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Felsic: Definition & Composition Video

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  • 0:00 Definition of Felsic Rocks
  • 0:30 Composition of Felsic Rocks
  • 1:44 Extrusive
  • 2:48 Lesson Summary
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Lesson Transcript
Instructor: Suzanne Rebert

Suzanne has taught college economics, geography, and statistics, and has master's degrees in agricultural economics and marine affairs (marine resource management).

In this lesson, you will read about felsic rocks and find out what they're made of - their name gives clues to their composition. You'll also take a quiz to test your understanding.

Definition of Felsic Rocks

Felsic rocks are igneous rocks that are rich in feldspar and silicon. The word 'felsic' was made up from parts of those words. Because they are made of lighter elements, they tend to be more buoyant than mafic rocks, which are rocks high in magnesium and iron, like basalt. Granite is the most common felsic rock. It may seem strange to think of a solid granite mountain as light weight, but compared to a basalt rock of the same size, it is.

Composition of Felsic Rocks

Feldspar, one of the key components of felsic rock, which is the name for a group of aluminum silicate minerals containing the light metallic elements potassium, sodium, or calcium. The word 'feldspar' comes from German words meaning 'field rock.' This may not sound very specific, and in fact there are many different kinds of feldspar. As a group, feldspars form about 60% of Earth's crust.

Quartz is a silicate mineral that comes in a vast array of different forms, from semiprecious gemstones to common sand. After feldspar, it's the second most important crust mineral.

Granite is a felsic rock containing feldspar and quartz along with mica and amphibole, which are also silicate minerals. Because there are so many forms that all these minerals can take, there are also many types of granite. The name 'granite' refers to the grainy quality of the rock. Granite is an intrusive, or plutonic, igneous rock, which means that it's formed when magma cools and hardens in spaces within the Earth's crust. Because the magma is not exposed to the open air or water, it cools slowly and has time to form the kinds of crystals seen in a typical piece of granite.

Extrusive

Rhyolite is a felsic rock that, like granite, is rich in silicate minerals. However, rhyolite is extrusive. It erupts from volcanoes and cools too quickly to form large crystals.

Obsidian is natural volcanic glass. Like more familiar kinds of glass, it's high in silica. The explosive volcanic eruptions that produce rhyolite are also sources of obsidian. Obsidian fractures into very sharp-edged pieces, and cultures around the world have taken advantage of this fact to make beautiful cutting tools and weapons.

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