2011 (in Russian)

If you are pre-registered for the 2014 General Assembly (Vienna, 27 April – 2 May), you can take part in our annual photo competition! Up until 1 March, every participant pre-registered for the General Assembly can submit up three original photos and one moving image on any broad theme related to the Earth, planetary, and space sciences in competition for free registration to next year’s General Assembly!  These can include fantastic field photos, a stunning shot of your favourite thin section, what you’ve captured out on holiday or under the electron microscope – if it’s geoscientific, it fits the bill. Find out more about how to take part at http://imaggeo.egu.eu/photo-contest/information/.

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