Geochemistry, Mineralogy, Petrology & Volcanology

Imaggeo on Mondays: Finger Rock


Standing proud amongst the calm waters of Golovnina Bay is ‘The Devil’s Finger’, a sea stack composed of volcanic sediments. Located on the Pacific coast of Kunashir Island -which is controlled by Russia but claimed by Japan – the stack is testament to the volcanic nature of the region. The island itself is formed of four active volcanoes which are joined together by low-lying geothermally active regions.

Sea stacks, tall columns of rocks which jut out of the sea close to the shore, are common across the world, with famous examples found in the UK, Australia, Thailand, Ireland and elsewhere. Sea stacks are formed naturally by erosion processes. Headlands which protrude out towards the sea are subject to many years of battering by wild winds and seas. Slowly, the force of the wind and sea weakens, cracks and breaks up the rock and a cave is formed. The process continues, particularly during stormy weather spells, when eventually an arch is formed. Given more time, the arch too breaks away, leaving a solitary tower of rock, such as that seen in this week’s Imaggeo on Mondays picture. In case you are looking to expand your Russian vocabulary, it might be useful to know that Russian term for sea stack is ‘kekur’!

GeoTalk: Explosive testing with Greg Valentine

GeoTalk: Explosive testing with Greg Valentine

Following his session at the EGU General Assembly, Greg Valentine (Buffalo University) spoke to Sara Mynott about how he creates model volcanoes, specifically maar-diatremes, and blows them up to better understand what goes on in an eruption…

So what is a maar-diatreme?

A diatreme is a vent-like structure, mostly made up of broken up bedrock and magma. Initially, you have a dyke that channels magma straight up to the surface, but somewhere along the line the magma interacts with groundwater. This causes explosions below ground, which start to build up zones of debris. Once you get a debris-filled zone, the magma comes up in fingers that probably facilitate further interaction with water.

Tell me a little about your experiments…

We have an experimental site near Buffalo, New York. It’s out in the countryside, so we can be messy and loud.

We dig a trench (20m long, 4m wide and 2m deep) and make craters between 1 and 2 metres in diameter. It’s like landscaping. We have a guy who normally works in gardens, he comes and digs the trench, and we go shopping at quarries and buy different types of sand and gravel to fill it, bringing in truckloads and truckloads of sediment. But first we add some explosives.

What we’re trying to do is relate what we see in a surface eruption to what goes in the subsurface. In the experiments we can completely control that – from what makes up the sediment layers to the amount of energy and where the explosion occurs.

How much explosive energy do you put into your experiments and how does that compare to what you would see naturally?

We’re working with about a million Joules – it’s about half a stick of dynamite, something like that. The natural explosions are maybe 3 to 6 orders of magnitude larger.

Valentine and his team were standing about 75m away from the explosion, but that didn’t stop a couple of stray rocks landing behind them! Here, the plume is about 15 m high and the clumps of sand that are being thrown out are several centimetres across. The scaled depth? Just right. Credit: Graettinger et al. (2014).

How does the depth and energy of the explosion affect what happens in an eruption?

To tackle this we use this thing called scaled depth, which relates the depth of the explosion to the energy involved. This way of characterising underground explosions has been used for over a hundred years, by people who do mining, geotechnical engineering and weapons testing.

We know that if we have a very small scaled depth, most of the energy goes into the atmosphere. You get a big bang – it’s very dramatic, it’s very fun, you can see the camera shaking when the shockwave hits the camera, but it doesn’t excavate that much of a crater. If you get it too deep, so the scaled depth is really large, nothing comes out. This is because more of the energy is being absorbed by the ground. So there’s this intermediate scaled depth where you get the most crater excavation.

How often do these explosions occur at a particular location or at different depths? Is there any regular pattern at a particular volcano?

We don’t understand enough yet to be able to say that, except that there are probably many – perhaps hundreds – of explosions at a natural maar-diatreme. We suspect that probably more of the explosions happen near the surface because there’s less confining pressure – something that acts against the explosion.

The depth? Too deep. The ground goes up when the shockwave hits it from below, but then it sinks. Look closely – the tennis balls on the surface are used as ballistic markers, but rather than being thrown out in the explosion, they sink as the ground subsides to form a small crater. Credit: Graettinger et al. (2014).

What proportion of the world’s volcanoes are maar-diatremes?

If you just counted each volcano on Earth, not including under the ocean, maar-diatremes would be the second most abundant. They tend to be what we call monogenetic – they have one eruptive episode that may be a few weeks to a few years long and they’re dead.

Usually, they occur in what we call volcanic fields, which, instead of having one central cone, have many small volcanoes over an area. There are many such fields in Europe, including Chaîne des Puys in France and Campi Flegrei in Italy. A volcanic field may have an eruptive period every 1,000 years or so, some more frequent and some less frequent.

What risk do maar-diatremes pose to the human population?

Many of these volcanic fields are inhabited. If an eruption occurred close to Naples, there would be tens to thousands of people affected. Auckland, New Zealand, is almost entirely built on a young volcanic field. It’s dormant right now, but sometime something will happen. Mexico City is another one.

If an explosion were to occur close to the surface in one of these places, what impact would it have?

It could make a crater that is 100-200 metres in diameter and throw blocks – big rocks – 100s of metres from the volcano, probably generate a lot of ash and pyroclastic flows.

What do you love most about your job?

The flexibility to pursue different lines of research wherever they take me.

Finally, what advice would you give a young scientist wanting to get into experimental volcanology the way that you have?

They should make sure they have a good physics and math background, try to get an internship with somebody and just dive in!


By Sara Mynott, Press Assistant at the 2015 General Assembly and PhD student at the University of Exeter.



 Valentine, GA, Graettinger, AH and Sonder, I.: Phreatomagmatic explosive eruption processes informed by field and experimental studies. Geophysical Research Abstracts, Vol. 17, EGU2015-1896, 2015.

Valentine, G. A., Graettinger, A. H., Macorps, É., Ross, P. S., White, J. D., Döhring, E., & Sonder, I.: Experiments with vertically and laterally migrating subsurface explosions with applications to the geology of phreatomagmatic and hydrothermal explosion craters and diatremes. Bulletin of Volcanology, 77(3), 1-17, 2015.

Graettinger, A. H., Valentine, G. A., Sonder, I., Ross, P. S., White, J. D. L., and Taddeucci, J.: Maar‐diatreme geometry and deposits: Subsurface blast experiments with variable explosion depth. Geochemistry, Geophysics, Geosystems, 15(3), 740-764, 2014.

Valentine, G. A., Graettinger, A.H. and Sonder, I.: Explosion depths phreatomagmatic eruptions. Geophysical Research Letters, 41 (9), 3045-3051, 2014.

Imaggeo on Mondays: Pyroclastic flow, Montserrat

Below the warm and tranquil waters of the Caribbean, some 480 km away from Puerto Rico, the North America Plate is being subducted under the Caribbean Plate. This has led to the formation of the Lesser Antilles volcanic arc; the result of the formation of reservoirs of magma as fluids from the down going North America Plate are mixed with the rocks of the overlying Caribbean Plate.

The continued magma generation is expressed violently at the surface on Monserrat Island, which has been the subject of extensive scientific scrutiny since the mid-1990s. This is all because of Soufrier Hills volcano, a Pele’ean type lava dome complex. This means that rather than explosive eruptions taking place, very viscous lava is slowly erupted from the volcano’s vent. The lava is so sticky and gooey that instead of flowing away, down the flanks of the volcano, it accumulates in the vent area and forms a large plug. Lava domes come in a range of shapes and sizes, in the case of Soufrier Hills, it tends to be circular and quite spiky.

Just because the eruptions on this Carbbien Island aren’t generally as spectacular, as for instance at Mt Etna in Italy, they are no less deadly! A common hazard associated with the building up of a dome by the continued accumulation of volcanic material means they can become dangerously unstable and collapse. The volcanic material careers down the flanks of the volcano in the form of pyroclastic density currents (PDCs). The largest such collapse ever observed took place in July 2003 and numerous smaller flows have occurred since. One rather large collapse happened in early 2010, when the dome atop Soufrier Hills had grown to be 1150 m asl (above sea level). After a period of unrest which started in late 2009 and was characterised by seismicity and extrusion of lava from the vent, there was a catastrophic dome collapse in February which reduce the summit height by almost 100m!

Pyroclastic flow, Montserrat. Credit: Alan Linde (distributed via

Pyroclastic flow, Montserrat. Credit: Alan Linde (distributed via

“The photo is taken from a spot at the water’s edge (just behind me) that was previously about 200 m out to sea. A PDC pushed the shoreline out by as much as ~600 m,”

says Alan Linde, who took this photograph of the smoking black landscape in April 2010.

Alan and the research team from the Department of Terrestrial Magnetism (DTM, Carnegie Institution for Science) have been involved with studying Soufrier Hills since 2003. By installing a network of very sensitive instruments in small shafts dug into the ground in and around the volcano, known as borehole strainmeters, they can measure changes in the size and volume of the ground as a result of dome collapses and explosive eruptions.

 “One of our borehole sites, very close to the coast, was almost destroyed by the hot ash. There is a clear change (from before to after the flow) in the tidal signals recorded by that site because an area of ocean loading has been removed as a result of the ash filling in and moving the coastline. The volcano is behind the small mountains, obscured by cloud.”


Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at


Imaggeo on Mondays: An explosive cloud

Imaggeo on Mondays: An explosive cloud

One of the world’s most volcanically active regions is the Kamchatka Peninsula in eastern Russia. It is the subduction of the Pacific Plate under the Okhotsk microplate (belonging to the large North America Plate) which drives the volcanic and seismic hazard in this remote area. The surface expression of the subduction zone is the 2100 km long Kuril-Kamchatka volcanic arc: a chain of volcanic islands and mountains which form as a result of the sinking of a tectonic plate beneath another.  The arc extends from Hokkaido in Japan, across the Kamchatka Peninsula, through to the Commander Islands (Russia) to the Northwest. It is estimated that the Pacific Plate is moving towards the Okhotsk microplate at a rate of approximately 79mm per year, with variations in speed along the arc.

There are over 100 active volcanoes along the arc. Eruptions began during the late Pleistocene, some 126,000 years ago at a time when mammoths still roamed the vast northern frozen landscapes and the first modern humans walked the Earth.

Many of the volcanoes in the region continue to be active today. Amongst them is Karymsky volcano, the focus of this week’s Imaggeo on Mondays image. Towering in excess of 1500 m above sea level (a.s.l), the volcano is composed of layers of hardened lava and the deposits of scorching and fast moving clouds of volcanic debris knows as pyroclastic flows. You can see some careering down the flanks of the volcano in this image of the July 2004 eruption. The eruptive column is the result of a

“strong Vulcanian-type explosion, with the cloud quickly rising more than 1 km above the vent. The final height of the eruption cloud was approximately 3 km and in the image you can clearly see massive ballistic fallout from multiple hot avalanches on the volcanoes slopes,”

explains Alexander Belousov, a Senior Researcher at the Institute of Volcanology and Seismology in Russia and author of this week’s photograph.


USGS map of the Kuril-Kamchatka trench, showing earthquake locations and depth contours on downgoing slab. Credit: USGS, USGS summary of the 2013 Sea of Okhotsk earthquake, via Wikimedia Commons.

USGS map of the Kuril-Kamchatka trench, showing earthquake locations and depth contours on downgoing slab. Credit: USGS, USGS summary of the 2013 Sea of Okhotsk earthquake, via Wikimedia Commons.

If you pre-register for the 2015 General Assembly (Vienna, 12 – 17 April), you can take part in our annual photo competition! From 1 February 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


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