Yorkshire Philosophical Society

Event Information

Mapping Mountains & Microstructures – How Atomic Scale Crystal Defects Influence Global Tectonics.

Dr Eddie Dempsey – Senior Lecturer in Structural Geology & Geohazards at the School of Environmental Sciences, University of Hull.

John and Anne Phillips Lecture with preentation of the John and Anne Phillips 2024 Awards to University of Hull final year students.

We are all familiar with “young” towering mountain ranges such as the Himalaya, the Rocky Mountains, and Southern Alps of New Zealand. We are also familiar with ancient mountain ranges such as the Caledonian mountains which stretch from Norway, through Britain and Ireland and on into North America. All these geological wonders are formed through the actions of plate tectonics. These immense forces not only shape the surface of our dynamic planet but leave a distinct microscopic fingerprint of deformation within the rocks. By analysing these microstructural fingerprints and relating them to large crustal scale feature we can glean a great insight into how plate tectonics operate. In particular, how the earth’s crust to deforms allowing it to rise into immense mountain ranges which in turn influence climate, waterways, populations and politics. Ultimately, we find that if it wasn’t for atomic scale defects in crystals, plate tectonics as know it could not happen.

This talk will cover microstructural analysis and mapping expeditions I have carried out over the last 20 years across the globe from New Zealand to Scotland. I will introduce you to an atomic scale process known as dislocation creep, how we identify it in the rocks that make our mountains. We will look at how this process is so critical to our understanding of global tectonics and ponder how life might be rather different if it wasn’t for microscopic mistakes in crystals.

Landscape at three different scales:-

2.30pm in the Tempest Anderson Lecture Theatre in the Yorkshire Museum.

Member’s report

Dr Dempsey has published papers on geological surveys in many parts of the world but this lecture concentrated on his studies of faults in New Zealand and northern Scotland.

The west coast of New Zealand’s South Island is on the boundary of the Australian and Pacific tectonic plates where major earthquakes have occurred about every 300 years – interestingly, the next one is due! The fault line was examined in some detail and it was concluded that ultra-mylonite and mylonite (comprising biotite, quartz and feldspar) above the fault gave critical evidence. Under microscopic examination the quartz in particular showed very distinctive patterns and shapes –  including rods, foliation, ribbons, blotchy, amoeboid and polygonal. These changed with distance from the fault and demonstrate why apparently similar rocks can exhibit different flow and other characteristics.

Similar findings were found in faults in northern Scotland though feldspar behaved differently. In very localized areas some rocks had melted. This was explained by frictional forces from rocks moving at more than 1 m/s – only plausible during an earthquake.

Rocks are of many different chemical compositions, crystal form and crystal size. They therefore exhibit different microstructural changes and at different temperatures. During fault formation the temperature differs with both depth and distance from the fault so it is now possible to develop a multi-dimensional understanding of the mechanisms involved at the microscopic scales.

Dr Dempsey summarized what was happening at the atomic level, both within crystal grains (eg diffusional dislocations) and at grain boundaries. The intense forces they are under cause many point, planar and linear defects in individual crystals – these cause increases in energy levels and in resistance to flow. Given sufficient time and temperature the crystals can readjust to lower their energy levels and regain their flowability eg by a mechanism called ‘dislocation creep’. This was demonstrated in a series of slides showing schematically how each type of defect can correct itself over time and also by a film of ‘bubble raft’ experiments.

At the macro-scale plate tectonics are driven by convection currents in the earth’s upper mantle. This has caused the earth’s crust to break up into seven or eight large ‘tectonic plates’; the currents cause slow movement (up to 10cm pa) of these plates across the earth’s surface with inevitable collisions. Along convergent plate boundaries subduction of the denser oceanic plate under the less dense continental plate occurs where it is consumed into the mantle. The lost surface is replaced by seafloor spreading as new oceanic crust is formed by upwelling magma along divergent margins.

The major conclusion is that if it were not for mechanisms such as ‘dislocation creep‘ in crystal defects at the atomic scale plate tectonics as we know it could not happen. Subduction zones (in which dense oceanic plates slide under the less dense land masses) would not occur and there would be no immense mountain ranges that influence our climate and many other aspects of our lives. The new understandings will also inform the current debate on when subduction occurred in the history of the earth.

Rod Leonard