Volcanology The study of volcanoes
Magma
forms wherever temperature and pressure are
high enough to melt rock.
Some magma forms at the aesthenosphere
Magma also forms at plate boundaries, where
intense heat and pressure develops from friction
between the plates.
Volcanoes
Melted rock expands as it heats up,
becoming less dense than
surrounding rock.
Magma moves upwards through
fractures;
If magma reaches the surface, it
erupts through an opening called a
volcano.
Types of Magmas
Felsic magmas: high silica,
thick, light-coloured, slow-
moving
Mafic magmas: low silica,
thinner, dark-coloured, flow
more easily
Volcanic Eruptions
Magmas contain dissolved gases (i.e., water vapour, carbon dioxide)
As magma reaches the surface, pressure is reduced and dissolved gas comes out of solution as bubbles of gas.
Bubbles expand and explode…therefore…
LOTS OF GAS = MORE EXPLOSIVE ERUPTIONS!
Mafic magmas are more fluid and let gases
escape more easily than felsic magmas
MAFIC MAGMAS = LESS EXPLOSIVE
FELSIC MAGMAS = MORE EXPLOSIVE
Lava and Lava Fragments
If magma reaches the surface of
the earth, it is called lava
Explosive eruptions produce lava
fragments called TEPHRA
TEPHRA CLASSIFICATION:
Ash < 2mm
Lapilli 2 mm – 64 mm
Bombs >64 mm
Pyroclastic Flow
in explosive eruptions, tephra
combines with gases to form a
dense, superheated cloud
traveling at very high speeds
(> 100 km/h)
Volcano Hazards
1. Ash
Gritty sand-sized particles blasted
from erupting volcano
Can reach very high altitudes.
Large amounts can block sunlight,
causing world temperatures to drop.
Can destroy crops, buildings, clog
rivers, damage machines.
Volcano Hazards
2. Lava
Molten rock may flow over large
areas, destroying everything in its
path.
Volcano Hazards
3. Pyroclastic Flow
Destructive mix of
superheated gas, ash and
debris which can move faster
than 100 km/h
Volcano Hazards
4. Lahar
Flow of mud, water, ash and debris
that can result when snow-covered
volcanoes erupt
Types of Volcanoes
There are three main ways volcanoes are formed:
Convergent boundaries
Divergent boundaries
Hot Spots
1. Convergent Boundaries
(Composite Volcanoes)
As ocean plate sinks beneath the
continental plate, increasing heat
melts the rock, forming magma.
Magma is lighter than the
surrounding solid rock, so it rises up
through the edge of the continental
plate to form a volcano.
1. Convergent Boundaries
(Composite Volcanoes)
Form classic, cone-shaped volcanoes that
erupt ash and lava
Cone shape results from layers of ash and
lava building up over time
Magma is usually felsic (very thick), and
traps gas producing explosive eruptions
i.e, Mt. Baker
2. Divergent Boundaries
(rift eruptions)
Magma flows out of volcanoes at
mid-ocean ridges;
occasionally, volcanoes grow high
enough to rise above the surface of
the ocean and produce islands
i.e, Iceland
3. Hot Spots
(Shield Volcanoes)
Sometimes extremely high temperatures are found beneath
the middle of oceanic plates, in areas called hot spots.
At these locations, the melting of crustal rock produces
magma that rises up to form a volcano.
3. Hot Spots
(Shield Volcanoes)
As the plate moves, it carries the
hot spot with it, so that chains of
volcanic islands are formed
Magma is usually mafic, so
eruptions are not explosive and
volano’s sides have a shallow slope.
i.e., Hawaii
Plutons and Volcanism
Magma squeezes through fractures and
rock layers as it rises.
Igneous rock masses that form inside
other rocks are called PLUTONS.
Structures are named according to
size and orientation.
Dikes
sheets of igneous rock that cut across the rock
layers they intrude (usually mafic)
Sills
sheets of igneous rock that are parallel to the
rock layers they intrude (also usually mafic)
Laccoliths
Dome-like masses of igneous rock formed by stiff
magma which, instead of spreading, bulges
upwards (usually felsic)
Volcanic Neck
Plug of magma that hardens inside the vent of a
volcano
Batholiths and Stocks
Batholith: Largest of intrusions; forms the core of
many mountain ranges (usually felsic)
Stock: Small batholith (less than 100 square
kilometres exposed at surface)