Rate of Cooling
Igneous rocks form through crystallization of minerals from magmas (molten rocks). Their textures refer to the size and arrangement of the minerals that make up the rock and reveal about how the rock formed. For example, the size of crystals in igneous rocks primarily reflects the rate of cooling. Rapid cooling creates tiny crystals and freezes the liquid to glass, while slower cooling forms coarse-grained igneous rocks.
Obsidian – volcanic glass
Glassy texture indicates quenched lava
Size: 10 cm
Green Obsidian – Volcanic glass
Glassy texture indicates instant cooling
Size: 10 cm
Obsidian with Feldspars Microlites
Two phases of cooling: one slow, one instant
Size: 10 cm
Pitchstone – Volcanic Glass with dull pitchlike luster
Contains microcrystalline embryonic crystals indicating rapid cooling
Size: 10 cm
Rhyolite - Fine-grained volcanic rock
Fine-grained crystalline texture indicates fast cooling
Size: 11 cm
Porphyritic Rhyolite with Euhedral Feldspars
Two phases of cooling: one slow, one fast
Size: 12 cm
Equigranular Granite – coarse-grained plutonic rock
Coarse-grained crystalline texture indicates slow cooling
Size: 12 cm
Pegmatite – plutonic rock with high water content
Large mineral grain sizes indicates very slow cooling
Size: 20 cm