Volcanoes

Volcanoes

What is a volcano?

A volcano is a vent in Earth’s crust through which melted—or molten—rock flows. Molten rock below Earth’s surface is called magma. Have you heard of some famous volcanoes such as Mount St. Helens, Kilauea, or Mount Pinatubo? All of these volcanoes have erupted within the last 30 years. Volcanoes exist in many places around the world. Some places have more volcanoes than others

Multiple Choice

Molten rock beneath earths surface is called lava

• true

• false

How do volcanoes form?

Volcanic eruptions constantly shape Earth’s surface. They can form large mountains, create new crust, and destroy anything in their path. Scientists have learned that the movement of Earth’s tectonic plates causes volcanoes to form and to erupt.

How do they form continued....

Volcanoes can form along convergent plate boundaries. When two plates collide, the denser plate sinks, or subducts, into the hot mantle. The thermal energy below the surface and fluids driven off the subducting plate melt the mantle and form magma. Magma is less dense than the mantle and rises through cracks in the crust. This forms a volcano. Molten rock that erupts onto Earth’s surface is called lava.

Multiple Choice

Magma is less dense than the mantle

• true

• false

How do they form continued....

Lava also erupts along divergent plate boundaries. As the plates spread apart, magma rises through the vent or opening between them. More than 60 percent of all eruptions occur at divergent plate boundaries along midocean ridges. There, the lava forms new oceanic crust.

Hot spots

Not all volcanoes form on or near plate boundaries. Volcanoes that are not associated with plate boundaries are called hot spots. Geologists hypothesize that hot spots form above a rising current of hot mantle materials, called a plume.

Plumes do not move. As shown in the figure below, a volcano forms as a tectonic plate moves over the plume. As the moving plate carries a volcano away from the hot spot, the volcano becomes dormant, or inactive. As the plate continues to move, a chain of volcanoes forms. The oldest volcano will be the farthest away from the hot spot. The youngest volcano will be directly above the hot spot. 

Multiple Choice

Geologists hypothesize that hot spots form above a rising current of hot mantle materials, called a

• hot spot

• boundary

• plume

• ash

Where do they form?

If you were to map out the locations of most volcanoes in the world you will notice they most often occur on or along plate boundaries

There is also an area around the Pacific Ocean where there is alot of earth quake and volcanic activity, this is called the Ring of Fire.

Most volcanoes that are located in the United States are near the Pacific Coast.

Multiple Choice

Where do most volcanoes in the world occur?

• in the ocean

• in the middle of a plate

• at a plate boundary

• at a hot spot

Fun Facts about Unity States volcanoes

The United States has 60 active volcanoes. Most are part of the Ring of Fire. Alaska, Hawaii, Washington, Oregon, and northern California all have active volcanoes.

Mount Redoubt in Alaska is an active volcano. Mount St. Helens in Washington is also an active volcano. It exploded with a violent eruption in 1980.

The United States Geological Survey (USGS) operates volcano observatories. Because many people live near volcanoes, scientists monitor earthquake activity, changes in the shape of volcanoes, and gas emissions. Scientists also study the history of past eruptions to determine the possibility of future eruptions

Multiple Choice

What do scientist study to determine the possibility of a future eruption?

• How old it is

• history of its past eruptions

• How tall it is

The three types of volcanoes

Scientists classify volcanoes based on the shape and size of the volcano. The magma composition and eruption style of a volcano contribute to its shape.

Read on to learn about the three types!!!!!

Multiple Choice

How many main types of volcanoes are there?

• 1

• 2

• 3

• 4

Cinder Cones

The cinder cones are small, steep-sided volcanoes that erupt gas-rich, basaltic lavas. Cinder cones are made from mildly explosive eruptions.

They are built from particles and blobs of lava ejected from a single vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders or tephra around the vent.

Multiple Choice

The fragments of rock that are ejected, blown into the air, and fall on the side of cinder cone volcano are called?

• magma

• tephra

Composite volcanoes(also called Stratovolcanoes)

The composite volcanoes are large, steepsided volcanoes. They result from explosive eruptions of andesitic and rhyolitic lava and ash along convergent plate boundaries.

They are built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8,000 feet above their bases. Some of the most conspicuous and beautiful mountains in the world are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in California, Mount Hood in Oregon, and Mount St. Helens and Mount Rainier in Washington.

Multiple Choice

Composite volcano layers are

• all the same

• alternating

Shield Volcanoes

The shield volcanoes are common along divergent plate boundaries and oceanic hot spots. They are large, with gentle slopes of basaltic lavas.

They are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of lava after lava layer.

Volcanic Eruptions

Sometimes magma surfaces and erupts as a lava flow. Lava flows are often slow, and rarely are deadly. They can continue on for months sometimes!

Other times, magma erupts explosively. Explosive eruptions send volcanic ash high into the atmosphere up to 40 k into the air! . An example of an explosive eruption is the violent Mount St. Helens eruption in 1980.

Mudflows form when heat from an erupting volcano melts snow. The meltwater can mix with mud and ash from the volcano, and it flows downhill. This can be very damaging and devastating to people, towns, livestock, and crops.

Multiple Choice

Lava flows are usually

• fast

• slow

Continued....

Explosive volcanoes can produce pyroclastic flows—which are avalanches of hot gas, ash, and rock. These flows can travel at speeds of more than 100 km/h and reach temperatures above 1,000°C. A pyroclastic flow from Mount St. Helens killed 58 people

Volcanic eruptions can be predicted when Geologists study changes in the ground and patterns of earthquakes that could signal a brewing eruption. Moving magma can deform ground features, change a volcano’s shape, or set off a series of earthquakes called an earthquake swarm. A volcano might emit more gas before it erupts.

Multiple Choice

a series of earthquakes are called an earthquake

• flume

• swarm

• explosion

• event