Friday, April 26, 2013

Scafell: An old volcano's exposed guts

We all love the photos of perfect conical stratovolcanoes and eruptions that appear on TES, but geologists don't only study the Earth's surface, they need to know what's happening underneath. There are several ways to do this. Geophysics has technological and interpretative limitations, along with being expensive. Drilling cores is also pricey, and only limited sampling can be done, even if oil or gold are at stake. The best and cheapest is to use erosion, and go sniff around a place where weathering has exposed the type of structure you wish to study. This allows a thorough survey of a large area, easy rock sampling, and allows us to assemble high resolution reconstructions of past events and processes. One of the best locations to study the insides of a volcanic caldera is at Scafell, in England's scenic Lake District.

Calderas develop when large amounts of quartz rich felsic magma are erupted and the ground sinks into the newly emptied magma chamber below, creating a large bowl shaped crater. This process often repeats itself, as the magma keeps on rising after each collapse, resulting in a complex set of nested bowls. Famous examples include Santorini in Greece, Krakatoa in Indonesia and Mammoth Lake in the USA. Understanding calderas is important to geohazard researchers, who wish to learn how to predict these destructive eruptions in order to protect threatened populations. As Lyell said in a proverb that became a keystone of Earth science, the first step to predicting the future is always understanding the past.

Back in the Ordovician, the proto-Atlantic (called the Iapetus ocean) was closing, and was later to weld Scotland to the rest of Britain. As this slow motion continental crash approached, an arc of volcanic islands formed in Iapetus, close to the oceanic plate's subduction zone under Avalonia (the England of that epoch, Japan is a modern analogue). The process is well understood: Water is baked out of the subducting plate, rises, and makes the mantle wedge above the sinking slab melt.

Scafell started life as a group of nested calderas in this island arc, became a mountain range twice as America and Europe collided (called the Caledonian and Variscan orogenies) and was later carved open by Pleistocene glaciers. This last event uncovered some very interesting geology for us to enjoy. The mountain is England's highest peak at 964 metres. William Wordsworth loved it some centuries ago, and it is a staple with modern hikers. Cumbria's poet was born and lived in the area, and acquired his love of the natural world 'wandering lonely as a cloud' around the Lake District.

The caldera is also England's best known volcano, and thanks to its exposure and well layered deposits, the reconstruction of the sequence of events at its fiery birth is one of the most detailed in the world. Unlike most calderas, where the collapse occurs piston style within a ring fault, Scafell repeatedly sank in a very complex piecemeal manner down a network of faults. Since geology's early days it has been the type locality for the study of this rare kind of subsidence.

The Borrowdale volcanics that compose the peak show us the succession of events as an arc forms and welds itself to a continent as an ocean closes. Several kilometres of mixed volcanics boiled out of the Earth, starting with repeated andesitic lava flows (the usual arc rock), with the odd ash spewing explosion. Then came enormous explosive rhyolitic eruptions whose pyroclastic flows deposited a couple of kilometres of welded ignimbrites. An wide variety of deposits has been recognised over decades of study, giving us a high resolution dataset to add to in the future. These rocks repeatedly sank into the crust accompanied by massive earthquakes, forming the structural complexity of the nested calderas.

The original pretty volcanoes have now been eroded into dust, making them impossible to find, but the underlying structures are exposed for our edification. The area has been repeatedly surveyed and sampled, and still provides new insights into how such complex calderas form.

For a geologist the most informative places aren't always the most spectacular sights, but the knowledge we acquire from places like Scafell sure inspires poetry in my soul...

Image credit: Sean McMahon of
Our story on Krakatoa:

Our past story on ignimbrites:

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