Igneous rocks: formation and classification

Here’s a quick link to a good explanation of igneous (“formed by fire”) rocks: Igneous Rocks: Formation and Classification, by by amelianavarro.

Briefly, igneous rocks are formed from melted rock. Higher water content makes it easier for rocks to melt.

Conditions of formation:

  • Intrusive rocks harden inside the earth and tend to cool slowly, forming larger crystals.
  • Extrusive rocks harden on the surface or in air. Usually, it is volcanoes that extrude them as lava, ash, or flying rocks. Extrusive rocks may expand explosively and cool rapidly. They may even be cooled by sea-water.

Classification by texture:

  • Phaneritic rocks have large crystals that are easy to see without magnification, e.g. granite.
  • Aphaneritic rocks have small, sand-like crystals. They may feel smooth. An example is basalt. Their crystals are so small that the human eye cannot see them [easily?] without a lens.
  • Porphryitic rocks have both small and large crystals. One may impulsively assume that large crystals make them phaneritic, so  looks can be deceiving. An example is andesite.
  • Glassy rocks always look shiny. They may feel smooth or greasy. An example is obsidian. They form when molten rock cools very quickly.
  • Vescular rocks have many holes, with holes formed by expanding gasses. They are usually extrusive. A well-known example is pumice.

Classification by silica content :

  • Felsic rocks have more silica. They are usually light in color.
  • Mafic rocks have more iron and magnesium. They are usually dark in color.

I wonder how much silica (SiO2) it takes to be truly “felsic”.

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