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244

igneous rocks

OLogy Series
geology
card
244

igneous rocks

OLogy Series
geology

Igneous rocks form when magma, or melted rock from inside the Earth, cools and hardens. They can form above ground when magma erupts from a volcano as lava. They can also form deep inside the Earth's crust and then be exposed at the surface by the overlying rocks wearing away. The crystals found in igneous rocks have angular shapes. They have not been worn like sedimentary rocks or squished like metamorphic rocks.

Black Sand Beaches
A black beach might be a strange sight, but it's not uncommon on the big island of Hawaii. The black sand covering some of its beaches is actually black igneous rock that has been broken into tiny, smooth pieces. The igneous rocks are erupted as lava from Hawaii's volcanoes. Currently, Hawaii has two active volcanoes. These volcanoes are steadily erupting, but they don't have violent explosions. Instead, these volcanoes send lava flowing through vents and cracks in the ground. When the lava cools, it forms a black igneous rock called basalt. Since the volcanoes are so close to the sea, sometimes the lava reaches the cold ocean before it cools. If this happens, the lava suddenly explodes into tiny, sharp black pieces. In time, these pieces become sand grains rounded by waves. These black beaches are often partially covered by lava flows. The black sand also forms when rivers and streams erode or break up lava flows and carry the pieces to the ocean. There are even green beaches¬ómade mostly of the mineral olivine, which is contained in some lava flows.

"Intrusive" igneous rocks form from magma that cools slowly underground. Rocks formed from magma that erupts from volcanoes are called:

explosive rocks

extrusive rocks

extrovert rocks

Are you right?

Correct!

Extrusive rocks form when magma erupts from volcanoes. When magma cools quickly, there is only time for small crystals to form, and some of the magma freezes to volcanic glass. When the magma cools slowly, there is more time to form bigger crystals and you never get glass.

"Igneous" comes from the Latin word ignis, which means:

fire

rock

iguana

Are you right?

Correct!

The Latin word ignis means "fire." This is also the root of the word "ignite," which means "to set fire."

Rosamond Kinzler, Earth scientist

Igneous rocks are important evidence. They tell us that there are places inside the Earth where the temperatures get so high that solid rock actually melts!

Most of the Earth's crust is made of igneous rocks.

Fact
or
Fiction
?

Fact

About 75% of the Earth's crust is made up of igneous rocks. Basalt makes up most of the crust under the sea. Granite makes up most of the continental crust.

An igneous rock could never float on water.

Fact
or
Fiction
?

Fiction

Pumice is an igneous rock that floats. During a volcanic eruption, the lava it forms from cools so quickly, lots of gas bubbles get trapped inside. These holes make the rock very light.

Around the world, people live on volcanoes.

Fact
or
Fiction
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Fact

Many islands, like Hawaii and the Galapagos, were formed by volcanic eruptions. Over 17,000 volcanic islands make up Indonesia, a densely populated country in Southeast Asia.

Definition: rock that forms when magma cools, hardens, and crystallizes
Where it forms: on the Earth's surface or inside the Earth
Two types: intrusive or extrusive
Common examples: pumice, basalt, granite
Where it's found: on every continent and on the ocean floor
Cool fact: Volcanoes and most mountains are made of igneous rocks.

Image credits: courtesy of AMNH; Rosamond Kinzler: AMNH.