Download Why Volcanoes Form

Intro: define volcano & define tectonic plate
boundaries.
*many volcanoes form on plate boundaries
*a few processes cause rock to melt, forming
magma
*because magma is not as dense as the rock around
it, it moves up & erupts to form a volcano
What is a volcano? A crack in the Earth’s surface
where melted rock (lava/magma) and gases pass.
Why Volcanoes Form
Where Volcanoes Form
Divergent Boundary
 Boundary between two tectonic plates that are pulling away
from each-other.
 Molten rock also forms submarine volcanoes
 The denser plates forms to make a trench
 Fissures form
 Mid-ocean ridges are formed
 Molten rock flows through the fissures onto the ocean floor.
 Divergent boundaries under water chains known as mid-ocean
ridges.
Convergent Boundary
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A boundary when plates collide and it gets denser and bends
One plate moves under the other (this is called subduction)
As a plate moves down it gets greater heat & pressure
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Plates release fluid which causes surrounding rock to melt
When subduction occurs, the denser plate moves underneath
the lighter plate---creating a trench
As the plates subduct, the rock receives greater pressure /
this pressure causes the plate to give off fluid.
Hot Spots
 Places that experience a great deal of volcanic activity, but do
not lie on plate boundaries
 Sit over columns of hot rock that come up that come up through
the Earth’s mantle
 The columns do not move and are called mantle plumes
 The plate slips over this column of hot rock, and as the magma
comes up, it creates a chain of volcanic islands. The Hawaiian
Islands are an example of this
How Magma Forms
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Magma is made deep in the Earth’s crust and in the upper
sections of the mantle
Temperature is high and there is a great deal of pressure
When temperature /pressure change, magma forms.
Specifically temp. up and pressure down---makes rock melt
Water can also make the melting temp. of rock lover, causing it
to melt
Temperature causes some minerals to melt and form magma
When pressure on rock goes down, it creates space around rock.
The rock then expands & melts, creating magma adding fluids
In the lithosphere of oceanic plates, there is sediment and
volcanic rock that holds water and other fluids
When an oceanic plate subducts, the fluids come into contact
with the surrounding rock
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When fluids enter, the melting temp. decreases and rock begins
to melt.