Lesson 7.1: Volcanoes and Plate Boundaries

Volcano What is a volcano? How do volcanoes form? Where do volcanoes occur?

What? A land or underwater feature that forms when magma reaches the surface of Earth. Volcano is an opening on Earth’s surface for things to come up from the interior (inside) of Earth. Heat pushes up land—magma rises up to the Earth’s surface because it is less dense than the rocks around.

Magma comes up to the surface is called lava. Volcanoes can erupt liquid, gas, or solid materials.

How? Magma must first reach Earth’s surface. Magma is less dense than the rocks above it, so it pushes up. Different plate boundaries results in volcanoes forming.

Where? Volcanoes do not form just anywhere on Earth’s surface. They occur along divergent and convergent plate boundaries, and hot spots. Pg. 295, Ring of Fire Ring of fire: a line of active volcanoes that circles the Pacific Ocean.

Review Plate boundaries: Earth’s crust is not one whole piece. It is broken up into different pieces we call “plates.” These plates—think of egg shells—move in different directions because of heat. Convergent plates—two plates coming towards each other. Divergent plates—two plates move away from each other.

Convergent Plate Boundaries If two plates move towards each other, one has to give and bend and move underneath. If the one that bends goes deep enough, the heat from Earth’s interior will melt the rocks and it will float back up again, pushing cooler rocks down. Volcanoes form on surface and an opening on the earth’s surface, where the magma comes out as lava.

Volcanoes that form along convergent plate boundaries tend to erupt more violently than other volcanoes. Volcanic Arcs: Oceanic plate subducts (goes underneath) beneath a continental plate, a volcanic arc is formed. Island Arcs: When two oceanic plates move toward each other, an island arc is formed.

Oceanic plates are more dense than continental plates – So oceanic plates will sink down underneath the continental plates

Divergent Plate Boundaries Divergent plates mean that the plates are moving apart. Think of huge cracks or rifts on Earth’s surface. It can happen with two ocean plates, like the mid-ocean ridge. It can happen with two continental plate, and we have a continental rift valley.

With these rifts (like holes or gaps, ripped apart, think cookie when you rip it), heat and lava flows come up and it cools off quickly. It builds up on the seafloor. Sometimes volcanoes rise above sea level, forming islands such as Iceland. Pg. 297 Define Vent.

Hot Spots Some volcanoes do not form along plate boundaries—called hot spots. Hawaii (pg. 298)