1. Igneous Rocks

2. Formation
The word igneous comes from a Latin word meaning fiery which is why igneous rocks are those that formed from the solidification of molten rock (Figure 1).
Molten rock beneath the surface is known as magma and is called lava when it erupts on the surface.
Figure 1. The table from the Earth Science Reference Tables (ESRT) shows the process that results in the formation of igneous rocks.

3. Types of Igneous Rocks
Intrusive igneous rocks form from the slow cooling of magma deep beneath the surface of the earth.
Because the surrounding rocks insulate the magma, the rate of cooling is very slow. Because crystals take time to grow, the slow cooling rate results in coarse-grained (large crystals) minerals.
Extrusive igneous rocks form from the rapid cooling of lava as a result of volcanism.
Because the lava cools within in weeks to hours, the crystals are smaller or non-existent creating textures known as fine-grained and glassy.

4. As you can see from Scheme for Igneous Rock Identification chart found on page 6 in your ESRT, the names and textures of the two types of igneous rocks can be found by looking at the chart.

5. Examples of Igneous Rocks
Extrusive Intrusive

6. Intrusive Rock Under a Microscope

7. Extrusive Rock Under a Microscope

8. Texture
The texture highlighted in Figure 2 refers to the crystal size of the minerals that make up the igneous rock.
Figure 2. Section from Scheme for Igneous Rock Identification chart in the ESRT.

9. Texture
The magma that feeds some explosive volcanoes contains a high amount of dissolved gases. However, because this magma is deep below the surface it is under tremendous pressure and the gases remain dissolved.
When this magma erupts the pressure drops allowing the dissolved gases to escape.
If the ejected lava cools rapidly, this “frothy” lava will develop a vesicular texture (Figure 3).
Figure 3. Section from Scheme for Igneous Rock Identification chart in the ESRT above and below is the rock scoria.

10. Magma Composition
Because minerals have different compositions, and magma is made up of melted minerals, not all magma is composed of the same elements.
Earth’s crust, seen in Figure 4, can be broken up into two different compositions, basaltic and granitic.
The continental crust is thicker, less dense, and primarily composed of granite while the ocean crust is more dense, thinner, and primarily composed of basalt.
Figure 4. The diagram of the Earth’s crust show’s both the continental and oceanic crust. The primary minerals that make up the oceanic crust are seen on the right in the red box. Starting from the top the minerals are amphibole, pyroxene, and olivine. The primary minerals that make up the continental crust are shown in blue which are the feldspars (left) and quartz.

11. Rhyolitic Andesitic Basaltic
From the Scheme for Igneous Rock Identification chart in the ESRT, the major difference between the rocks in the different highlighted areas is the mineral composition.

12. Terms Associated with Igneous Rocks
Felsic igneous rocks contain high amounts of quartz (Si) and aluminum (Al) and tend to be lighter in color.
Mafic igneous rocks contain high amounts of iron (Fe) and magnesium (Mg).

13. Examples of Felsic and Mafic Rocks
Felsic Mafic

14. Terms Associated with Igneous Rocks
Porphyritic texture, shown in Figure 5, refers to an igneous rock that contains both coarse and fine-grained minerals.
This textures reflects two different cooling rates.
Originally this rock cooled slowly, but because different minerals solidify at different temperatures, the entire rock wasn’t solid when the larger crystals were forming. At a later time this rock cooled rapidly, resulting in the fine-grained crystals.
Figure 5. This rock contains both small and large crystals as a result of two different cooling rates.