2006 Prentice Hall Science Explorer- Earth ScienceLiz LaRosa http://www.middleschoolscience.com 2009Images from Geology.com unless otherwise noted

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

http://www.fi.edu/fellows/payton/rocks/create/igneous.htm

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.

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.

http://www.windows.ucar.edu/tour/link=/earth/geology/ig_intrusive.html&edu=high&fr=t

Origin of Igneous Rock

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

Classification of Igneous Rocks by

Color and Texture

Felsic

Mafic

Coarse-Grained

Fine-Grained

Granite

Gabbro Basalt

Rhyolite

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.

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

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.

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.