Do Now:

Group your samples into two categories of your choice using their physical characteristics. (Avoid size and volume)

Igneous Rocks

AIM: What are the characteristics of igneous rocks?

Igneous rocks are classified by

Origin of formation which determines:– Texture (grain size) ** most important**– Composition (what they are made of)– Color (you know what that means)– Density (a physical property of m/v)

Igneous rocks are formed by

Cooling of molten rock above or below the Earth’s surface

Molten rock is melted liquid rock made of different minerals (magma or lava)

1. Texture: Grain size

Determined by where the rock formed. – Magma (molten rock that

cools below the surface of the Earth) forms Intrusive or plutonic igneous rocks

– Lava (molten rock that cools above the surface of the Earth) forms Extrusive or volcanic

INTERGROWN CRYSTALS

Texture: Grain size continued

Intrusive rock cools slower and forms larger grain sizes (coarse crystals).

Extrusive cools faster and forms smaller grain sizes (fine crystals)

Granite

Obsidian

What is the relationship between depth and grain size in igneous rock formation?

The deeper below the surface you go, the hotter the temperature. This causes slower cooling of magma, so larger crystals can form.

Can intrusive igneous rocks be fine grained?

Yes…those that form closer to the surface cool more rapidly than those deeper underground.

Coarse grained granite

Finer grained gabbro

2. Composition, color , density

Determined by minerals that solidify from the magma or lava during cooling

Felsic – most common – light in color – contain mostly silicates

like quartz and feldspar (ex. granite)

Composition, color, density cont…

Mafic – dark in color from iron– more dense– Ex. Basalt found in the

ocean crust

3. Extrusive (volcanic) rock

Lava from volcanic eruptions Mostly mafic (dark colors) Rapid cooling on surface forms

fine crystals– Glassy texture:

shiny black obsidian

– Porphyry (frothy appearance) texture:pumice has holes from gases. Known as vesicular on ESRT.

obsidian

pumice

Question #1 -

Which is a fine grained igneous rock made primarily of pyroxene and plagioclase feldspar?

1) gabbro 2) basalt 3) granite 4) rhyolite

2) BASALT

Question #2

Sand collected at the beach contains:pyroxene, olivine, amphibole, and plagioclase feldspar. The rock from which this mixture came is described as:

1)Dark colored, mafic

2)Dark colored, felsic

3)Light colored, mafic

4)Light colored, felsic

1) dark, mafic

Question #3

Ryholite and granite are alike in that they are both

1) fine grained2) dark colored3) mafic4) felsic

4) Felsic

Question #4

What is the percentage of mineral quartz in granite?

1) 5% 2)10% 3) 35% 4) 75%

4) 75%

Summary:

1. How do mafic rocks differ from felsic rocks?

2. What factor affects the texture of an igneous rock?

3. How is crystal size related to its place of formation?

4. What characteristic of an igneous rock tells the most about where it was formed?