CHAPTER TWELEVEVolcanoes

If green dots are earthquakes and red triangles are volcanoes, What are two things that you notice about their locations??

Section 1: Volcanoes and Earth’s Moving Plates

A volcano is an opening in Earth that erupts gases, ash and lava.

Volcanic mountains form when layers of lava, ash and other material build up around these openings.

What Are Volcanoes?

Section 1: Volcanoes and Earth’s Moving Plates

Kilauea (kee low AY ah), located in Hawaii, is the world's most active volcano.

The most recent series of eruptions began in 1983 and still continues.

What Are Volcanoes?

Section 1: Volcanoes and Earth’s Moving Plates

Magma is forced upward because it is less dense than the rock around it.

After many thousands or even millions of years, magma reaches Earth's surfaces and flows out through an opening called a vent.

How Do Volcanoes Form?

Section 1: Volcanoes and Earth’s Moving Plates

As lava flows out, it cools quickly and becomes solid, forming layers of igneous rock around the vent.

The steep walled depression around a volcano's vent is called a crater.

How Do Volcanoes Form?

Section 1: Volcanoes and Earth’s Moving Plates

Divergent Plate Boundaries The mid-Atlantic ridge is an

area where the plates are moving apart.

When plates separate, they form long, deep cracks called rifts.

Lava flows from these rifts and is cooled quickly by sea water.

Where Do Volcanoes Occur?

Section 1: Volcanoes and Earth’s Moving Plates

Sometimes, the volcanoes and rift eruptions rise above sea level, forming new islands such as Iceland.

In 1963, the new island Surtsey was formed during a volcanic eruption.

Where Do Volcanoes Occur?

Section 1: Volcanoes and Earth’s Moving Plates

Convergent Plate Boundaries Forms volcanoes where an oceanic

plate slides below a continental plate.

Magma forms when the plate sliding below another plate gets deep enough and hot enough to melt partially and the magma is then forced upward to the surface.

Volcanoes that form at convergent plate boundaries erupt more violently than other volcanoes do.

Where Do Volcanoes Occur?

Section 1: Volcanoes and Earth’s Moving Plates

Hot spots Form when areas between the

Earth's mantle and core are unusually hot and rock at these areas is forced toward the crust where it melts partially.

Magma breaks through the crust to form several volcanoes.

Not at a plate boundary Formed Hawaiian islands

Where Do Volcanoes Occur?

Section 2: Types of Volcanoes

Trapped gases like water vapor and carbon dioxide build up in volcanoes.

If the gas escapes easily, it is a quiet eruption.

If gas and water vapor build up, eruptions can be explosive

What Controls Eruptions?

Section 2: Types of Volcanoes

Quiet eruptions: magma that is low in silica and is basaltic. Has lava that pours from volcanic vents

and runs down the sides of a volcano

Composition of Magma

Section 2: Types of Volcanoes

Explosive eruptions: magma that is high in silica and is andesitic. Has magma that is thick and when

pressure builds up an explosion occurs. It often carries pieces of lava.

Composition of Magma

Section 2: Types of Volcanoes

Shield Volcano Broad, gently sloping volcano formed by

quiet eruptions of basaltic lava. The basaltic lava flows over Earth's

surface covering large areas with deposits of basaltic igneous rocks when it cools.

Forms of Volcanoes

Section 2: Types of Volcanoes

Shield Volcano Accounts for the greatest volume of

erupted volcanic material. Much of the new seafloor that originates

at the mid-ocean ridge forms as underwater flood basalts.

Forms of Volcanoes

Section 2: Types of Volcanoes

Cinder Cone Volcano Steep-sided, loosely packed

volcano formed when tephra falls to the ground.

Tephra: bits of rock or solidified lava dropped from the air after an explosive eruption.

Vary in sizes from volcanic ash, cinders, large rocks and blocks.

Forms of Volcanoes

Section 2: Types of Volcanoes

Composite Volcano Volcano built by alternating explosive and

quiet eruptions that produce layers of tephra and lava.

Found mostly where Earth's plates come together and one plate sinks below the other.

Forms of Volcanoes

Section 3: Igneous Rock Features

Batholiths: large intrusive rock body that forms when magma being forced upward toward Earth's crust cools slowly and solidifies underground.

Dikes: magma that is forced into a crack that cuts across rock layers and hardens.

Sills: magma that is forced into a crack parallel to rock layers and hardens.

Intrusive Features

Section 3: Igneous Rock Features

Forms of Volcanoes

Section 3: Igneous Rock Features

Volcanic neck: solid igneous core of a volcano left behind after the softer cone has been eroded. When a volcano stops

erupting, the magma hardens inside the vent.

Erosion, usually by water and wind, begins to wear away at the volcano.

Other Features

Section 3: Igneous Rock Features

Caldera: large circular-shaped opening formed when the top of a volcano collapses. Crater Lake in OR is a caldera that filled

with water and is now a lake.

Other Features

Section 3: Igneous Rock Features

Examples of destruction: Lava flows destroy

everything in their path. Falling volcanic ash can

collapse buildings, block roads and can cause lung disease in people and animals.

Volcanic ash and debris rush down the side of a volcano: pyroclastic flow.

Effects of Eruptions

Section 3: Igneous Rock Features

Examples of destruction: Temperature inside a pyroclastic flow can

be high enough to ignite wood. Sulfurous gases from volcanoes can mix

with water vapor in the atmosphere to form acid rain.

Effects of Eruptions

In your notes, create a diagram of a volcano. It must include the following features (with

correct lables):

• Ash cloud• Pyroclastic flow• Tephra• Layers of ash • Layers of lava• Crater• Main vent• Dike• Sill• Magma chamber