This page ARCHIVED - August 2008
Most of what we have considered so far has been extrusive in nature: magma from beneath the surface has reached the surface.
There are many locations where magma cools off before reaching the surface, or where it cannot find a way through the crustal layer and solidifies into a series of shapes within the COUNTRY ROCK.
In these cases, the magma plume will enter the crust, and try to force its way through to the surface. It is an intruder, hence the name.
The rising magma makes space for itself in 3 ways:
Ø Wedging open the overlying rock – lifts the rock, which fractures, and it moves into the cracks and repeats the process
Ø Breaking off large blocks of rock – breaks blocks off the crust, which then sink into the magma
Ø Melting the surrounding rock
PLUTON
A large igneous body ranging from 1 cubic kilometre to hundreds of cubic kilometres.
The largest size of pluton is the BATHOLITH
Defn:
" a large, irregular mass of coarse grained igneous rock which covers at least 100 square kilometres"
(smaller areas are called STOCKS)
They are DISCORDANT intrusions because they cut across the layers of the existing crust. Coarse grain is a result of slow cooling at depth (can extend down to 10-15 km below the surface) and formation of crystals (can be valuable..) Most of the SW Peninsula is a granite batholith, and explains the structure of Dartmoor through to Land’s End and the Scilly Isles.
Where the structure is close to the surface it may cause the surface rocks upwards in a dome: e.g the Eildon Hills in the Scottish borders. This is a LACCOLITH.
SILLS
Defn:
"a tabular, sheetlike body formed by injection between parallel layers of pre-existing bedded rock"
These are CONCORDANT intrusions – boundaries lie parallel to the layers.
Range in thickness from a single centimetre to hundreds of metres.
e.g Great Whin Sill (dolerite) on which (and of which) Hadrian’s Wall was built in NE England
DIKES
The major route of magma transport in the crust. They cut across the layers of bedding rather than running parallel and are vertical features.
Sometimes form by forcing open existing fractures (cleavage line), or more likely create new cracks by the force of their intrusion. Width varies from a few centimetres to tens of metres. Rarely do you see just one dike, they tend to occur in SWARMS. Can also get a circular feature called a RING DIKE e.g An t-Sron (the Nose) at the head of Glencoe
Texture varies from coarsely crystalline to more fine grain – depends on where the intrusion occurred.
Nearer the surface = finer grain.
There can also be VEINS of minerals deposited at the same time because of the melting of the rock.
These often branch off the main intrusive body and can run for several kilometres.
e.g Mother lode of 1849 Californian Gold rush was vein of quartz containing gold
Where these rocks outcrop at the surface it is usually as a result of denudation: weathering which removes the overlying layers, often because the doming upwards has produced cracks which are exploited by the weathering processes.
Rocks are then quarried for use in road construction e.g. Aberdeenshire granite taken from a batholith, and Victorians used it to build town halls up and down the country.