VOLCANISM

VOLCANISM Volcanoes are fueled by magma.

Once magma forms, it rises towards Earth’s surface because it is less dense than the surrounding mantle and crust. Volcanism describes all the processes associated with the discharge of magma, hot fluids, and gases.

Volcanoes will erupt in about 60 different places on Earth in any given year. However, these places on Earth’s surface are not random. Most volcanoes form at plate boundaries – both divergent and convergent. Only about 5% of volcanoes erupt far from a plate boundary.

CONVERGENT VOLCANISM Most volcanoes that form on

land result from oceanic-continental subduction. These volcanoes are characterized by explosive eruptions. (Ex: Mount St. Helens)

The volcanoes associated with convergent plate boundaries forms a major belt known as the Circum-Pacific Belt (Pacific Ring of Fire). Ex: Mount St Helens, Mount Fuji,

Mount Pinatubo Mediterranean Belt – Mount

Etna, Mount Vesuvius

DIVERGENT VOLCANISM Mostly occurs along

ocean ridges. Here, the lava takes the form of giant pillows and it is called pillow lava. Unlike convergent volcanism, volcanism along divergent boundaries tends to be non-explosive with effusions of large amounts of lava.

HOT SPOTS Volcanism that occurs far

from any plate boundaries. These hot spots are unusually hot regions of Earth’s mantle where high temperature plumes of magma rise to the surface. Ex: Hawaii (explain how the

island chain has formed) Meiji is near Kamchatka 80 million years old.

Daikakuji bend – change in the direction of the Pacific plate about 43 million years ago.

HOT SPOTS

FLOOD BASALTS Sometimes, hot spots occur

beneath continental crust. Flood basalts will form when lava flows out of long cracks in the Earth’s crust. Over hundreds or even thousands of years, lava can flow out of fissures and form large plateaus.

Ex: Columbia River Basalts – Northwestern United States has over 170,000km3 of basalt.

Ex: Deccan Traps – Occurred 65 mya in India has over 512,000km3 of basalt. Many geologists think that the eruption of the Deccan traps might have caused a global change in climate. Is the timing of this event significant?

FLOOD BASALTS

ANATOMY OF A VOLCANO Magma Chamber – Origin

of magma for a volcano Conduit – Magma reaches

the surface where it becomes lava via a conduit

Vent – Emerges through an opening

Crater – Around the vent at the top of the volcano

Caldera – a large depression that often forms after a large eruption. Occurs when the summit collapses into the empty magma chamber

CRATER LAKE

TYPES OF VOLCANOES Smallest of the 3

types of volcanoes Steep-sloped and

cone-shaped Usually composed of

basaltic lava Explosive eruptions Usually form at the

edges of larger volcanoes

Ex: Lassen Volcanic Park cinder cone

TYPES OF VOLCANOES Shield Volcano

Largest of the three types of volcanoes

Long, gentle slopes Composed of layers of

solidified basaltic lava Quiet eruptions Ex: Mauna Loa,

Hawaii

TYPES OF VOLCANOES Composite Volcanoes

Considerably larger than cinder cones

Tall, majestic mountains Composed of layers of

rock from explosive eruptions and lava flows

Cycle through periods of quiet and explosive eruptions

Ex: Mount Augustine, Mount St Helens

ERUPTIONS Often, the activity of a volcano depends on the

composition of the magma within it.

Temperature – Most rocks begin to melt at temperatures between 800 -1200 degrees Celsius. Temperature increases with depth.

Pressure – Pressure also increases with depth. As pressure increases, the temperature required to melt a rock also increases. This effect of pressure explains why most of the rocks in the lower crust and upper mantle do not melt.

COMPOSITION OF MAGMA The composition of magma depends on the following

factors: Temperature Pressure Amount of dissolved gas

As the amount of gases in magma increases, the magma’s explosivity also increases

Important gases in magma include: water vapour, carbon dioxide, sulfur dioxide, and hydrogen sulfide

Amount of silica The higher the silica content, the thicker and more viscous the magma Due to the higher viscosity, magma with a lot of silica tends to trap more

gases and, as a result, the eruptions are more explosive. Magma with low silica content tends to flow easily and produce quiet,

non-explosive eruptions.

TYPES OF MAGMA Basaltic Magma

Often occurs when rocks from the upper mantle melt

This magma is less than 50% silica Low-viscosity magma The resulting volcanoes are characterized

by quiet eruptions

TYPES OF MAGMA Andesitic Magma

Magma is 50-60% silica Andesitic magma is generally found along

oceanic-continental subduction zones Source material is either oceanic crust or

oceanic sediments Resulting volcanoes have intermediate

viscosity and, therefore, intermediate explosivity

TYPES OF MAGMA Rhyolitic Magma

Occurs when molten material rises and mixes with the overlying continental crust that is rich in silica and water.

Magma is >60% silica Due to its high viscosity, large volumes of

gas are trapped in this type of magma. This results in very explosive volcanoes

The dormant volcanoes in Yellowstone National Park are good examples

EXPLOSIVE ERUPTIONS This occurs when lava

is too viscous to flow freely from the vent of a volcano. The result is that pressure builds up until the volcano explodes throwing lava, rock and ash into the air. The erupted materials are called tephra.

EXPLOSIVE ERUPTIONS

PYROCLASTIC FLOWS Some tephra can cause a lot of

damage and kill many people. Violent volcanic eruptions can send clouds of ash and other tephra down a slope at speeds in excess of 80km/h. These rapidly moving tephra and clouds of hot suffocating gases are called pyroclastic flows. These flows can have internal temperatures of more than 700 degrees Celsius.

A deadly pyroclastic flow occurred in 1902 on the island of Martinique in the Caribbean Sea. More than 29,000 people suffocated or were burned to death.

PYROCLASTIC FLOWS