Type: JPEG image
Description: This table shows different extrusive (volcanic) igneous rocks. Left, from top: scoria, basalt, gabbro. Right, from top: obsidian, rhyolite, granite. Extrusive igneous rocks form when molten rock (magma) is cooled outside the ground, either in the air or underwater. Each rock's properties depend on how fast it cools and how much silica is present. The faster it cools, the finer the grains within the rock. Here the left arrow shows the speed of cooling (size of grain), where red is slow (coarse grained) and blue is fast (fine grained). The top arrow shows increasing silica content from left (mafic) to right (felsic). Credit: Gary Hinck/Science Photo Library, http://www.sciencephoto.com/media/88178/enlarge
Uploaded by: MaryAnn McGarry
Created: Tuesday, 25 October 2011, 1:00 PM
Last modified: Tuesday, 25 October 2011, 1:07 PM
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