Volcano / Test Review

What is a Volcano? The landform, often a mountain, built from repeated eruptions.

Parts of a Volcano magma chamber Vent crater pipe

Inside the Volcano Crater A bowl-shaped area that forms around a volcano's main opening. Vent The opening where molten rock and gases leave a volcano. Pipe A long tube through which magma moves from the magma chamber to the Earth’s surface. Magma Chamber The pocket beneath the volcano where magma collects.

Classifying Volcanoes by How They Erupt Geologists classify volcanoes as explosive or quiet. Some volcanoes tend to be explosive when they erupt= Mount St. Helens. Others tend to be quiet, loosely flowing, and nonexplosive= Hawaii's Kilauea.

For example, honey is more viscous than water What is Viscosity? Viscosity is an internal property of a fluid that offers resistance to flow. For example, honey is more viscous than water Low viscosity=runny High viscosity=thick The more fluid, (runny), a magma, the more likely it is to erupt. On the other hand, when more viscous, (thick), (higher viscosity) lavas do erupt, they usually do so explosively.

Explosive Eruption= High Viscosity If magma is viscous (as is Mount St. Helens), the gases cannot escape easily; pressure builds inside the magma until the gases sometimes escape violently. In an explosive eruption, the sudden expansion of gases blasts magma into airborne fragments called tephra, which can range in size from fine particles of ash to giant boulders.

What Volcano is Most Dangerous? Explosive Volcanoes Can belch out deadly gases, ash, cinder, and bombs. Volcanic ash can bury entire towns. If volcanic ash becomes wet, it can cause roofs to collapse. A jet plane that sucks ash into its engine may stall. Eruptions can cause landslides and avalanches of mud, melted snow, and rock. Quiet Volcanoes Lava flows from vents, setting fire to everything in its path. Can bury a large area with a thick layer of lava.

Classifying Volcanoes by Their Shape Repeated volcanic eruptions build volcanic mountains of three basic types, or shapes. The type of volcano depends on the kind of materials deposited by the eruption. Shied Volcano Cinder Cone Volcano Composite Volcano

Shield Volcanoes Form by quiet eruptions of fluid lava. Lava flow upon lava flow slowly builds a broad, gently sloping volcanic shape that resembles a warrior's shield. Example: Kilauea (Hawaii)

Cinder Cone Volcanoes Form primarily from explosive eruptions of lava. The erupting lava breaks apart into fragments called cinders. The fallen cinders accumulate into a cone around the volcano's central vent. Cinder Cones are the smallest volcanoes. Example: Sunset Crater in Arizona

Composite Volcanoes Form from both explosive and quiet eruptions. Layers of ash, cinder, and bombs from explosive eruptions alternating with layers of viscous lava flows create steep-sided, often symmetrical cones that we think of as the classic volcano shape. Example: Mount St. and Mount Fuji

Why Do Volcanoes Occur? PRESSURE!

Why do Volcanoes Occur? 70 to 200 miles below the Earth's surface, in the Earth's mantle, temperatures are hot enough to melt rock and form a thick, flowing substance called magma. Lighter than the solid rock that surrounds it, magma is buoyant much like a cork in water; being buoyant, it rises. As the magma rises, some of it collects in large reservoirs, or magma chambers that fuel volcanoes. As the rising magma nears the Earth's surface, pressure decreases, which causes the gases in the magma to expand. This expansion propels the magma through openings in the Earth's surface: a volcanic eruption occurs.

Where do Most Volcanoes Occur? The greatest number of the Earth's volcanoes are hidden from view, occurring on the ocean floor, where two oceanic plates converge along spreading ridges.

The Ring of Fire More than half of the volcanoes that are exposed on land form a chain along the converging plates that encircle the Pacific Ocean. This chain is called the "Ring of Fire." Mount St. Helens is located in the "Ring of Fire."

How do Volcanoes Form When Plates Diverge? Two plates can pull away from each other, opening a rift in the crust. Rift volcanoes allow magma to ooze up from the mantle along the rift.

Hot Spots It's also possible for volcanoes to form right in the middle of a crustal plate. Hot-spot volcanoes result from magma melting a hole in the plate and oozing upward. All of the Hawaiian Islands, for example, are actually just huge volcanic rocks, produced by lava flowing, cooling, and hardening over millions of years.

Magma and Lava Magma and lava are basically the same substance. Both are melted rock. The different names help scientists know where the melted rock is located. It is called Magma when it is located within the Earth (forms in the asthenosphere (major ingredient of magma is silica) It is called Lava when it is located outside the Earth

very small minerals when cooled large minerals when cooled Magma and Lava outside the earth inside the earth cools quickly lava magma cools slowly (molten) melted rock very small minerals when cooled large minerals when cooled

Volcanic Ash Volcanic ash consists of bits of pulverized rock and glass, (less than 2 millimeters (0.079 in), in diameter created by volcanic eruptions.

Pyroclastic Flow A pyroclastic flow occurs when an explosive eruption hurls out a combination of hot gases, ash, cinders, and bombs.

Active U.S. Volcanoes Found mainly in Hawaii, Alaska, California, Oregon and Washington. The greatest chance of eruptions near areas where many people live is in Hawaii and Alaska. The danger area around a volcano covers about a 20-mile radius.

Mount Saint Helens The most destructive eruption ever, in the United States, was when Mount Saint Helens, a volcano in Washington, exploded on May 18, 1980 on a Sunday morning. Its ash cloud rose twelve miles into the sky. Winds carried the cloud as far as 250 miles away, darkening the daytime sky. It killed 58 people and caused more than $1 billion in property damage.

Mount Saint Helen