MagmaMagma MagmaMagma is a mixture of molten rock, suspended is a mixture of molten rock, suspended

mineral grains, and dissolved gases deep mineral grains, and dissolved gases deep beneath Earth’s surface.beneath Earth’s surface.

Pressure and water Pressure and water are two factors that are two factors that determine whether rock will melt to form determine whether rock will melt to form magma.magma.

A liquid’s resistance to flowing is called its A liquid’s resistance to flowing is called its viscosityviscosity..

Substances such as honey, liquid soap, and Substances such as honey, liquid soap, and motor oil have a higher viscosity than water, motor oil have a higher viscosity than water, vinegar, and gasolinevinegar, and gasoline..

Rhyolitic magma Rhyolitic magma has the greatest viscosity.has the greatest viscosity.

Types of MagmaTypes of Magma Basaltic magma Basaltic magma has low silica content has low silica content

and the least explosive eruption.and the least explosive eruption. Andesitic magma Andesitic magma has intermediate has intermediate

silica content and an intermediate silica content and an intermediate eruption.eruption.

Rhyolitic magma Rhyolitic magma has high silica has high silica content and an explosive eruption; content and an explosive eruption; greatest viscosity. greatest viscosity.

Anatomy of a VolcanoAnatomy of a Volcano An opening in the Earth’s surface An opening in the Earth’s surface

where magma escapes is called a where magma escapes is called a ventvent..

A bowl-shaped depression around the A bowl-shaped depression around the vent of a volcano is called a vent of a volcano is called a cratercrater..

If a volcano collapses, the very large If a volcano collapses, the very large crater that is left is called a crater that is left is called a calderascalderas. .

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Types of VolcanoesTypes of Volcanoes Shield VolcanoShield Volcano::Mountain with broad, gently sloping Mountain with broad, gently sloping

sides and a nearly circular base. sides and a nearly circular base. Shield volcanoes form when layer Shield volcanoes form when layer upon layer of basaltic lava upon layer of basaltic lava accumulates during non-explosive accumulates during non-explosive eruptions.eruptions.

Cinder-cone VolcanoCinder-cone Volcano Forms when material ejected high Forms when material ejected high

into the air falls back to Earth and into the air falls back to Earth and piles up around the vent. piles up around the vent. Cinder-coneCinder-cone volcanoes have volcanoes have steep sides and are generally steep sides and are generally small. Cinder-cone volcanoes are small. Cinder-cone volcanoes are more explosive in nature than more explosive in nature than shield volcanoes.shield volcanoes.

Composite VolcanoesComposite Volcanoes Forms when layers of volcanic fragments Forms when layers of volcanic fragments

alternate with lava. The magma that alternate with lava. The magma that forms forms composite volcanoes composite volcanoes commonly commonly contains large amounts of silica, water, contains large amounts of silica, water, and gases. They are larger than cinder-and gases. They are larger than cinder-cone volcanoes, and, because of their cone volcanoes, and, because of their violently explosive nature, they are violently explosive nature, they are potentially dangerous to humans and potentially dangerous to humans and the environment.the environment.

Hot SpotsHot Spots Hot spots Hot spots are unusually hot areas are unusually hot areas

in Earth’s mantle that are stationary in Earth’s mantle that are stationary for long periods of time, where high for long periods of time, where high temperature plumes of magma temperature plumes of magma material rise toward the surface.material rise toward the surface.

Some volcanoes are located far Some volcanoes are located far from plate boundaries and these from plate boundaries and these form as a result of hot spots.form as a result of hot spots.

Where Do Volcanoes Where Do Volcanoes Occur?Occur? Most volcanoes form at plate Most volcanoes form at plate

boundaries.boundaries. 80% of all volcanoes are found 80% of all volcanoes are found

along convergent boundaries.along convergent boundaries. 15% are found on divergent 15% are found on divergent

boundaries.boundaries. Only 5% occurs far from plate Only 5% occurs far from plate

boundaries.boundaries.