2.  Volcanoes erupt and eject a flow of molten rock material, called magma.  Magma flows when it is hot and becomes a solid when it cools and hardens.  When hot magma cools and hardens, it forms igneous rock.

3.  Why do volcanoes erupt?  Where does the molten material come from?

4.  What comes our of erupting volcanoes?

5.  Temperature and pressure in some areas beneath the Earth’s surface allow for rocks to melt.  Temperature of magma can reach approximately 650 ˚ C to 1200 ̊ C.  Because the magma is less dense than surrounding solid rock, it is forced upward toward the surface.  When magma reaches the Earth’s surface and flows from volcanoes, it is called lava.

6.  Rocks that form from magma below the Earth’s surface are called intrusive igneous rocks.  It takes a long time for intrusive rocks to cool.  Intrusive rocks are found at the surface only after the layers of rock and soil that have once covered them have been removed by erosion.

7.  Extrusive igneous rocks are formed as lava cools on the surface of Earth.  The lava is exposed to air and water.  The lava will cool faster. This prevents mineral grains from growing large.

8.  These rocks are cooled so quickly that few or no mineral grains will form.  Gases may become trapped in the molten material when it cools.  Some of the gases may escape the rock causing holes to form around it.  Examples:  Pumice, Obsidian, and Scoria

9.  Igneous rocks are classified depending on how they are formed:  Intrusive  Extrusive

10.  Basaltic Rocks – Igneous rocks that are dense, dark colored rocks.  Rich in iron and magnesium  Poor in silica  Granitic Rocks - Igneous rocks that are of a lower density than basaltic rocks and are light-colored.  High in silica  Found in regions of violent volcanic eruptions  Andesitic Rocks - Igneous rocks have a mineral composition between that of basaltic and granite rocks.  Page 96: Table 1

11.  The violent nature of some volcanic eruptions is partly explained by the composition of the magma that feeds them.