Igneous Rocks, pt.2

What is magma made of?

Review1. What are extrusive rocks?

Made from magma that cools outside the earth.2. What are intrusive rocks?

Made from magma that cools inside the earth.

Felsic rocks

Magma in intrusive rocks tend to have high silica (SiO2) content.

The high silica magma is thick, and flows slowly.

FELSIC rocks = LIGHT-colored, high-silica rocks; tend to be found in Intrusive rocks.

Mafic rocks

Have lower silica content, higher content of magnesium, iron, and calcium content.

Formed from hotter, thinner, and more fluid magma than felsic magma.

Magnesium, iron, and calcium are common in DARK colored minerals.

Called mafic rocks.MAFIC rocks tend to be EXTRUSIVE, and

dark-colored.(See basalt in next slide)

Again…

Felsic rocks: tend to be lighter-colored, intrusive, from slow flowing magma.

E.g. granite (contains quartz, feldspar)

Mafic rocks: tend to be darker, extrusive, from smoother flowing magma.

E.g. most volcanic rocks.

Felsic rocks (such as granite) are commonly found on continents

mafic rocks (such as basalt) are more commonly found on the ocean floor.

Textures of igneous rocks

Textures of igneous rocks can be smooth, rough, or anywhere in between.

CRYSTAL SIZE is the most important factor affecting texture.

…and the crystal size depends on how fast magma hardens.

The slower the magma cools, the longer time its atoms have to move around and arrange into crystals.

More time to cool = larger crystals.

Granite

Dissolved gases in magma

When magma has a lot of dissolved gas, crystals grow faster. Why?

When gas moves around, ions move around faster in the magma.

What happens when lava cools?

Lava cools much faster than magma.Less time for larger crystals to form: tiny

crystals. Fine-grained textures. E.g. basalt

Extrusive or Intrusive?

A: Extrusive.

This basalt specimen cooled rapidly in a lava flow, causing the crystals to be small and invisible without the help of a microscope. Note the big holes in this specimen, giving it an appearance of black Swiss cheese. This rock cooled so rapidly that gas bubbles had no time to escape the lava and were frozen in to place.

Q: extrusive or intrusive?

A. Intrusive

granite is an example of an igneous rock that formed by slow cooling over tens of thousands of years 30-40 km below the earth's surface. Various geological processes and erosion of the overlying rocks allows us to find granite at the Earth's surface today. Slow cooling led to the large crystals visible in this specimen.

Quiz

You are hiking through several states, and come across samples A and B.

1. Which has larger (coarser) crystals? 2. Which has finer crystals?3. What are crystals?4. Which sample is an extrusive rock?5. Why?

A.

B.