Formation of Igneous Rocks

It is often seen as the "beginning" of the rock cycle.





This is because it's the biggest change in the rock cycle.

While sedimentary rocks form under pressure of sediment layers and metamorphic rocks form under pressure and/or due to changing temperatures, none of them actually melts and turns into hot, liquid magma. Igneous rocks do.

 
 By Greg Bishop via Flickr.com


As part of the rock cycle, rocks are built, eroded and some end up being pushed deep down into the interior of the Earth. The deeper down they go, the hotter are temperatures - the core of the Earth is very hot.

As the conditions (pressure and temperature) change, chemical reactions start happening, because rocks, as other things, consist of chemicals.

All chemicals strive to reach stability so chaniging conditions make them to react to become more stable in the new conditions.

As the chemical reactions happen, the mineralogy of the rock changes. If that happens in the solid state - metamorphic rock is formed.

Igneous rock, on the other hand, is formed after the previous rock has undergone such extreme temperatures and pressures, that it has melted and turned into hot liquid. When that liquid is under the Earth's surface, it is called magma. When it sprinkles out of a volcano, it is called lava.

Igneous rock is formed when that magma, or lava, freezes and turns into a rock. That happens when the rock is lifted closer to the Earth's surface again, and the temperatures and pressures decrease; or, when lava sprinkles out of a volcano and turns from a liquid to a solid substance.

That is also when the minerals in the rock are formed. Different minerals form at different pressures and temperatures. That's why different rocks have different mineralogical composition.

Intrusive Igneous Rock

Extrusive Igneous Rock