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IGNEOUS ROCKS
Lesson 3 - Page 3

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A glassy texture is when the rock looks like glass. Rocks like obsidian are formed when lava cools within minutes to hours. This usually occurs as thin layers as the lava flows over the land. The lava contains silica dioxide which has not had time to form crystals. This amorphous fluid hardens without forming any true minerals. Obsidian is actually composed of SiO2, but has no crystalline structure.

Pumice, which has a frothy look is also considered glassy, because if you look at it closely, the dividing walls between the cells are actually glassy.

A pyroclastic texture is composed of fragments, like tephra, which includes bombs and ash. These rocks take on an irregular shapes created during volcanic eruptions. Hardened ash flows with other pyroclastic material are grouped as "tuffs." 


Pumice


Obsidian

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