6/6 Igneous Rocks

What mainly controls igneous rock texture?

Rate of cooling (slow = big crystals, fast = small crystals).

Big crystals usually mean what cooling rate?

Slow cooling (lots of time).

Small crystals usually mean what cooling rate?

Rapid cooling.

List 3 factors that affect cooling time of magma.

Depth of magma; size/shape of the magma body; presence of groundwater.

What are volcanic textures?

Textures that form in rocks that crystallize at Earth’s surface.

Glassy texture forms when…

Molten rock cools so rapidly that ions can’t form crystal grains.

Glassy texture typically forms where?

On the crust of lava flows and in viscous magma.

Two examples of glassy-textured rocks.

Obsidian and pumice.

What is aphanitic texture?

Mineral grains are too small to see without a microscope (rapid cooling).

Give 3 examples of aphanitic rocks.

Basalt, rhyolite, andesite.

What is vesicular texture?

Gas bubbles get trapped, leaving many cavities.

Vesicle sizes can range from…

Almost microscopic (pumice) to a few millimeters (scoria).

Two examples of vesicular rocks.

Pumice and scoria.

What is pyroclastic (fragmental) texture?

Formed when crystals/rock fragments/glass are blown out as ash and accumulate in layers; hot ash flows can fuse fragments.

A common rock example of pyroclastic texture.

Tuff (tuffs).

What are plutonic textures?

Textures that form in rocks that crystallize beneath Earth’s surface (slow cooling, large crystals).

What is phaneritic texture?

Grains large enough to see with the unaided eye; interlocking crystals; roughly similar size.

Example rock with phaneritic texture.

Granite.

What is pegmatitic texture?

Extremely slow cooling produces huge crystals; many gemstones/minerals form this way.

Example rock with pegmatitic texture.

Granite pegmatite.

What is porphyritic texture?

Two cooling rates: large crystals form during slow cooling, then smaller crystals form during faster cooling (can be porphyritic aphanitic or porphyritic phaneritic).

Porphyritic texture indicates what cooling history?

Two-stage cooling (slow then fast).