igneous Rocks
All rocks on earth were originally igneous rocks. They are formed when magma cools and hardens on the Earth's surface after erupting onto the surface from a volcano. In this case the hot magma that has erupted is called lava. However, it doesn't always cool on the Earth's surface. sometimes the magma cools underground, deep in the earth. This causes the crystals to be much larger for example in the case of granite. The rocks that form above ground cool within several hours. they end up looking shiny and glass-like with crystals that are microscopically small. Since there are two different types of igneous rocks, scientists decided to put them into two different categories. Those which cool underground deep beneath the Earth's surface are called intrusive igneous rocks and those which cool on the surface of the Earth are called extrusive igneous rocks.
Characteristics of Igneous Rocks
Intrusive rocks:
Extrusive rocks:
Examples of Igneous Rocks:
Characteristics of Igneous Rocks
Intrusive rocks:
- Typically coarse grained
- Many large crystals
Extrusive rocks:
- Typically fine grained
- Extremely microscopically small crystals
- Glassy texture if cooling is very rapid
- May contain holes
Examples of Igneous Rocks:
- obsidian - It is a volcanic glass without gas bubbles and usually black or dark brown.
- pumice - It is a volcanic glass filled with gas bubble holes. It floats on water because of the large number of holes in it. it comes in many colours but the most common colour is grey.
- rhyolite - It is a high-silica, fine-grained rock with mineral grains not visible to the naked eye. It may come in grey, light brown, tan, pale yellow, pink and other earth colours. there may sometimes be a few small crystals but most of the rock is fine-grained.
- andesite - It is a fine-grained igneous rock which is commonly grey or medium brown.
- basalt - It is a fine-grained igneous rock rich in iron which gives it its blackish brown colour.
- granite - It is a coarse-grained igneous rock often with a pink to reddish colour. It is made up of small crystals.