Chapter 5 Lesson 3 Volcanoes
Vocabulary
1. Volcano
2. Lava
3. Shield volcano
4. Cinder-cone volcano
5. Composite volcano
6. Island chain
7. Hot spot
8. Island arc
NOTES
Where are volcanoes found?

Volcanoes form on land and on the ocean floor.

Found on certain places on Earth’s surface.

Most are found where plates meet.

Ring of Fire-circle of volcanoes surrounding the Pacific Ocean.

More likely to erupt at plate boundaries than anywhere else on Earth.

An eruption is an outpouring of melted rock, ash, gases, or a combination of these.

Scientists concluded that volcanoes tend to erupt where one plate is pushed under another
plate.

When rocks in the plate that is being pushed down reach the heat and pressure in the mantle,
they melt.

Magma forms and pools in a chamber underneath the crust.

Magma may rest quietly for 100s or 1000s of years.

A crack forms above the chamber or the pressure in the chamber grows too great to be held in
by the rock above it.

The magma rushes toward the surface.

More than 80% of Earth’s volcanic activity occurs on the ocean floor.

All volcanoes have at least one vent, or opening.

Lava, ashes, and gases erupt through vents.

A cup-shaped depression called a crater may form around the vent.
o
This is usually found at the top of a volcanic mountain.

Sometimes the magma chamber beneath a volcano is emptied.

The volcano may then collapse inside itself. The hole that forms is called a caldera.
How do volcanoes build land?

Sometimes magma cools and hardens before it reaches the surface.

A dike forms when magma hardens in vertical or nearly vertical cracks.

A sill forms when magma hardens between horizontal layers of rock.

Sometimes the magma pushed into a sill does not spread horizontally, but pushes upward.

o
It forms a dome shape called a laccolith.
o
Laccolith may raise the rock layers above it when formed.
The largest and deepest of all underground magma formations are batholiths.
o
Batholiths are huge and irregularly shaped.
o
Reaches deep into the crust.

When lava comes out of a vent, it is liquid.

Lava forms a solid layer of rock as it hardens.
o

Layers increase the height of a volcano and form a volcanic mountain.
Active Volcano-currently erupting or has erupted.

Dormant Volcano-doesn’t erupt for a long time.

Extinct Volcano-volcano that stops erupting or is dead.

Active volcanoes differ in how they form and in the shapes of the mountains they build.
o
Shield volcano, cinder-cone volcano, and composite volcano. (SEE VOCABULARY)
How do volcanoes build islands?

The Hawaiian Islands are an island chain.

They rest on a slowly moving tectonic plate.

As it moves, the plate passes over a stationary pool of magma call a hot spot.

Lava erupting from the hot spot formed a mountain that eventually becomes a volcanic island.

As the plate moved, that island moved away from the hot spot and a new island began to form.

In areas where an ocean-floor plate is pushed under another ocean-floor plate an island arc is
formed.
o
As the plate is pushed down it melts.
o
Magma forms, rises upward, and erupts through the ocean floor.
o
The eruptions form a series of volcanic islands along the plate boundary.
o
The Aleutian Islands in Alaska form an island arc.