The Earth Story

An Sgurr

Some 58 million years ago the area we now know as the Hebrides (a group of Islands off Scotland) were a place of fire and smoke rather than the present windy sea lashed locale of the present era. The North Atlantic Ocean was opening, completing the bustup of Pangaea. This sundering of the European and North American plates was accompanied by huge outpourings of lava over a wide region, whose remnants still dot the landscape from Greenland to Ireland, Canada to Scandinavia.

This 5km long ridge on the isle of Eigg is what remains of a huge pyroclastic flow from a large eruption, whose main vent was located at the present isle of Rum. An Sgurr is also a textbook example of what geomorphologists (those who study the surface shape of the Earth and how it came to be) call inverted topography, a process whereby what were once lowlands turns into highlands due to an interaction between the constraints imposed by the underlying geology and the forces of erosion.

When the silica rich rhyodacitic lava erupted, the hot molten ash and rock flowed from the vent down a river valley, filling it to a height well above the 400 metres of the uneroded remnants that we see today. The rock is dark (unlike most silica rich volcanic rocks) because it is a quick cooled variety known as pitchstone with a high glass content, but coarser and with more crystallinity than the more familiar obsidian. It looks like a glassy matrix, filled with angular crystals. Our ridge is the largest exposure of such rock in the UK, and it displays typical lava cooling features (such as columnar jointing similar to the Giant's Causeway).

Pitchstone also happens to be very resistant to erosion, the result of its peculiar part glass part crystal structure, and over the aeons since it was emplaced, the basaltic rocks (also from the growing continental rift) around it have melted away under the assault of wind, frost and wave, leaving the fossilised impression of a whole Palaeocene river bed standing proud as a tall ridge, complete with its rounded boulders and tree fossils at the base. Solitary mountains like An Sgurr (the notch) are known as inselbergs, but they form by a variety of processes, and most are not inverted.

Loz

Image credit: James Gray

http://www.geograph.org.uk/snippet/11009

Ephemeral inverted topography

Inverted topography occurs when low lying features become high ones as a result of geological processes (it is also known as topographic inversion). There are many kinds, for example a lava flowing down a riverbed carved into softer rocks. As the forces of erosion work their way through the millennia, the harder lava is more resistant, and eventually the softer surrounds become valleys on either side of an upraised paleo-riverbed. Rivers laying down tougher sediments than the surrounding rocks would create another similar example. Cementing of sediments with tougher minerals making them more resistant than their surrounds such as happens near hot springs can also produce this effect.

This example is more ephemeral, showing that geological processes are fractallic ie they occur and resemble each other at many different scales in both space and time. The photo shows coyote or dog tracks preserved in ice. The marks were originally made in fresh powder snow, when the falling feet created the tracks by compressing the snow beneath them. Sometime later, the wind removed the layer of powder snow, leaving behind the more resistant compressed snow in the tracks, which if you look carefully even include a couple of claw imprints.

Loz

Image credit: Mike Zawaski via EPOD

http://geology.teacherfriendlyguide.org/index.php?option=com_glossary&id=150&Itemid=157 http://science.jrank.org/pages/4486/Mountains-Inverted-topography.html

An Sgurr Some 58 million years ago the area we now know as the Hebrides (a group of Islands off Scotland) were a place of fire and smoke rather than the present windy sea lashed locale of the present era. The North Atlantic Ocean was opening, completing the bustup of Pangaea. This sundering of the European and North American plates was accompanied by huge outpourings of lava over a wide region, whose remnants still dot the landscape from Greenland to Ireland, Canada to Scandinavia.  This 5km long ridge on the isle of Eigg is what remains of a hug pyroclastic flow from a large eruption, whose main vent was located at the present isle of Rum. An Sgurr is also a textbook example of what geomorphologists (those who study the surface shape of the Earth and how it came to be) call inverted topography, a process whereby what were once lowlands turns into highlands due to an interaction between the constraints imposed by the underlying geology and the forces of erosion.  When the silica rich rhyodacitic lava erupted, the hot molten ash and rock flowed from the vent down a river valley, filling it to a height well above the 400 metres of the uneroded remnants that we see today. The rock is dark (unlike most silica rich volcanic rocks) because it is a quick cooled variety known as pitchstone with a high glass content, but coarser and with more crystallinity than the more familiar obsidian. It looks like a glassy matrix, filled with angular crystals. Our ridge is the largest exposure of such rock in the UK, and it displays typical lava cooling features (such as columnar jointing similar to the Giant's Causeway). Pitchstone also happens to be very resistant to erosion, the result of its peculiar part glass part crystal structure, and over the aeons since it was emplaced, the basaltic rocks (also from the growing continental rift) around it have melted away under the assault of wind, frost and wave, leaving the fossilised impression of a whole Palaeocene river bed standing proud as a tall ridge, complete with its rounded boulders and tree fossils at the base. Solitary mountains like An Sgurr (the notch) are known as inselbergs, but they form by a variety of processes, and most are not inverted.  Loz Image credit: James Gray http://www.geograph.org.uk/snippet/11009 http://www.isleofeigg.net/an_sgurr.html http://www.summitpost.org/an-sgurr/195034