How & Where Do Volcanoes Form


    1.    Analyze how magma forms as a result of plate movement.

    2.    Describe conditions that cause volcanoes.

    3.    Describe the relationship between volcanoes and Earth's moving plates.

Vocabulary words:

    volcano    Pacific Ring of Fire    hot spot



Notes: (9-1)

Volcanoes and Earth’s Moving Plates

What causes volcanoes:

One of the most impressive visual representations of Earth's internal energy is the eruption of a volcano.  The eruption is the result of the less dense magma from the earth’s core rises toward the earth’s surface.  The hot liquid magma escapes through the lithosphere in weakened areas of the lithosphere called vents  

    Eruption column at Redoubt Volcano, Alaska

( coast/HMM_1967.gif)    ( Redoubt/dds39_063_med.jpg)

Magma Formation

Volcanoes are the result of magma rising to the surface.  Even though the asthenosphere is very hot, it is able to remain mostly solid due to the enormous pressure exerted on it by the lithosphere and crust.  The magma that causes volcanoes originates in the semisolid asthenosphere as a result of the following conditions:

Volcanoes are mostly found at plate boundaries

Divergent boundaries – mid-Atlantic ridge, rifts

( krafl.jpg)

Convergent  - Pacific and N.American plates

Mt St Helens eruption    ring-o-fire

( valerie/erupt.jpg)                                       



( images/ring.jpg)


Hot Spots  

Not all volcanoes are the result of plate boundaries.  Some are the result of areas of thin asthenosphere.  This thin area allows for the formation of hot plumes of magma.  Island chains are the formed as the lithosphere moves over the asthenosphere.


( volcano%20picture5.jpg)