Arthur’s Seat, Edinburgh

Edinburgh recently held a volcanology conference (VMSG), which is quite fitting seen as an ancient volcanic complex can be found at the centre of Edinburgh! The following photos are just a few photos from a  trip up Arthur’s seat whilst at VMSG…

Edinburgh from  Arthur's seat
Edinburgh from Arthur’s seat. This ancient volcano  existed around 350 million years ago in a period know as the Carboniferous.
Columnar jointing above the road on Arthur's Seat
Columnar jointing above the road on Arthur’s Seat. This feature forms when the the molten rock cools causing it to fracture in a regular pattern. The intersection of these fractures forms the polygonal columns. They are a common feature found in igneous rocks worldwide.
Columnar jointing above the road on Arthur's Seat
More columnar jointing above the road on Arthur’s Seat…
Arthur's Seat at sunset
Arthur’s Seat at sunset. I think this area is currently closed due to rockfall.
View from Arthur's seat at sunset
View from Arthur’s seat at sunset
Advertisements

Oil Shale Experiment

Another stop on the trip to Scotland involved looking at the oil shale’s exposed beneath the Forth Road Bridge.

The Forth Bridge
The Forth Bridge

Unfortunately the tide was in meaning we couldn’t look at the outcrops we had intended to, but we did get to see some of the sand produced from these rocks.

The black sand here is fragments of the oil shale. Unfortunately the tide was in so we couldn't see the actual outcrop.
The black sand here is fragments of the oil shale. Unfortunately the tide was in so we couldn’t see the actual outcrop.
Oil shale sand beneath the Forth Bridge
Oil shale sand beneath the Forth Bridge

If you take some of this black sand and heat it up (we used a candle), it is possible to release the hydrocarbons in the rock. Nothing particularly scientific, but its a fun experiment to determine if the rock contains abundant hydrocarbons!

Heating up the oil shale
Heating up the oil shale
Smelling the hydrocarbons released from the oil shale
Smelling the hydrocarbons released from the oil shale

An ancient ocean floor in the Alps?

After the adventures in Italy and Switzerland it was time to start the fieldwork part of my trip in Briancon. Part of this involved a visit to the Chenaillet Ophiolite in the French/Italian Alps.

Looking north with the large pillow lavas outcrop in the foreground
Looking north from the Chenaillet Ophiolite

Some Basics

To think about what an ophiolite is we first need to go back to some basic geology…

The outer layer of the Earth is made up of numerous large pieces which can move relative to one another in a theory known as plate tectonics, we use this to explain many of the phenomenon that we observe on the Earth. There are two types of crust making up the outer layer of the Earth; so called oceanic and continental crust. Oceanic crust is thinner, yet denser than continental crust.

This animation gives some idea of how a mid-ocean ridge may look and function but I'm not entirely convinced. http://upload.wikimedia.org/wikipedia/commons/c/c0/Mid-ocean_ridge_topography.gif
This animation gives some idea of how a mid-ocean ridge may look and function but I’m not entirely convinced…
http://upload.wikimedia.org/wikipedia/commons/c/c0/Mid-ocean_ridge_topography.gif

Oceanic crust is created at Mid ocean ridges. These are essentially underwater volcanic mountain chains where the erupted material produces new crust. They mark the boundary between different plates, and are referred to as a divergent boundary as the plates are moving away from one another.

http://en.wikipedia.org/wiki/File:Earth_seafloor_crust_age_1996.gif
Ages of the ocean floor. Oceanic crust is created at the Mid-ocean ridges and destroyed at subduction zones, because of this the crust closest to the mid-ocean ridges is youngest and the oldest is found closest to the edges of the Oceans. http://en.wikipedia.org/wiki/File:Earth_seafloor_crust_age_1996.gif

The difference in density between different crustal units is crucial to allow a process called subduction to take place. Subduction occurs when two plates move towards each other and the denser of the two goes underneath the less dense plate. Subduction zones are the opposite of mid-ocean ridges in that they are a convergent boundary, meaning that the plates move towards one another. As a result of subduction oceanic crust is essentially destroyed, as it is sent back down into the Earth.

Looking out over the Chenaillet Ophiolite in the Franch–Italian Alps. The blue areas of scree represent weathered outcrops of the serpentinised peridotite.
Looking out over the Chenaillet Ophiolite in the Franch–Italian Alps. The blue areas of scree represent weathered outcrops of the serpentinised peridotite.

When we observe rocks which have the characteristics of the oceanic crust on top of continental crust we call these abnormalities ‘ophiolites‘. The word Ophiolite apparently literally translates to ‘snake rock‘ referring to the similar aesthetics of the rocks found in ophiolites and snake skin. Ophiolites have taught us a lot about oceanic crust and tectonics but there is a catch; if normal oceanic crust is subducted back into the Earth then for ophiolites to have been preserved must mean that they do not represent “standard” oceanic crust.

So oceanic crust is created at mid ocean ridges and destroyed at subduction zones, but how did we end up with a segment of oceanic crust high up in the French Alps? Why was this segment of oceanic crust not subducted?

How do we know that the rocks here in the Alps are in fact oceanic crust?

Firstly, we have got abundant outcrops of Peridotite. Peridotite is a ultramafic igneous rock (contains less than 45% silica and solidified from melted rock). Peridotite is the main rock type in the upper mantle and it wouldn’t just form here, it needs to have been produced elsewhere and put here.

A small fragment of the serpentinised peridotite
A small fragment of the serpentinised peridotite

Another major piece of evidence is that the peridotite has undergone a process known as serpentinisation. This is a chemical reaction which occurs through contact with sea water, and has produced many of the minerals in the fragment shown above.

Angular fragments of serpentinised peridotite incorporated into a carbonate
Angular fragments of serpentinised peridotite incorporated into a carbonate rock

The photo above shows that some of the peridotite has been reworked and incorporated into sedimentary rocks (rocks made up of fragments of older rocks). The darker parts are the peridotite and the lighter surrounding material is limestone. Limestone of this type is produced under the sea which is further evidence towards the oceanic origin of these rocks.

Pillow lavas
A spectacular outcrop of Pillow lavas

Finally at the Chenaillet Ophiolite we observe large outcrops of pillow lavas. Pilow lavas are volcanic rocks which were produced underwater, providing further evidence towards the oceanic nature of these rocks.

How did these rocks get here?

These observations of rocks which seem out of place confused early geologists, and it wasn’t until the theory of plate tectonics was established that these suits of rocks were understood to represent small fragments of the oceans attached to the continents.

The process of ophiolite emplacement is termed obduction, and is defined as the overthrusting of oceanic material on top of continental.  Obduction occurs at convergent tectonic boundaries (i.e. where two plates are moving towards one another), this explains why we often find ophiolites incorporated into the mountain chains produced at these boundaries.

Numerous mechanisms have been proposed to explain why obduction occurs rather than the expected subduction but they often struggle to be applied to all cases. It is therefore generally regarded that there is no singular mechanism responsible for all ophiolite occurances, and that more work is required to understand the formation of ophiolites…