An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly cools into a solid, the different parts of the magma crystallize into minerals.
Molten rock, or magma, does not always succeed in reaching the earth’s surface. Forcing a way through layered strata is not easy. Sometimes magma finds joints or fault cracks to travel towards the surface; at other times it seeks an easier route along the bedding between layers of strata. Such diversions can allow basalt magma to cool before reaching the surface. The magma cools and solidifies into rock called dolerite. This layered sandwich of hard intruded magma between beds of softer sedimentary strata is called a sill.
By far the most spectacular one in the area is Salisbury Crags, where the intrusive form of the hard sill forms the crags and the strata above and below can be clearly seen. Quarrying the crags to pave the streets of London in the nineteenth century left their faces sharper than nature fashioned them. Conservation interests fortunately halted this practice
Intrusions are one of the two ways igneous rock can form; the other is extrusive, that is, a volcanic eruption or similar event. Technically speaking, an intrusion is any formation of intrusive igneous rock; rock formed from magma that cools and solidifies within the crust of the planet. In contrast, an extrusion consists of extrusive rock; rock formed above the surface of the crust.
Intrusions vary widely, from mountain-range-sized batholiths to thin veinlike fracture fillings of aplite or pegmatite. When exposed byerosion, these cores called batholiths may occupy huge areas of Earth's surface. Large bodies of magma that solidify underground before they reach the surface of the crust are called plutons.
Intrusions can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes:
- Batholith: a large irregular discordant intrusion
- Dike: a relatively narrow tabular discordant body, often nearly vertical
- Laccolith: concordant body with roughly flat base and convex top, usually with a feeder pipe below
- Lopolith: concordant body with roughly flat top and a shallow convex base, may have a feeder dike or pipe below
- Phacolith: a concordant lens-shaped pluton that typically occupies the crest of an anticline or trough of a syncline
- Volcanic pipe or volcanic neck: tubular roughly vertical body that may have been a feeder vent for a volcano
- Sill: a relatively thin tabular concordant body intruded along bedding planes
- Stock: a smaller irregular discordant intrusive
A schematic geological cross-section of a sequence of sedimentary rocks that are later intruded by igneous rocks accompanied by volcanic activity. Source: United States Geological Survey - Geologic History of Lake Mead National Recreation Area
Quartz-dolerite is very common in central Scotland, in intrusive formations, sills and dykes, and is widely quarried for roadstone. Other dolerite sills form most of the hills on the west side of Edinburgh such as Corstorphine Hill, Turnhouse Hill, Mons Hill, Dalmahoy Hill and Binny Craig, all pocked with ancient or modern quarries for building and road stone. Quarrymen call this rock ‘whin’. It is no coincidence that in spring these hills are ablaze with yellow gorse or ‘whin’ which thrives on the thin stony dolerite soil.
Dykes are narrow vertical intrusions that fill cracks in the strata. Like all intrusions, the rock cracks on cooling to give a joint pattern as at Beecraigs. Quartz-dolerite contains many cooling fractures and weathers badly, becoming unstable. It is not uncommon for large boulders to break loose, and significant rockfalls are not uncommon
To log this earthcache, please email me answers to the following questions. Do not put the answers in your log or it will be deleted. A picture of you at the site would be great too.
- What type of intrusion do you think this is? (from the list above)
- Estimate the height of this intrusion?
- Several rockfalls have occurred over the last few years and the larger rocks remain. Describe their shape
- In what direction does this section trend? East-West or North-South?
A picture of you at the intrusion would be great, but not obligatory.
- PLEASE DO NOT CLIMB ON THE INTRUSION. IF YOU WANT TO USE THE CLIMBING FACILITIES THEN VISIT THE RANGERS FOR MORE INFORMATION.
Thanks to the Beecraigs Country Park Rangers for encouraging Geocaching.