lesson 1 - now

1. Once a rock forms as part of a mountain, it does not change.

2. Some rocks, when exposed on Earth’s surface, undergo weathering and erosion.

Do you agree or disagree?

• How do igneous rocks form?• What are the common types of

igneous rocks?

Igneous Rocks

• extrusive rock• volcanic glass• intrusive rock

Igneous Rocks

• When volcanic material erupts and cools and crystallizes on Earth’s surface, it forms a type of igneous rock called extrusive rock.

• Volcanic glass is rock that forms when lava cools too quickly to form crystals.

• Among the most noticeable features of some extrusive igneous rocks are holes that are left after gas escapes.

Igneous Rock Formation

Igneous Rock Formation (cont.)

intrusivefrom Latin intrudere, means “to push in”

Igneous rocks that form as magma cools underground are called intrusive rocks.

Intrusive and Extrusive Rocks

How do igneous rocks form?

Igneous Rock Formation (cont.)

Geologists determine whether an igneous rock is extrusive or intrusive by studying the rock’s texture.

• If the crystals are small, the rock is extrusive.

• If all the crystals are large enough to see and have an interlocking texture, the rock is intrusive.

Igneous Rock Identification

• Igneous rocks are also classified, in part, by magma composition, such as silica content.

• Magma composition, the location where the lava or magma cools and crystallizes, and the cooling rate determine the type of igneous rock that forms.

Igneous Rock Identification (cont.)

Igneous Rock Identification (cont.)

How are extrusive and intrusive rocks different?

• An extrusive igneous rock cools and crystallizes from volcanic material erupted on Earth’s surface.

• When lava cools fast, volcanic glass forms.

• An intrusive igneous rock forms as magma cools and crystallizes deep inside Earth.

A. sediment B. volcanic glassC. pumiceD. magma

What erupts onto Earth’s surface during a volcanic eruption?

A. extrusive rockB. intrusive rockC. volcanic glassD. magma

Which of these forms when lava cools too quickly to form crystals?

A. intrusive B. extrusiveC. crystallizedD. mineral

What kind of rock forms when magma cools underground?

3. Large crystals form when lava cools quickly on Earth’s surface.

4. Igneous rocks form when cooling magma crystallizes.

Do you agree or disagree?