VOLCANOES TO THE FRONT, LEFT AND RIGHT
Here, 350 million years ago, you would be overlooking a coastal plain of forest and lagoons, with rivers meandering from the north or north east to the sea. Flat! Yet today you see a landscape of hill ranges and strange isolated rock shapes, evidence of volcanic activity over a long period, more that 300 million years ago during the Carboniferous Period. . These eruptions were caused by tectonic movements that stretched the Earth's crust and caused melting of deeper rock: magma formed and erupted upwards, resulting in volcanic activity that contributes to the variety in today's landscape.
Choose a reasonable day for this EarthCache or you may not be able to see all the features listed below.
To the north and Edinburgh you can see Arthur’s Seat (Compass bearing from Viewpoint: 315°) with its distinctive lion’s head and body shape. These are the remains of a volcanic eruption probably under water, and eroded over the past 300 million year leaving us with the main lava flows and exposing the internal volcanic structure.
Slightly to the west and formed much later you can see Salisbury Crags (Compass bearing from Viewpoint: 310°) starting with a sheer ascent in the west and tapering to Arthur’s Seat in the east. This is a “sill”, part of the internal structure of a volcano. Sills are formed when magma, instead of erupting through the vent, is forced sideways through the rock, and eventually cools to form very hard rock. Erosion has exposed this sill, and local tilting has lifted the west end high above the plain.
A bit further to the west is a famous stump of rock, once an active volcano, with Edinburgh Castle (310°) perched on the top. This is a volcanic “plug” created when volcanic activity ceased and the magma in the volcanic “pipe”, the passage between the magma chamber below and the “vent” on the surface, cooled to form a “plug”. This plug, of a very hard type of basalt, resists erosion, During the last Ice Age, which ended only about 10,000 year ago, the ice sheets advancing from the west were forced to pass by or over the hard plug leaving us with a “tail “structure , the gentle descent down the High Street to Holyrood Park.
Then, on a clear day, looking north, you should be able to see over Fife and more volcanic plugs. Most prominent are Largo Law (10°) and Kellie Law (20°).
Over to the east is Berwick Law (39°), another plug, eroded to a distinctive conical hill, also with a “crag and tail” stretching 1.5 km east and down nearly to sea level. Then behind and to the east you should be able to make another volcanic plus, the distinctive bun- shape of the Bass Rock (43°).
Then more evidence of volcanic activity, but not another plug, further east is Traprain Law (60°). This is known as a laccolith, and these occur when, deep down, some magma has been forced between two layers of sedimentary rock and the pressure is high enough to force the overlying strata upwards into a dome or mushroom-like shape, and then eroded to give us the long flattish hill of today.
And finally, over in the west lie the many peaks of the Pentland Hills, evidence of volcanic activity about 410 million year ago. The highest, Scald Law (260°), is yet another volcanic plug.
Q1. Why are plugs such common relics of volcanic activity?
Q2. What has here been the main eroding force over the last two million years?
Q3. The last volcanic eruption in Britain was about 60 million years ago. What is the nearest place to Britain which is tectonically active, with erupting volcanoes?
Q4. Which of the volcanic plugs is pictured above?
Q5. With which one of the following volcanic structures do you associate “Crag and Tail”?
- Sill, Volcanic Plug, Concordant Pluton, Glacial remains
Q6. Please draw a field sketch of the landscape which includes all the named features including
Title and date
The features should be labelled by
Name
Volcanic structure
including
bearing in degrees and approximate distance in km from viewpoint
Please email me or send me your answers via the Message Centre, and a photo is always welcome. Your sketch needs to be photographed/ scanned and attached to your message.
This earthcache has been provided by the Lothian and Borders GeoConservation and in cooperation with the ranger service of Vogrie Country Park http://www.midlothian.gov.uk/info/200142/vogrie_country_park
For more information about the geology of this area why not go on the following short walk http://www.edinburghgeolsoc.org/downloads/GeologyWalk-VogrieCountryPark.pdf
http://www.edinburghgeolsoc.org/downloads/GeologyWalk-Crichton.pdf