IN THE PATH OF A GIANT LAVA FLOW
Torphin Quarry
Imagine standing here about 410 million years ago; low carpet-like vegetation stretches before you, there is little animal life, just insects and a few amphibians, like frogs. No dinosaurs, of course; they are still over 200 million years into the future.
Then there is a terrible noise in the distance - perhaps flames shoot into the air - and a giant sheet of glowing lava flows slowly down the hillside towards you and stops here.
About 490 million years ago, in the Ordovician Period, two pieces of the Earth's crust started to come together to form what we call Britain. This event is called the Caledonian Orogeny. Later, around 410 million years ago, early in the Devonian Period, the southern piece of crust, which continued to push northwards under the piece containing Scotland, melted with the intense pressure to produce hot liquid rock, called magma.
When magma reaches the surface it erupts, sometimes with explosive force sending ash high into the sky, or else as a runny liquid. There are many of these volcanoes in this part of Scotland. In the Pentlands area the first magma to reach the surface, where it is called lava, was sticky and did not flow very far, but later on the composition changed to runny basalt lava creating the great lava flows you see here. There were explosive eruptions as well.
Here, at the face of Torphin quarry, you can see several distinct lava flows. The first flow, dark grey lava, was probably about 30 metres thick but here you can only see the top 3 metres, the rest is buried under the quarry face.
Then there is a second flow about 3 metres thick but the composition is different. There are lots of fragments and signs of water presence.
The third flow reaches to the top of the quarry and is the same composition as the first flow. Don’t be confused by the line running about 3 metres from the top which was created during the quarrying process.
After the volcanic activity, earth movements broke up the lava flows in several places, displacing the layers. Some time later, hot fluids passing through the fractures depositing minerals, including the pink and white mineral, called barite, which can be found in the quarry.
Because of the commercial and other activities to the east of Torphin quarry you are asked to approach from the west. There is a car park near Harlaw House Centre at GR 183655 about 4 kms from the quarry. Or else walk/bike from Currie and through Easter Kinneith, about 3 kms. Alternatively please consult Pentland Hills Hotline 0131-529-2401 for latest Health and Safety issues.
QUESTIONS AND ANSWERS
1. How did these lava flows originate before they erupted?
2. Lava Flow 2 contains volcanic debris and evidence of water. Explain this different composition.
3. The volcanic rock of these lava flows is very hard and difficult to work. For what purpose was the rock quarried ?
4. Draw a picture , with scale , identifying the following with labels
- the dark grey lava flows
- the ashy flow with water presence
- where barite is visible
- sign of rupture
- the quarryman line
Please send your answers via the message centre, where attachments can be added. Please do not add your answers to the cache page.
Now go ahead and log this cache, and a photo is always welcome.
For more information about the geology of the Pentland Hills please go to Pentland Hills Regional Park
This geocache has been provided by the Lothian and Borders GeoConservation