Earth Science 10.2A The Nature of Volcanic Eruptions

The Nature of Volcanic Eruptions

Earth Science 10.2 The Nature of Volcanic Eruptions

On May 18th, 1980, Mount St. Helens erupted with tremendous force. The blast blew out the entire north side of the volcano.

The eruption ejected nearly a cubic kilometer of ash and other debris.

Earth Science 10.2 The Nature of Volcanic Eruptions

The primary factors that determines whether a volcano erupts explosively or quietly include characteristics of the magma and the amount of dissolved gases in the magma.

Magma that has reached the surface is called lava. Lava cools and hardens to form solid rock.

Earth Science 10.2 The Nature of Volcanic Eruptions

Magma’s viscosity , the thickness of the fluid, affects the type of eruption that occurs.

Viscosity is a substance’s resistance to flow.

For example; maple syrup is more viscous than water; it flows more slowly when you pour it.

or The thicker the fluid, the slower

and more viscous. The grade (30-5w) of a motor oil measures the viscosity of the oil, it’s thickness.

Earth Science 10.2 The Nature of Volcanic Eruptions

Magma from an explosive eruption may be thousands of times more viscous than magma that erupts quietly.

The temperature and chemical composition determine the magma’s viscosity.

The affect of temperature on viscosity is easy to see. If you heat maple syrup or honey it becomes more fluid and less viscous.

In the same way, the viscosity of lava is strongly affected by temperature. As a lava flow cools, it becomes more viscous; thickening until it finally stops moving and hardens into rock.

Earth Science 10.2 The Nature of Volcanic Eruptions

The chemical composition of magma has a more important effect on the type of eruption.

In general, the more silica in magma, the greater it’s viscosity (the thicker it stays when heated)

Because of their high silica content; rhyolitic lavas are very viscous and erupt explosively.

Earth Science 10.2 The Nature of Volcanic Eruptions

Dissolved gases:

During explosive eruptions, the gasses trapped in magma provide the force to propel molten rock out of the vent, an opening to the surface.

These gases are mostly water vapor and carbon-dioxide.

As magma moves nearer the surface, the pressure in the upper part of the magma is greatly reduced.

The reduced pressure allows dissolved gases to be released suddenly.

Earth Science 10.2 The Nature of Volcanic Eruptions

Dissolved gases:

Very fluid basaltic magma allows the expanding gases to bubble upward and escape relatively easily.

Therefore, eruptions of fluid basaltic magma , such as those that occur in Hawaii, are relatively quiet.

Earth Science 10.2 The Nature of Volcanic Eruptions

Dissolved gases:

But, highly viscous magma slows and traps the upward movement of expanding gases.

The gases are trapped, collect in pockets that increase is size and build pressure.

These pockets than explode when the pressure gets to a critical level and magma is ejected from the volcano in an explosive display.

Earth Science 10.2 The Nature of Volcanic Eruptions

Volcanic Material:

Lava may appear to be the main material produced by a volcano but this is not always the case.

Just as often, explosive eruptions eject huge clouds of broken rock, lava bombs, fine ash, and dust.

Depending on the type of eruption, volcanoes may produce lava flows or eject “pyroclastic” materials or both.

All volcanic eruptions also emit large amounts of gas.

Earth Science 10.2 The Nature of Volcanic Eruptions

Volcanic Material:

Lava Flows: Silica content and temperature

affect the characteristics of lava flows.

Hot basaltic lavas are usually very fluid because of their low silica content.

Flow rates of 10-300 meters per hour are common.

In contrast, the movement of silica-rich rhyolite lava is too slow to be visible.

Earth Science 10.2 The Nature of Volcanic Eruptions

Lava Flows:

Temperature differences produce

two types of basaltic lava:

Pahoehoe

Aa

Pahoehoe is a hotter, faster-moving, basaltic lava.

Aa is cooler, slower-moving basaltic lava.

Pahoehoe resembles braids in a twisted rope where Aa forms a surface of rough, jagged blocks of sharp, spiny projections.

Pahoehoe lava

Aa lava

Earth Science 10.2 The Nature of Volcanic Eruptions

Gases:

Magmas contain varied amounts of dissolved gasses held under pressure in the molten rock, just as carbon dioxide is held in a soft drink.

Just like a soft drink, as soon as the pressure is reduced, the gases begin to escape.

The gaseous portion of most magmas is only about 1 to 6 percent of the total weight.

The percentage of gas may be small, but the actual quantity of emitted gas can exceed thousands of tons each day.

Earth Science 10.2 The Nature of Volcanic Eruptions

Gases:Samples taken during one Hawaiian

eruption consisted of about

70 % water vapor 15% carbon dioxide 5 % nitrogen 5% sulfur And smaller amounts of

chlorine, hydrogen, argon

Earth Science 10.2 The Nature of Volcanic Eruptions

Pyroclastic Materials: Particles produced in volcanic

eruptions are called pyroclastic materials.

When basaltic lava is extruded, dissolved gases propel chunks of lava to great heights.

Some of this ejected material may land near the vent and build a cone-shaped structure.

The wind will carry particles great distances. Viscous rhyolitic magmas are highly charged with gases.

As the gases expand, pulverized rock and lava fragments are blown from the vent.

Earth Science 10.2 The Nature of Volcanic Eruptions

Pyroclastic Materials: The fragments ejected during

eruptions range in size from very fine dust and volcanic ash to pieces that weigh several tons.

Particles that range in size from small beads to walnuts (2-64 millimeters) are called lapilli, or cinders.

Particles larger than lapilli are called blocks when they are made of hardened lava and bombs when they are ejected as glowing lava.

Computer Lab:

Use the internet to research the following topic and write a short 3 paragraphs summarizing the topic. DONOT COPY CUT OR PASTE:

“The plaster casts at right were taken from the archeological dig at Pompeii. How were these made by a volcanic eruption?”