Earth Parts #28 - Igneous Rock Origins & Textures
Channel: Earth.Parts
Date: 2018-01-23

In which I discuss how the occurrence and texture of igneous rock provides information useful in interpreting its history. 

Our planet's crust has always been the product of igneous violence erupting molten rock onto the surface to form new crust. Deeper magma crystallizes in the crust to form granite and similar rocks. 

Igneous rock can form as extrusive rock at Earth's surface or as intrusive rock that cools from molten rock to solid igneous rock deep in the crust, allowing it to freeze slowly and grow large crystal grains in the molten mush before it solidifies. 

Rock that cools from a molten state at depth inside Earth or at the surface leaves telltale signs of its origin, such as crystal grain size and porosity resulting from volcanic gasses boiling out of cooling rock. Igneous rocks with large crystal grains grew slowly as their parent magma rose through the crust. Igneous rock like obsidian with negligible crystal growth froze quickly upon eruption onto Earth's crust. For igneous rocks grain size and mineral composition tell a lot about how that rock formed.