Volcanoes: The Eruptions, The Lava, and The Types

Volcanoes form around

vents that release magma onto the Earth’s surface.

VentsLava

Magma chamber

What causes volcanoes?

Where Volcanoes Form• Tectonic Plate Boundaries!!!

~75% world’s active volcanoes in Ring of Fire

What causes volcanoes?

What causes volcanoes?

Hot Spots

Types of VolcanoesShield volcano

Cinder cone volcano

Composite/Strato volcano

Are all Eruptions Equal?

• A quiet eruption • An explosive eruption

Why are They Different• All magma is not equal• One key factor that controls whether or not an eruption is

quiet or explosive is the amount of trapped gases and water vapor in the magma

• If gases can escape easily, the eruption is quiet. If the gas builds up high pressures, it eventually will cause an explosive eruption.

• The more trapped water vapor in the magma, the more explosive the eruptions

• The second factor is the amount of silica in the magma

Quiet Eruptions• Some eruptions, are quiet, with lava slowly oozing from a

vent.

• Magma that is relatively low in silica and other fluids is called Basaltic magma and produces quiet eruptions like those on Kilauea (Hawaii)

• Trapped gases and water vapor can easily escape sometimes forming lava fountains

• Quiet eruptions occur over hot spots (Hawaii) and divergent boundaries (Iceland)

Explosive Eruptions

• Some eruptions are very violent. Gases from within the earth's interior mix with huge quantities of dust and ash and rise into the air as great, dark, poisonous clouds called a plume.

• Andesitic magma has more silica than basaltic so it erupts more violently

• Silica-rich (Granitic) or Rhyolite magma produces explosive eruptions like those at Soufriere Hills volcano.

• Rhyolite magma is thick and can trap gas and water vapor causing and buildup of pressure

Buried in Ash and Mud

Car buried in mud 17 miles away from Mt. St. Helens

Hot ash covered

everything and traveled hundreds of

miles

Escaping the ash on a beautiful afternoon in May

This photo was taken 2 years after the eruption, this once great forest was still trying to recover

Balsaltic Lava3 types

• A'aPronounced "ah-ah", this is a basaltic lava that doesn't flow very quickly. It looks like a slowly moving mass of hot Jello. Once it hardens, the sharp spiny surface of A'a lava is extremely difficult to walk across. These types of lava erupts at temperatures above 1000 to 1100 degrees C.

• PahoehoePronounced "pa-ho-ho", this type of lava is much thinner and less viscous than A'a. It can flow down the slopes of a volcano in vast rivers. The surface of the lava hardens into a thin crust that looks very smooth. Pahoehoe lava can also form lava tubes, where the rock hardens around a fast-moving liquid core. When that core flows out of the tube, a long tunnel remains. Pahoehoe erupts at temperatures of 1100 to 1200 C.

• Pillow LavaPillow lava is typically found erupting from underwater volcano vents. As soon as the lava contacts the water, it's cooled down and forms a hardened shell. As more lava issues from the vent, the shell of lava cracks and more "pillows" come out of these cracks.

Blocky lavaPahoehoe

Aa Pillow lava

RHYOLITIC LAVA

• Relatively high viscosity (flows like wet concrete)

• Rhyolite can look very different, depending on how it erupts. Explosive eruptions of rhyolite create pumice, which is white and full of bubbles. Quieter eruptions of rhyolite often produce obsidian, which is bubble-free and black.

• Some of the United States' largest and most active calderas formed during eruption of rhyolitic magmas (for example, Yellowstone in Wyoming, Long Valley in California and Valles in New Mexico).

ANDESITIC LAVA          • Intermediate viscosity* (does not flow readily)

• Andesite magma commonly erupts violently from stratovolcanoes creating a pyroclastic flow

• The word andesite is derived from the Andes Mountains, located along the western edge of South America, where andesite rock is common.

• Andesite was the main rock type that erupted during the great Krakatau eruption of 1883.

*Viscosity is the resistance to flow (Molasses vs Water)