How Igneous Rock Is Formed

Igneous rocks form when melted rock (lava or magma) cools above ground or underground. They look different depending on how fast they cool and what minerals they contain.

How do you know if a rock is igneous?

Some igneous rocks form when lava flows at the earth's surface and cools quickly. Rapid cooling causes the minerals in these rocks to form tiny crystals or natural glass. These rocks may look shiny or glassy. Obsidean is an example of this kind of igneous rock. Some Native Americans used obsidean to make arrowheads.

Obsidian

Some igneous rocks form when magma cools slowly deep inside the earth. These rocks may look sparkly because the mineral in them had plenty of time to form large crystals. Granite is an example of this kind of igneous rock. Granite is used in buildings, countertops and monuments.

granite

Some igneous rocks have lots of air holes that make them lightweight rocks. Air bubbles trapped in the cooling lava made these holes. Scoria and pumice are examples of this kind of igneous rock. Because they don't weigh much, they are often used in as decorations in yards.

scoria Pumice