EXPLORATION: Types of Rock

1. Major Rock Types

Rocks are conglomerations of minerals, and they form the bulk of the Earth. The main classification of crustal rocks is based on their origin.

Igneous rocks form from molten rock called magma, which is emplaced into and onto the crust during volcanic activity. Uplift and exposure of rocks at the Earth's surface can be affected by weathering causing the rocks to break down into smaller grains which are transported and deposited as sediments. The sediments are lithified (compacted and cemented), and sedimentary rocks are formed. Changes in temperature, pressure, and/or rock or fluid chemistry can allow igneous and sedimentary rocks to change physically or chemically to form metamorphic rocks. At higher temperatures, metamorphic (or any other rock type) rocks may be partially melted, and crystallization of this melt will create igneous rocks. Uplift and erosion can expose all rock types at the surface, re-initiating the cycle. The approximate volume proportions of these three rock types throughout the Earth’s crust are: igneous 65%: metamorphic 27%; and sedimentary 8%.

Look at the graphic of the rock cycle below to get a sense of how rocks can change form.

Text and graphics on this page from University College London - Earth Sciences

https://www.ucl.ac.uk/earth-sciences/impact/geology/london/glossary/rocks