VolcanicDegassing

Afar

One year of volcanicdegassing

One year of volcanicdegassing

One year has passed since I first wrote a post for this occasional blog. Now, 12 months, 22 posts and 7500 page views later, here’s a quick look back. For me, this has been a way of using some of my back catalogue of field photographs, of fleshing out a bit of context around papers I have been working on, and adding a little commentary on some more topical aspects. I am pleased with the results so far, and will aim to keep it going for a little longer. In the meantime, thank you for reading and sharing the posts!

Sunset behind Mt Lokon, Sulawesi, Indonesia

Sunset behind Mt Lokon, Sulawesi, Indonesia

Top three posts of the past year.
A tribute to Barry Dawson
Sea-floor spreading: magmatism in the Afar, Ethiopia
An episode of volcanic unrest beneath Santorini, Greece

Sea-floor spreading, on land

Sea-floor spreading, on land
Looking north, up the Dabbahu rift, Afar.

Aerial view, looking north along the Dabbahu-Manda-Hararo rift, Afar, Ethiopia. The stratovolcano, Dabbahu, lies in the background. Our new work shows that this rift is opening at about 20 mm/yr. Over the past 200,000 years, opening of this part of the rift has accounted for all of the motion between the Nubian and Arabian tectonic plates. The topography of this rift is thought to be similar to that of the mid-Atlantic ridge. Note the prominent fault scarps that cut through young basalt lava flows.  The rift axis, and locus of current faulting and magmatism lies to the right hand side of the photo.

One piece of evidence that helped to establish the theory of Plate Tectonics in the early-1960’s was the recognition of patterns of magnetisation in the basalts of the seafloor that were symmetrical about the global oceanic ridge system. Fred Vine and Drummond Matthews recognised that this pattern had to be fixed in place as the lavas, that were erupted along the ocean ridge, cooled through a critical temperature (the Curie point). Below this point, they would become weakly magnetized, with a signature reflecting the local magnetic field. On long timescales (millions of years) Earth’s magnetic field changes in both intensity and direction, and it is the major changes in polarity, during which the field reverses, that lead to the striped ‘bar-code’ signature of magnetic stripes on the sea floor.  In turn, the symmetrical pattern of this magnetic chart-recorder requires that most of these seafloor lavas are erupted in a narrow region at the oceanic ridge, which marks the point of separation between two tectonic plates. These observations led to the now well established theory that tectonic plates form at oceanic ridges, where magmas from the mantle below rise, and freeze to form the trailing edges of  the separating plates.

While these processes are well understood for the sea-floor, we still don’t have a very good idea of what happens when a continental plate begins to split apart to form a new plate boundary. Think of the analogy of pulling apart a Mars bar. When you start to pull it from both ends, a number of fractures start to form in the brittle chocolate coating. But at what stage does the stretching become concentrated into the one major fracture, and what controls where that fracture forms? To get a closer understanding of how continents break up, there have been several concerted research efforts in the north-eastern parts of Africa (notably Ethiopia and Eritrea) over the past few decades.  These have teased out the geological and geophysical events of the past 30 million years as the Ethiopian Rift Valley developed, and evolved to its current status, with a ‘triple junction’ forming at the boundaries of three plates (the Nubian Plate, beneath north Africa; and the Somalian and Arabian Plates).

Afar, Ethiopia: en route to Digdigga

Afar, Ethiopia: en route to the field camp at Digdigga

A part of this evolving plate boundary now lies within the Afar region of Ethiopia.  This is an area that is both remote and challenging to work in, but also a place that is geologically and culturally fascinating. It is the location, for example, of the oldest Hominid fossils known including Lucy (Australopithecus afarensis).  With the sort of serendipity that geological fieldwork sometime relies upon, an opportunity to re-examine how a continent breaks up arose in late 2005, when a major segment of the plate boundary ruptured. In a series of events which are now very well documented (see references at the end),  the Dabbahu-Manda-Hararo rift lurched into life. Over the next six years, the rift has experienced at least 14 further rifting events, three of which were associated with eruptions of basaltic magma. A multi-partner and multi-national research team has been working in the Afar since 2005, with the aim of using this active rifting episode to understand better how continents rift apart, and how new basaltic crust, which is typical of the ocean floor, begins to form.

Fieldwork near Badi volcano, Afar

Fieldwork near Badi volcano, Afar

One part of this effort was undertaken by David Ferguson for his PhD research at the University of Oxford. David collected many samples of basalt lava from a section that crossed from the present active rift, and out onto the older margins of the rift.  This meant travelling on foot, 4×4 and helicopter, and the collection of many hundreds of kilogrammes of rock, which were then shipped back to the UK.

Access to remote parts of the field required helicopter support

Access to remote parts of the field required helicopter support

David then identified a small number of samples for dating, using the potassium (K) -argon (Ar) method. This method relies on the fact that all lavas contain small amounts of radioactive (and naturally occurring) K-40, which decays over time Ar-40. Since argon is a gas, is will naturally escape from lavas as they erupt; but once a lava is frozen, any Ar that subsequently forms by the decay of potassium will remain trapped within the finely-crystalline structure of the rock. By carefully breaking the rocks apart, pulling out the finely-crystalline  fraction of the frozen melt and then heating the samples up in a controlled environment, David Ferguson and Andy Calvert, from the US Geological Survey, were able to extract and measure the tiny amounts of young Argon that had accumulated since eruption. Since we can measure the amount of potassium present in each rock; and since the  rate of decay of potassium-40 is well known, these measurements tell us how much time that has passed since each lava sample erupted.

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Packing up rock samples at the Digdigga field camp

As we might expect, David found that the lava samples got progressively older the further away they were from the presently-active part of the rift.  But interestingly, the lava samples all have the same chemical signatures as the present day lavas that have been erupted from the rift – suggesting that each of the dated lava flows were also erupted from the centre of the active rift at the time. This may all sound a little bit obvious, but the conclusions are actually quite interesting: they tell us that for the past 200,000 years most of the volcanic activity in this part of Afar has occurred in a narrow rift, as we see today. The older lavas now lie a few kilometres away from the active rift – and this tells us the rate at which the rift has been opening over this period of time. Here, we find that the spreading rate of the Dabbahu-Manda-Hararo rift is about 20 mm/year (or about 4 km in 200,000 years). This matches quite closely with the independent estimates of the rate at which the Nubian Plate is separating from the Arabian Plate. So in this location, we can be fairly sure that main fracture of the ‘Mars bar’ formed at least 200,000 years ago, and this has effectively become the place where the trailing edges of the two plates are forming and pulling apart. So here we have it – the seafloor-spreading process, and the formation of new basaltic crust, identified on land.

Reference: DJ Ferguson et al., 2013a, Constraining timescales of focussed magmatic accretion and extension in the Afar crust using lava geochronology, Nature Communications, 4, 1416.

Update. July 4, 2013.

David Ferguson has now published a new paper in Nature that brings together information both from petrology (the nature of the erupted lavas, and their chemical compositions) and from seismology to develop a self-consistent explanation of the deep structure of the rift. This new work shows that the compositions of geologically young lavas requires them to have formed relatively deep (greater than 80 km or so), and at relatively higher temperatures than expected for ‘normal’ mantle. This can only be reconciled with other geological and geophysical constraints if the plate beneath the active Afar rift is still relatively thick. This, in turn, is consistent with the idea that the slow rate of spreading at the rift means that plate remains thick because of conductive cooling, while volcanism persists because the mantle at depth is suffiiciently hot to partially melt at depths just a little deeper than the bottom of the plate. In turn, this fits in with ideas about the long-lasting and slowly waning influence of a ‘hot spot’ or plume beneath the region, which is the favoured contender for the burst of magmatism in the region about 30 million years ago.

Reference: DJ Ferguson et al., 2013b, Melting during late stage rifting in Afar is hot and deep, Nature 499, 70-73.

Further reading:

Ebinger, C. et al., 2010, Length and timescales of rift faulting and magma intrusion: the Afar Rifting Cycle from 2005 to present. Annual Reviews of Earth and Planetary Sciences 38, 439–466.

Wright, T. J. et al. 2006, Magma‐maintained rift segmentation at continental rupture in the 2005 Afar dyking episode. Nature 442, 291–294.

Acknowledgements: this work was funded by NERC as a part of the Afar Rift Consortium. It would not have been possible without the excellent support of our colleagues, collaborators and logistical support teams in Ethiopia and elsewhere. We are very grateful to the people of Digdigga for graciously permitting us to share their school buildings and to set up a field camp in their village, and to the people of Digdigga and Teru districts for allowing access to the region.

Polygons, columns and joints

Over on her Georney‘s blog, Evelyn Mervine has recently posted a nice piece with some spectacular images of columnar jointing. This seemed like a good opportunity to dust off some field photos, with some more examples of polygonal joint sets in lavas from a variety of settings, to illustrate the diversity of forms that cooling-contraction joints may take in volcanic rocks.

The first example is a late Pleistocene lava flow from the Afar, Ethiopia.

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Berihu Abadi, with an example of columnar jointing in a late Pleistocene basalt, Afar, Ethiopia, exposed in the face of recently-opened fracture. Fieldwork carried out as part of the NERC-funded Afar consortium.

Young lavas in the Afar are predominantly fissure-fed basalts, erupted across the topography and air-cooled. Columnar jointing is pervasive in the upper surfaces of these young lava flows, and seems to develop immediately beneath the surficial glassy and vesicular crust that forms on the top surfaces of the modern lava flows.

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Uppermost surface of a fresh basaltic lava flow, Afar, Ethiopia. Field of view is about 30 cm. Beneath the surface layer formed by the scales of glassy, vesicular lava, the lava is weakly polygonally-jointed (not visible in this picture). Fieldwork carried out as part of the NERC-funded Afar consortium.

Moving across to Europe, and southern Spain. Here, in the Cabo de Gata, there are some fabulous examples of polygonally-jointed andesitic lava domes, emplaced in a shallow submarine environment. These are Miocene in age, and formed in a transient volcanic arc, which has now been faulted against the Spanish mainland.

At Playa Monsul, polygonally-jointed dykes can be found criss-crossing a fabulous series of hyaloclastite bodies. Monsul is also well known as the location in Indiana Jones and the Last Crusade, where Sean Connery fends off an aerial attack with the help of an umbrella and a flock of seagulls.

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Polygonal jointing in the vertical face of a dyke, intruding a submarine sequence of wet sediment and hyaloclastite lavas. Polygons are typically 10-20 cm across. Photograph taken on a University of Oxford undergraduate field trip.

The southern-most cape of the Cabo de Gata has a wealth of collonaded andesites; presumably submarine domes.

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Columnar joints in an andesite ‘dome’ of Miocene age, southernmost Cabo de Gata, Spain. This locality offers some spectacular views both of polygonal joints in planform, and also large-scale structures showing the fanning of columns that reveal the orientations of the original cooling surfaces. Location visited on an undergraduate field trip.

The final examples of polygonal jointing come from higher (southerly) latitudes, and higher elevations, where jointing has developed as a result of ice- or snow-contact volcanism. The first example is from the flanks of volcano Osorno, Chile, where characteristic hackly jointing has developed on the outer margins of an andesitic lava flow. This differs from the regular pattern typical of columnar jointing, and is thought to be one characteristic of rapid quenching of lava.

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Jose Naranjo, SERNAGEOMIN, and the hackly-jointed lavas from volcan Osorno, Chile. Fieldwork carried out as part of a NERC-funded project by Sebastian Watt.

A little further South, again in Chile, and here is another classic example of a small sub-glacial ridge of andesite, this time near the summit of volcan Apagado, on the Hualaihue peninsula.

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Complex polygonal jointing in a ridge of andesite, from the margins of volcan Apagado, Chile. Fieldwork carried out as a part of a NERC-funded PhD project by Sebastian Watt.

A final example is from volcan Sollipulli, a little further North in the Chilean Lake District. This example is of a dyke, thought to have intruded along the contact between a glacier and the volcanic edifice, near the present-day crater edge. Here, the portion of the dyke that was originally in contact with the ice has developed a very strong platy fabric, and is falling apart to form a scree of what looks like slate.

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Platy jointing developed in the margins of an originally ice-bounded dyke, volcan Sollipulli, Chile. View from above, looking down the dyke margin. Platy scree now partially fills the channel which was once filled with ice. View – about 1 m across. Fieldwork was carried out as a part of a IAVCEI-funded project on the hazards of snow-capped volcanoes.

While there is general agreement that these patterns of jointing form because of the contraction of magma as it cools, there is not yet a concensus model for what it is that controls the size of the polygonal joints, or the typical number of sides. Theoretically, it is argued that a hexagonal jointing pattern would be the lowest-energy, and favoured, solution. But in reality, cooling rates might be too fast for full energy minimisation, and this might explain why many polygonal joints have fewer than six sides. This is the conclusion of the most complete study to date, in which Gyoergy Hetenyi and colleagues argue that field evidence points to two major controls being the size of the cooling body, which influences how fast it cools, and composition, which influences the physical properties of the cooling magma.