Volcanoes

3.  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

4.  Magma – liquid rock that
forms under Earth’s
surface
 Comes from melted
crust and mantle

5.  Magma naturally
rises to the surface
because it is less
dense
 The amount of
magma increases
because it melts
rock along the
way

6.  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

7.  Like earthquakes, volcanoes tend to occur
near convergent or divergent plate
boundaries

8.  Where one plate
goes beneath
another
 Oceanic crust +
oceanic crust =
island arc chain
 Oceanic crust +
continental crust =
mountain range

9.  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

10.  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

11.  Magma that cools and
solidifies under the
ground (intrusion)
 Forms plutons
 Dikes (thin areas)
 Batholiths (large areas
>100 km2)
Devil’sTower
Wyoming

12.  2 types of magma
 Mafic – magma or rock
rich in magnesium and
iron; dark colored
 Felsic - magma or rock
rich in silicates; light
colored

13.  Quiet Eruptions
 Usually oceanic volcanoes
(ex. Hawaii)
 Lava has a low viscosity
(runny)
 Gases easily escape from the
lava

14.  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)

15.  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

16.  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

17.  Can form when the
magma chamber
empties and the cone
collapses
 Lakes can form in the
caldera basin
Crater Lake (Mount Mazama)
Oregon

18.  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

19.  Shield volcano
 Broad base; gently sloping sides
 Covers wide area
 Quiet eruption
 Layers of mafic lava build up
cone
 Ex: Hawaiian Islands

21.  Cinder cones
 Very steep slopes
 Rarely less than a few
hundred meters high
 Explosive eruptions
 Cone made of pyroclastic
material
 Ex: Parícutin, Mexico

23. Paracutin

24.  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

27. Mt. Fuji -- Fuji-san, on the island of Honshu, is Japan's highest
volcano and one of the world's most picturesque. It's classic
stratovolcano shape has been depicted in Japanese art for centuries.
Unlike, most stratovolcanoes, Mt. Fuji is dominantly basaltic in
composition. It last erupted in 1707 when it ejected andesitic pumice
blocks and created a new crater on the middle of its southeastern flank.

30. Today