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”.

Chimpanzee caches, and makes, throwing stones

A chimpanzee at the Furuvik Zoo has been observed collecting stones and leaving them in little piles to use later, for throwing at tourists. Ed Yong at Not Exactly Rocket Science has the story: “Chimpanzee collects ammo for ‘premeditated’ tourist-stoning.”

Even more interesting, when zookeepers removed the stones, the chimp, Santino, collected more and then began making them by breaking off chunks of concrete.

We already know that chimps were making stone tools 4,000 years ago.

Rocks at about.com

About.com has a lot of nice articles about rocks and rock formation by Andrew Alden. Already this morning, I’ve learned how to tell siltstone from shale or sandstone or mudstone, what a thunder egg is, that agate and chert are both types of chalcedony, that alabaster is a kind of travertine, and how banded iron formations were formed. They date back to when the earth’s atmosphere mostly free of oxygen, and any oxygen produced by photosynthetic bacteria quickly bonded with iron in the earth’s crust.

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