Volcanoes

Classification of Volcanoes Structure Tectonic Region Type of Eruptive Material Level of Activity Location Dangers Posed

Volcanoes Volcano: anywhere magma, ash, and gas

erupts onto the surface

Factors Affecting Eruptions

1. Magma Composition

2. Magma Temperature

3. Amount of Dissolved Gases

Magma Types Rate at which magma moves is determined by its

silica content

Felsic: High silica content, light colored, slow moving Bigger Eruptions

Mafic: Low silica content, dark colored, fast moving Calmer Eruptions

Volcanic Material Lava (magma that reaches the surface)

Gases: H20, CO2, Nitrogen, Sulfur…

Tephra: fragments of lava

1. Ash (less than 2mm)

2. Lapilli (2-64mm)

3. Bombs (larger than 64mm)

Lava Flows

Mafic Pahoehoe: Flow with smooth ropelike surface,

quick flowing basalt AA: Rough jagged surface Pillow Lava: Rounded pillow shaped

Felsic Pyroclastic Flow: Superheated flow of gas and rock

from the eruption

Eruption Types

Depending on the eruption type, different volcanoes are formed

Rift Eruptions Occur at long, narrow fractures in the crust

where lava flows out smoothly (mafic) In the ocean, it occurs at spreading centers

(mid-ocean ridges)

Hot Spots Areas of volcanic activity in the middle of a plate Caused by heat source in the mantle that makes

magma rise Creates a chain of volcanic islands overlying plate

moves over it.

Shield Cone Volcanoes Rift Eruptions & Hot Spots make Shield Cone

Volcanoes A broad base and gently sloping sides Usually low amounts of gas, mafic lava that

flows long distances before it cools

Shield Cone Volcanoes

Subduction Boundary Eruption Magma is thick and gas-rich. Plates are subducted, they melt, which floods

the mantle with silica, making it felsic Explosive eruptions of mainly tephra and

pyroclastic flows Form cinder cone volcanoes and

stratovolcanoes

Cinder Cone Volcano A volcanic, circular cone

with steep sides that is mainly made of tephra

Gas rich, felsic lava

Stratovolcano Tall with gentle slope near base and steep

slope near summit. Alternating layers of felsic lava and tephra

deposits. Usually gas rich, felsic lava Lots of pyroclastic flows

Anatomy of a Stratovolcano

Plutons and Volcanism Volcanoes and lava flows are the surface

activities of volcanism Magma below the crust, and its “behavior”

impacts the earth in a variety of ways Plutons: the rock masses that form when

magma cools inside other rocks

Dikes Sheets of igneous rock

that cut vertically through the rock layers they intrude

Sills Sheets of igneous

rocks that are parallel to the layers they intrude

Laccoliths When magmas bulge

upwards forming domelike masses

Volcanic Necks When an extinct

volcano is almost completely eroded, a volcanic neck may be left

Plugs of hardened magma

Batholiths Largest of the plutons

Form the core of many mountain ranges

Erosion of the mountain will expose them