READ: Volcanoes

Volcanoes

Is Earth the only planet to have volcanoes?

On Earth, active volcanoes are found on all continents except Australia. Volcanoes even erupt under the ice on Antarctica! Volcanoes are also common elsewhere in the solar system. In fact, the biggest volcano is on Mars. This image is of Olympus Mons, which is a shield volcano. Olympus Mons is three times the height of Mt. Everest. But its slopes are very gentle.

Volcanoes

A volcano is a vent from which the material from a magma chamber escapes. This may include lava, rock fragments, ash, and gases. Volcanic eruptions can come from many types of structures. Most people think of volcanoes as large, peaky cones. But volcanoes can be large and broad, or tiny little cones. Volcanic eruptions can come from fractured domes, a vent in the ground, or from a giant hole in the ground.

Where They Are

Volcanoes are a vibrant manifestation of plate tectonics processes. Volcanoes are common along convergent and divergent plate boundaries. They are also found within lithospheric plates at hotspots. Wherever mantle is able to melt, volcanoes are likely to be the result.

The Pacific Ocean basin is a good place to look for different types of volcanoes. The light blue wavy line that goes up the right-center of the diagram is the East Pacific Rise. The rise is made of volcanoes at a divergent plate boundary. Trenches due to subduction are on the west and east sides of the basin. Subduction creates island arcs that are seen in the Western Pacific. Subduction also creates continental arcs seen along Central and South America. Hawaii is the line of volcanoes trending southeast-northwest near the center-top of the image (Figure below). The Hawaiian volcanoes are due to a hotspot.

Creating Magma

Volcanoes erupt because mantle rock melts. This is the first stage in creating a volcano. How can rock melt? One way is if the temperature rises. Another way is if the pressure on the rock decreases; this will lower the rock's melting temperature. The melting temperature of a rock also goes down if water is added. Every time there is a volcanic eruption, one or more of these things happens. For each type of volcano mentioned in these lessons, think about how and why melting occurs.

Stages

Of all the volcanoes in the world, very few are erupting at any given time. Scientists question whether a volcano that is not erupting will ever erupt again and then describe it as active, dormant, or extinct.

• Active: Currently erupting or showing signs of erupting soon.
• Dormant: No current activity, but has erupted recently.
• Extinct: No activity for some time; will probably not erupt again.

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Magma and Lava

Are all volcanic eruptions alike?

The type of eruption a volcano has depends on the type of magma in its chamber. Some eruptions are explosive. Some eruptions are quiet. All eruptions are amazing!

Magma

Magma forms deep beneath the Earth’s surface. Rock melts below the surface under tremendous pressure and high temperatures. Molten rock flows like taffy or hot wax. Most magmas are formed at temperatures between 600^oC and 1300^oC (Figure below).

Magma collects in magma chambers beneath Earth’s surface. Magma chambers are located where the heat and pressure are great enough to melt rock. These locations are at divergent or convergent plate boundaries or at hotpots.

The chemistry of a magma determines the type of igneous rock it forms. The chemistry also determines how the magma moves. The higher the amount of silica in the magma, the higher the viscosity. Viscosity is a liquid's resistance to flow. Viscous magmas tend to stay below the surface or erupt explosively. These are felsic magmas, which are high in silica. When magma is fluid and runny, it is not viscous. This magma often reaches the surface by flowing out in rivers of lava. These are low-silica mafic magmas.

Lava

The way lava flows depends on what it is made of. Thick lava doesn’t flow easily. It may block the vent of a volcano. If the lava traps a lot of gas, the pressure builds up. After the pressure becomes greater and greater, the volcano finally explodes. Ash and pyroclasts shoot up into the air. Pumice, with small holes in solid rock, shows where gas bubbles were when the rock was still molten.

Fluid lava flows down mountainsides. The rock that the flow becomes depends on which type of lava it is and where it cools. The three types of flows are a’a, pahoehoe, and pillow lava.

• A’a lava is the thickest of the non-explosive lavas. A'a forms a thick and brittle crust, which is torn into rough, rubbly pieces. The solidified surface is angular, jagged, and sharp (Figure below). A'a can spread over large areas as the lava continues to flow underneath.

• Pāhoehoe lava is thinner than a'a, and flows more readily. Its surface looks more wrinkly and smooth. Pāhoehoe lava flows in a series of lobes that form strange, twisted shapes and natural rock sculptures (Figure below). Pāhoehoe lava can form lava tubes. The outer layer of the lava flow cools and solidifies. The inner part of the flow remains fluid. The fluid lava flows through and leaves behind a tube.

• Pillow lava is created from lava that enters the water. The volcanic vent may be underwater. The lava may flow over land and enter the water (Figure below). Once in the water, the lava cools very quickly. The lava forms round rocks that resemble pillows. Pillow lava is particularly common along mid-ocean ridges.

Volcanoes Video Clip

Vocabulary and Summary

Vocabulary

1. active volcano: Volcano that is currently erupting or is just about to erupt.

2. dormant volcano: Volcano that is not currently erupting but has erupted in the recent past.

3. extinct volcano: Volcano that has not erupted in recorded history and is unlikely to erupt again.

4. magma chamber: Region below a volcano where magma and gases collect.

5. viscosity: Thickness of a liquid; its resistance to flow.

Summary

• Volcanoes are located along convergent and divergent plate boundaries. They can be found in the middle of plates at hot spots.
• Magma can be created when temperature rises, pressure lowers, or water is added.
• Volcanoes may be active, dormant, or extinct. This depends on whether there is the possibility of magma in their magma chambers.
• Magmas differ in composition, which affects viscosity. Magma composition has a large effect on how a volcano erupts.
• Felsic lavas are more viscous and erupt explosively or do not erupt. Mafic lavas are less viscous and erupt effusively.
• Different lava types create different rock types, such as pahoehoe and a'a. Lavas that cool underwater become pillow lavas.

CK-12 Foundation, Earth Science. http://creativecommons.org/licenses/by-nc-sa/3.0/