What is an igneous rock?

Granite and Microgranite

Granite and Microgranite from Scotland

Igneous rocks are hot molten rocks which rise towards the surface. The term igneous originated from the Latin term ignis which means ‘fire’. In order for them to begin to melt temperatures can be anything from 700°C.

They begin to melt down within the deep crust or upper mantle and because the molten rock has a lower density (is lighter) than the surrounding solid rock it begins to rise through cracks within the solid rock.

Once they are near the surface they either erupt through a fissure or volcano (such as in Hawaii or Iceland) or stop just below the surface within the crust where they crystallise. They are either intrusive (solidify quickly within the crust) or extrusive (solidify slowly on the surface). The most common extrusive igneous rock is basalt which is erupted through fissures or volcanoes and the most common intrusive rock is Granite.

Igneous rocks are likely to contain one of the common minerals associated with igneous  rocks; these include quartz, mica, feldspar, olivine, pyroxene and amphibole.


Obsidian (Volcanic Glass)

Today igneous rocks are still forming at locations such as Iceland and Hawaii or around the Pacific Ocean ‘Ring of Fire.’ Iceland is on a constructive plate boundary where two plates are pulling apart along the Mid Atlantic Ridge, this causes magma to rise. Iceland has formed by the build up of lava to the point it has risen above the sea. Hawaii is not on a plate boundary but is within the middle of a plate boundary which is known as an inter-plate hot spot. Both Iceland and Hawaii are believed to be above a mantle plume, which is a large molten body of magma which is slowly rising and causing basaltic volcanism at the surface.

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One Response to “What is an igneous rock?”

  1. Say, you got a nice article.Much thanks again. Awesome.

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