GMPV
Geochemistry, Mineralogy, Petrology & Volcanology

Magma

How do crystal aggregates form in magma chambers?

How do crystal aggregates form in magma chambers?

By Penny Wieser (PhD student at the University of Cambridge)

Clues into the inner workings of volcanoes can be gleamed from material which is erupted at the surface, or that which solidified at depth in the crust. Just before eruption, three main phases are present: a gas phase (containing water, carbon dioxide, sulphur, chlorine etc), a liquid melt phase (the magma), and a solid phase (consisting of lots of different kinds of crystals).

Olivine aggregate consisting of at least 17 separate crystals (taking with a normal camera). B-C) Backscatter electron images taken using an SEM showing attached olivine crystals (dark grey) surrounded by volcanic glass (solidified melt; lighter grey).

Petrologists spend their lives studying these crystals, and use them to determine how deep the magmas are stored in the crust, how long the magmas are stored, and how fast the magmas rise to the surface. However, many of the characteristics of erupted crystals are not fully understood, but may provide crucial insights into processes happening at depth within volcanoes. One example is individual crystals attaching together to form aggregates (also known as clusters, clots, or glomerocrysts). Despite these being really common in volcanoes, it’s still not exactly clear how they form. Previously it has been suggested that aggregates form during growth processes such as heterogeneous nucleation (where a new crystal nucleates on the surface of an existing crystal), dendritic growth (which occurs when crystals grow extremely fast, forming finger-like dendrites (this comes from the greek for ‘tree’)), or twinning (where two parts of a crystal grow together, with a specific orientation relative to one another). Or, crystals may “swim”  together as they settle through a liquid magma column (through a process known as synneusis), or land on top of one another after they settle to the solid base of the magma chamber. We wanted to have a look at these different possibilities in some more detail, we did this using a technique called electron backscatter diffraction (EBSD).

We took olivine crystals erupted at Kīlauea Volcano (Hawai’i), and chromite crystals from the Bushveld Complex of South Africa, and prepared flat, very well polished slices. These samples were then placed in the Scanning Electron Microscope (similar to a regular microscope, but with electrons instead of light). An electron beam was focused on the sample; electrons which enter the sample are diffracted by planes of atoms in the olivine and chromite crystals, and leave the sample as “backscattered electrons” (these are collected by an EBSD detector). Following a series of data processing steps, these measurements can be used to determine the orientation of the crystal lattice. This provides a quick, quantitative way to determine the relative orientations of neighbouring crystals within aggregates. These observations can then be compared to the orientations expected from the various different hypotheses.

EBSD-map, color coded according to the orientation of the separate crystals. The fact that all 6 attached crystals show similar colors indicates that their orientations are similar.

Crystals are lazy; they will grow in the way that uses the least energy. During crystal growth, any mismatches in the direction of the new part of the crystal lattice with respect to the existing lattice is energetically exhausting. So if a new crystal wishes to grow on the surface of an existing crystal, it should mimic the orientation of the existing crystal to conserve energy. Similarly, twinning only produces a finite number of possible geometries, called “twin laws” (1 in chromites, 3 in olivines). In contrast, physical aggregation processes can result in a much broader range of attachment geometries. The settling of crystals onto the floor of a magma chamber where they land on other crystals produces totally random aggregate geometries (analogous to a very bad tetris player). The swimming together of crystals in the liquid part of the magma chamber results in certain crystal faces sticking together (which can be predicted from observations of the way that crystals orientate themselves as they fall).

Olivine and chromite aggregates show too broad a range of attachment geometries to have formed during crystal growth. Instead, we observe that large numbers of olivine crystals are attached along the crystal faces predicted from settling. So, just before eruption, olivines in in the liquid part of the volcanic plumbing system at Kīlauea Volcano must “swim together” as they settle. Chromite aggregates from the Bushveld complex show completely random orientations. This is best explained by the settling of individual crystals onto a firm substrate at the base of the chamber, where they randomly land on other chromite crystals. The fact that crystals in liquid magma are present as aggregates in some volcanic systems (e.g. Kīlauea), but individual crystals in others (e.g. Bushveld) has implications for calculations of the rates of crystal settling. This, in turn, affects the viscosity of erupted lavas (as slower settling rates means more crystals, and “stickier” magmas), and is important to understand the formation of economic deposits of platinum group elements (which are concentrated within settled piles of crystals within the Bushveld Complex).

Want to know more? Read all about it here!

Penny Wieser is a PhD student in volcanology at the University of Cambridge, UK. She uses a variety of analytical techniques to interrogate erupted crystal cargoes and their melt inclusion records to gain insights into the pre-eruptive storage of magma beneath Kīlauea Volcano, Hawai’i.

Are mantle melts heterogeneous on a centimeter scale?

Are mantle melts heterogeneous on a centimeter scale?

The mantle makes up the majority of the volume of the Earth, but there is still a lot about it that we don’t understand. This is because we can’t observe it directly – forget ‘Journey to the center of the Earth’ – even our deepest drill holes (about 12 km deep) are merely tickling the surface of the planet (about 6400 km to the center).

The journey to the centre of the Earth, by Édouard Riou (it’s not that easy). Source: wikimedia.org

Most of what we know about the mantle comes from secondary sources of information. For example, we can use magmas (liquid rock, which later become lavas when they erupt), which form when the mantle melts slightly. When these erupt onto the Earth’s surface, they bring with them some information about the rock (the mantle) from which they formed. This is just like how our DNA can tell us something (but not everything) about our parents. So by analysing the magma, we can start to piece together what the mantle was like where it formed. The problem here is that magmas are liquid, and can easily mix with other magmas, so the original information gets lost.

One thing we do know about the mantle, though, is it is not the same everywhere, it is heterogeneous. Some of the differences might be left over from four and a half billion years ago when the planet formed, others are due to rocks from the surface being pushed down into the Earth. The question is the scale of this heterogeneity –  is it heterogenous on the scale of hundreds of thousands of kilometers, kilometers, tens of meters, or even smaller?

The drill used to sample the rocks. Open your mouth and say ‘aaahh’. Credit: Sarah Lambart

A new perspective comes from a team from Cardiff University and the Vrije Universiteit of Amsterdam, led by Sarah Lambart (now at the University of Utah), and published in Nature Geoscience. The team took samples drilled from below the bottom of the North Atlantic ocean, and from these they extracted some minerals – clinopyroxene and plagioclase. These were selected as the most primitive minerals – that means the ones that formed first from a magma, and so they should most closely mirror the melt’s composition. From these millimeter sized crystals, they drilled out tiny samples of powder (the drill is a lot like a dentist’s drill, but with less fear and pain), which were then dissolved and analysed, to measure the concentration of some different isotopes (the same element, with different mass) of strontium and neodymium.

What they found was startling – these tiny samples were extremely variable in their isotopic compositions. The range was so big that it represented almost the whole variability of the North Atlantic – a whole ocean-sized range of chemistry in samples totalling about the size of a grain of rice. This implies that the mantle is able to preserve heterogeneous melts over a scale that is far smaller than previously thought. So, why do we not see this in the erupted lavas? Because they get mixed up, and the tiny variations are lost – this mixing has to happen somewhere between the site from where the crystals were sampled, and the sea floor.

So what’s next? Sarah says the answer might lie in experiments. By recreating the conditions of mantle melting, and simulating the movement of melt through solid rocks, we might be able to understand better how such tiny scale variations can be preserved in the mantle and then be completely lost by the time the melts erupt onto the ocean floor.

Building a lava dome: one block at a time

Building a lava dome: one block at a time

Lava domes form when lava is extruded from a volcanic vent, but is too viscous to flow far away. Think of thick treacle that does not flow as easily as runny honey, and so when it is extruded, it forms a “lava pile” around the vent. Lava domes commonly form within the crater of a larger volcano (e.g. Mt. St. Helens), but can also stand alone or form part of a “dome complex”.

Soufrière Hills Volcano, Montserrat (taken in 2016). Credit Claire Harnett

A lava dome can take on a variety of shapes and sizes dependent upon many factors, including lava chemical composition, viscosity and extrusion rate. Some of the different types of lava domes range from “pancake domes”, generally short and wide, to “spiny domes”, where lava cools and breaks into blocks of rock. Volcanologists have observed over time that every lava dome is different, even at the same volcano or during the same eruption. The number of factors that influence the behaviour of a lava dome makes it difficult to understand the processes occurring during lava dome growth. There is still a lot of research necessary before we can think about forecasting behaviour during a dome-building eruption.

Lava dome hazards

There are many hazards associated with dome-building eruptions, and they are often linked to explosive volcanic activity. In the same way that landslides occur in a slope made of soil, lava domes can collapse if they become oversteepened. Lava dome stability can also be affected by intense bouts of rainfall, as the rainwater can be pressurised when it comes into contact with the hot dome (temperatures at eruption are generally in excess of 850˚C), thus turning the rainwater into steam and forcing the solidified lava to fracture apart, causing collapse. Other causes for collapse include a build-up of gas within the dome, or a change in direction of the lava extrusion.

Pyroclastic flows from dome collapse in Montserrat, June 1997. Credit BGS.

A number of things can happen when a lava dome collapses, ranging from small scale rockfalls, to debris avalanches, to pyroclastic density currents (turbulent mixtures of gas and rock that can travel at speeds of over 100 kilometres per hour). Any of these can be devastating to communities surrounding the volcano, as pyroclastic density currents cannot be redirected using the same methods that have been utilised for lava flow diversion.

Hazards for scientists

Despite how dangerous these events can be to populated areas, very little is known about various causes of a collapse. This is mainly because the internal structure of a lava dome is uncertain. We think there is a ductile, frictionless core that is surrounded by a solid, friction-controlled rind (think of a soft caramel encased by hard chocolate). A dome often also includes talus aprons, which are piles of loose rubbly material that accumulate due to rockfalls that occur during dome growth.

Research into lava dome collapse can be split into three main areas: field observations, analogue and laboratory experiments, and numerical modelling. Field observations of real life environments and landscapes are vital for creating conceptual models of a lava dome.

Conceptual diagram of internal structure, and photo of the dome at Mt. Unzen (2016). Credit: Claire Harnett

The missing particle of the puzzle

After fieldwork at both Mt. Unzen and Soufrière Hills Volcano, my research has focused on numerical modelling of lava dome collapse. The aim is to build a computer simulation of a lava dome, and then change conditions in the model to create various “model scenarios” for collapse. This allows us, for example, to change the gas pressure (something we know to cause collapse at a real dome) in our synthetic lava dome, and vary the time period over which forces are exerted upon the dome.

To do this, we use discrete element method (DEM) modelling. DEM considers the simulated lava dome as an assembly of individual particles/blocks, which means that the model can evolve dynamically and the particles act according to gravity and their interactions with neighbouring particles. We generate a particle based material that represents lava. We then give the particles a speed and a direction to simulate extrusion, and we control the point at which parts of the material start to behave as a solid (i.e. representing a realistic cooling/solidification process).

Discrete element model of a very simplistic lava dome. Credit: Claire Harnett

Although these models are still in development, we are able to replicate the growth patterns observed at lava domes. For example we see when a lava dome reaches its maximum height (determined by the rock properties), it starts to grow outwards rather than upwards. We aim to create software that can further our understanding of the processes occurring within a dome, with the ultimate goal of use in observatories. Scientists monitoring the volcano could then input what they see occurring during a volcanic eruption, and have a greater chance of predicting what will happen next.

These models are proving to be a really exciting and innovative way to study lava dome growth and collapse, and will reveal some of the mysteries of lava domes and the dangers they pose.

A movie of lava dome emplacement, where red material acts as ductile core, and grey material acts as solid rock. 

Blog post written by Claire Harnett. For more information, keep your eyes peeled for the paper coming soon! Or get in touch with Claire on Twitter.

Living with volcanic gases

Living with volcanic gases

Professor Tamsin Mather, a volcanologist in Oxford’s Department of Earth Sciences reflects on her many fieldwork experiences at Masaya volcano in Nicaragua, and what she has learned about how they effect the lives of the people who live around them. 

Over the years, fieldwork at Masaya volcano in Nicaragua, has revealed many secrets about how volcanic plumes work and impact the environment, both in the here and now and deep into the geological past of our planet.

Working in this environment has also generated many memories and stories for me personally. From watching colleagues descend into the crater, to meeting bandits at dawn, or driving soldiers and their rifles across the country, or losing a remotely controlled miniature airship in Nicaraguan airspace and becoming acquainted with Ron and Victoria (the local beverages), to name but a few.

I first went to Masaya volcano in Nicaragua in 2001. In fact, it was the first volcano that I worked on for my PhD. It is not a spectacular volcano. It does not have the iconic conical shape or indeed size of some of its neighbours in Nicaragua. Mighty Momotombo, just 35 km away, seems to define (well, to me) the capital Managua’s skyline. By comparison, Masaya is a relative footnote on the landscape, reaching just over 600 m in elevation. Nonetheless it is to Masaya that myself and other volcanologists flock to work, as it offers a rare natural laboratory to study volcanic processes. Everyday of the year Masaya pumps great quantities of volcanic gases (a noxious cocktail including acidic gases like sulphur dioxide and hydrogen chloride) from its magma interior into the Nicaraguan atmosphere. Furthermore, with the right permissions and safety equipment, you can drive a car directly into this gas plume easily bringing heavy equipment to make measurements. I have heard it described by colleagues as a ‘drive-through’ volcano and while this is not a term I like, as someone who once lugged heavy equipment up 5500 m high Lascar in Chile, I can certainly vouch for its appeal.

Returning for my fifth visit in December 2017 (six years since my last) was like meeting up with an old friend again. There were many familiar sights and sounds: the view of Mombacho volcano from Masaya’s crater rim, the sound of the parakeets returning to the crater at dusk, the pungent smell of the plume that clings to your clothes for days, my favourite view of Momotombo from the main Managua-Masaya road, Mi Viejo Ranchito restaurant – I could go on.

But, as with old friends, there were many changes too. Although in the past I could often hear the magma roaring as it moved under the surface, down the vents, since late 2015 a combination of rock falls and rising lava levels have created a small lava lake visibly churning inside the volcanic crater. This is spectacular in the daytime, but at night the menacing crater glow is mesmerising and the national park is now open to a stream of tourists visiting after dark. Previously, I would scour the ground around the crater for a few glassy fibres and beads of the fresh lava, forced out as bubbles burst from the lava lake (known as Pele’s hairs and tears after the Hawaiian goddess of the volcano – not the footballer) to bring back to analyse. Now the crater edge downwind of the active vent is carpeted with them, and you leave footprints as if it were snow. New instruments and a viewing platform with a webcam have been put in, in place of the crumbling concrete posts where I used to duct-tape up my equipment.

Footprints in the Pele’s hair. Image credit: Tamsin Mather

This time my mission at Masaya was also rather different. Before I had been accompanied solely by scientists but this time I was part of an interdisciplinary team including medics, anthropologists, historians, hazard experts and visual artists. All aligned in the shared aim of studying the impacts of the volcanic gases on the lives and livelihoods of the downwind communities and working with the local agencies to communicate these hazards. Masaya’s high and persistent gas flux, low altitude and ridges of higher ground, downwind of it, mean that these impacts are felt particularly acutely at this volcano. For example, at El Panama, just 3 km from the volcano, which is often noticeably fumigated by the plume, they cannot use nails to fix the roofs of their houses, as they rust too quickly in the volcanic gases.

The team was drawn from Nicaragua, the UK and also Iceland, sharing knowledge between volcano-affected nations. Other members of the team had been there over the previous 12 months, installing air quality monitoring networks, sampling rain and drinking water, interviewing the local people, making a short film telling the people’s stories and scouring the archives for records of the effects of previous volcanic degassing crises at Masaya. Although my expertise was deployed for several days installing new monitoring equipment (the El Crucero Canal 6 transmitter station became our rather unlikely office for part of the week), the main mission of this week was to discuss our results and future plans with the local officials and the communities affected by the plume.

Some of the team working on Masaya: Left to right Dave Lynch, Evgenia Ilyinskaya, Rachel Whitty, Peter Baxter, Tamsin Mather, Jennifer Le Bond, Gudrun Halla Tulinius and Sara Barsotti. Image credit: Tamsin Mather

Having worked at Masaya numerous times, mainly for more esoteric scientific reasons, spending time presenting the very human implications of our findings to the local agencies, charged with monitoring the Nicaraguan environment and hazards, as well managing disasters was a privilege. With their help we ran an information evening in El Panama. This involved squeezing 150 people into the tiny school class room in flickering electric light, rigging up the largest TV I have ever seen from the back of a pick-up and transporting 150 chicken dinners from the nearest fried chicken place! But it also meant watching the community see the film about their lives for the first time, meeting the local ‘stars’ of this film and presenting our work where we took their accounts of how the plume behaves and affects their lives and used our measurements to bring them the science behind their own knowledge.

Workshop at El Panama. Image credit: Tamsin Mather

Watching the film it was also striking to us that for so many of this community it was the first time they had seen the lava lake whose effects they feel daily. Outside the school house there were Pele’s hair on the ground in the playground and whiffs of volcanic gas as the sun set – the volcano was certainly present.  However, particularly watching the film back now sitting at home in the UK, I feel that with this trip, unlike my others before, it is the people of El Panama that get the last word rather than the volcano.

 

 

Blog written by Tamsin Mather (University of Oxford). You can follow the project on Twitter @UNRESPproject. UNRESP project is funded by GCRF Building Resilience programme.