1. Igneous Rocks 2006 Prentice Hall Science Explorer- Earth Science
Liz LaRosa
Images from Geology.com unless otherwise noted

2. Igneous Rocks
The first rocks to form on Earth probably looked like the igneous rocks that can be seen today.
Igneous rock begins as magma.
Magma can form:
When rock is heated
When pressure is released
When rock changes composition
Magma freezes between
700 °C and 1,250 °C
Magma is a mixture of
many minerals

3. Classifying Igneous Rocks
Igneous rock is any rock that forms from magma or lava.
The name igneous comes from the Latin word ignis, meaning “fire”.
Igneous rocks are classified according to their origin, texture, and mineral composition.

4. Origin of Igneous Rock
Intrusive Igneous Rocks: forms when magma pushes into surrounding rock beneath Earth’s surface and hardens. The most abundant intrusive rock on Earth’s continents is granite. Granite forms the core of many mountains ranges.
Extrusive Rocks: forms when magma erupts onto the Earth’s surface (lava), cools quickly with very small or no crystals formed. Basalt is one of the most common rocks on Earth. A layer of basalt forms much of Earth’s ocean floors.

5. Classification of Igneous Rocksby
Color and Texture
Felsic: light colored rocks that are rich in elements such as aluminum, potassium, silicon, and sodium
Mafic: dark colored rocks that are rich in calcium, iron, and magnesium, poor in silicon
Coarse-grained: takes longer to cool, giving mineral crystals more time to grow
Fine-grained: cools quickly with little to no crystals

6. Igneous Rocks Coarse-Grained Fine-Grained Felsic Granite Rhyolite
Mafic
Gabbro
Basalt

7. Mafic, fine grained, extrusive
Obsidian
What is Obsidian?
Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form.
Igneous Rocks
Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form.
Is this rock Felsic or Mafic?
Is it fine-grained or coarse-grained?
Is this rock Intrusive or Extrusive?
Mafic, fine grained, extrusive

8. Mineral Composition of Igneous Rocks
Most of Earth’s minerals contain silica, a material formed from oxygen and silicon.
The silica content of magma and lava affects the types of rock the form.
Lava that is low in silica usually forms dark-colored rocks such as basalt.
Magma that is high in silica usually forms light-colored rocks such as granite.

9. Uses of Igneous Rocks
People throughout history have used igneous rock for tools and building materials.
Granite has a long history of use in building statues, bridges, public buildings and paving streets with cobblestones.
Granite today is used in curbstones, floors, kitchen counters and is crushed to make gravel that is used in construction.
Pumice and obsidian are good abrasives used for cleaning and polishing. Obsidian is used to make sharp tools for cutting and scraping.
Perlite, formed from the heating of obsidian, is often mixed with soil for starting vegetable seeds.