Igneous rocks are formed when molten material from within the Earth, called magma, cools down and solidifies forming crystals. They are thus characterised by an interlocking crystalline texture. Igneous rocks can be classified in various ways: 1. There are two types of igneous rocks. Those are igneous rock formed from magma that hasn't left the crust, intrusive, and those formed from lava, extrusive. The study of igneous rocks enables us to understand the igneous part of geologic history. For example, at the end of the Triassic period, million years ago.


Author: Miss Jake Morar
Country: Mauritania
Language: English
Genre: Education
Published: 15 June 2017
Pages: 701
PDF File Size: 43.65 Mb
ePub File Size: 48.81 Mb
ISBN: 175-1-96163-312-8
Downloads: 3673
Price: Free
Uploader: Miss Jake Morar


So how do we know how it forms?

As a matter of fact, the fact that we never see it forming is one of the predictions of the theory that it is an intrusive igneous rock, and so tends to confirm the theory. We do not see granite or granite-like sediment geology igneous rocks by surface processes; what else can we conclude but that it is formed underground?

Pictures of Igneous Rocks

In the second place, as we have observed, granite has the same chemical composition as rhyolite, differing from it only in its texture.

Now, as we know that larger crystals form when cooling is slower, and geology igneous rocks the thermal properties of rock as opposed to air or water will lead to slower cooling underground, we must conclude that granite is exactly what we should expect to see if the magma that forms rhyolite when extruded onto the surface was to cool below the surface instead.

A close look at its texture through a microscope confirms the igneous nature geology igneous rocks its formation. The picture to the right is a photomicrograph of granite. Note how the crystals, however bizarre their shape, fit together perfectly.

We may compare this with the texture of sedimentary rocks such as sandstonewhich are clearly made of non-interlocking particles cemented together.

Then we may consider the structures formed by intrusive rocks.

It geology igneous rocks difficult to see how something such as a dike, which, as explained above, is a vertical or near-vertical sheet of rock, could form by any process except the intrusion of magma into a crack in pre-existing rocks.

Lava, at the surface, is exposed to air and water which causes the molten rock to cool rapidly.

Historical Geology/Igneous rocks - Wikibooks, open books for an open world

Solidifying rocks at the surface cool geology igneous rocks quickly for large crystals to form. Molecules in the lava do not have time to arrange themselves to form large crystals.

Extrusive rocks have crystals that are too small to see without magnification.

  • Types of Igneous Rocks - Rocks and Minerals / Geology - Geology for Kids
  • Igneous rock - Wikipedia
  • Igneous Rocks - Rocks and Minerals / Geology -Geology for Kids
  • Igneous Rocks
  • Historical Geology/Igneous rocks
  • Types of Igneous Rocks: Intrusive, Extrusive

A common example of an extrusive igneous rock is basalt. Some extrusive rocks, such as obsidian and pumice, cool so rapidly that they completely lack crystal structure and are considered a volcanic glass.

Pumice is just like obsidian except it is tiny shards geology igneous rocks glass.

Igneous rock

Texture Within the geology igneous rocks main categories of intrusive and extrusive, rock can be classified even further using texture and chemical composition. Scoria forms in the crust of lava or as chunks of lava fly through the air in more violent eruptions.


As magma still inside the earth slowly cools, it begins to crystallize to form rocks. This results in rocks that can be smooth and sometimes even glass-like.


Because the temperature decreases as you travel closer towards the crustthe magma hardens and crystallizes as it ascends. The result geology igneous rocks this crystallization is igneous rock.

Types of Igneous Rocks

If geology igneous rocks cooling of the lava happened more slowly, the rock would be coarse-grained. Because the minerals are mostly fine-grained, it is much more difficult to distinguish between the different types of extrusive igneous rocks than between different types of intrusive igneous rocks.


Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be determined by examination of thin sections of the rock under a microscopeso only an approximate classification can usually be made in the field. Classification Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body.

The classification of the many types of different igneous geology igneous rocks can provide us with important information about the conditions under which they formed. Two important variables used for the classification of igneous rocks are particle size, which largely depends on the cooling history, and the mineral composition of the rock.

Feldsparsquartz or feldspathoidsolivinespyroxenesamphibolesand micas are all important minerals in the formation of almost all igneous rocks, and they are basic to the classification of these rocks.

All other minerals present are regarded as nonessential in almost all igneous rocks and are called accessory minerals.