
Volcanic eruptions can be
caused by the movement of
tectonic plates

Plates are moved by the
internal heat of the Earth

A volcanic eruption can be
more powerful than an atomic
bomb

Magma – liquid rock that
forms under Earth’s
surface

Comes from melted
crust and mantle

Magma naturally
rises to the surface
because it is less
dense

The amount of
magma increases
because it melts
rock along the
way

Volcanism – any activity that includes the
movement of magma onto Earth’s surface

Lava – magma that has reached the surface

Volcano – the vent in Earth’s surface where
magma reaches the surface

Like earthquakes, volcanoes tend to occur
near convergent or divergent plate
boundaries

Where one plate
goes beneath
another

Oceanic crust +
oceanic crust =
island arc chain

Oceanic crust +
continental crust =
mountain range

The largest amount of
magma comes to the
surface at mid-ocean
ridges

Forms under water
volcanoes

Water quickly cools the
lava creating pillow lava

Volcanically active areas in
the interior of a tectonic
plate

The plate moves over the
hot spot, causing a volcano
to form

That volcano eventually
moves away and a new one
forms

Magma that cools and
solidifies under the
ground (intrusion)

Forms plutons
 Dikes (thin areas)
 Batholiths (large areas
>100 km2)
Devil’s Tower
Wyoming

2 types of magma
 Mafic – magma or rock
rich in magnesium and
iron; dark colored
 Felsic - magma or rock
rich in silicates; light
colored

Quiet Eruptions
 Usually oceanic volcanoes
(ex. Hawaii)
 Lava has a low viscosity
(runny)
 Gases easily escape from the
lava

Lava Flows
 Crust may form on top of
the lava
 If the lava keeps flowing,
it forms wrinkles called
pahoehoe
Aa lava – surface
breaks into
jagged chunks
Blocky lava –
lava is more
viscous
(thicker)

Explosive Eruptions
 Usually continental volcanoes (ex.
Mt. St. Helens)
 Lava has a high viscosity (thick &
sticky)
 Gases trapped inside lava
 Pyroclastic material – fragments
of rock that form during a volcanic
eruption
▪ Thrown into the air during an
explosive eruption

Volcanic ash
 <2mm in diameter

Lapilli
 <64mm in diameter
 “little stones”

Volcanic bombs
 Form when red hot lava is
thrown and cools in the air

Volcanic blocks
 Largest material
 Pieces of rock blasted from the
vent

Can form when the
magma chamber
empties and the cone
collapses

Lakes can form in the
caldera basin
Crater Lake (Mount Mazama)
Oregon

Increase in strength and
frequency of earthquakes may
be a sign of an eruption

Before an eruption, the surface
of a volcano may bulge
outward from the magma

Scientists compare a volcano’s
previous behavior to current
measurements

Shield volcano
 Broad base; gently sloping sides
 Covers wide area
 Quiet eruption
 Layers of mafic lava build up
cone
 Ex: Hawaiian Islands

Cinder cones
 Very steep slopes
 Rarely less than a few
hundred meters high
 Explosive eruptions
 Cone made of pyroclastic
material
 Ex: Parícutin, Mexico

Composite volcano
 Cone made of alternating
layers of…
▪ lava flows (quiet eruptions)
▪ pyroclastic material (explosive
eruptions)
 Form large volcanic
mountains
 Ex: Mt. Hood, Mt. St.
Helens, Mt. Rainier