Rock and minerals formations

Also see Sedimentary rocks, /Metamorphic rocks/, Igneous rocks and /Precipitate rocks/

The worlds’ surface is made of rock. The rock type in a specific area is dependent on the complex history of the Rock cycle for that area.

In the formation of rocks, there are two main types, Sedimentary and Igneous rock.

Another minor formation is the Precipitates out of a solution, similar to how salt is formed from saline water.

Metamorphic rock is made from any of these existing rock formations with the addition of heat and/or pressure.

Igneous rocks
First we will look at Igneous rock. This is rock that begins as molten rock or magma. It melts within the asthenosphere and rises through the lithosphere.

Intrusive rock either settles beneath the crust or solidifies within the country rock (crust) forming plutons.

Extrusive rock breaks through the crustal rock and extrudes onto the surface of the Earth through Volcanoes, spreading centres and mantle plumes.

Intrusive Igneous rock


For the rest of this page we will cover only on this topic and its effects on fossilization.

For a rock to be an intrusive igneous rock or plutonic rock it must have risen as a magma and solidified within the lithosphere.

Igneous rock makes up 95% of crustal rock and is one of the oldest types of rock the main rocks formed through intrusion are Granite, Diorite, Gabbro and Ultramafic Rocks. Of these the most common type is the Granite which is composed mainly of potassium and silicon rich Feldspars and quartz.

Because intrusive rock cools within the lithosphere rather than the atmosphere, the heat release is slow and the magma can cool slowly over thousands to millions of years. Mineral crystals form slowly within the cooling rock and the crystal structure can become quite large if there is enough room.

Intrusive rocks are named and classified according to:
 * 1) Whether the body large ar small.
 * 2) Whether it has a particular geometric shape.
 * 3) Whether the rock formed at considerable depth or was a shallow intrusion.
 * 4) The geometric relationship of the intrusion to the country rock? (McGeary and Plummer 1994, p.217)

Small intrusive bodies
Dikes, Sills and Laccoliths are small but important intrusions. Dikes are discordant and sills are concordant and tubular intrusions, while a laccolith is a domed concordant intrusion. A laccolith is also rare as an intrusion because of the pressure needed to force other rock apart and make its unique shape. These small intrusions are generally offshoots of banoliths and other magmatic hotspots.

Affects of intrusive bodies on fossilization
When an intrusive body penetrates a rock strata like siltstone, shale etc, the effects of heat from the intrusion on the country rock on the fossils it may contain can be drastic.

Any fossils directly in contact with the molten intrusion will be destroyed. Rock not directly melted but still receiving substantial heat, may be metamorphosed into another type of rock, eg. limestone into marble. This metamorphosis will likely distort any fossils’ appearance or destroy them.

Readings

 * Resources from the central Michigan University
 * Rock types and hardness By Kim Brannon
 * Igneous Rock by Dave Jessey and Don Tarman