Download b. Section 4.2 Volcanology (powerpoint)

Unit 4: Earth Science
Section 4.2 Volcanology
Volcanology: The Study of Volcanoes
Magma – forms wherever temperature and
pressure are high enough to melt rock.
• Some magma forms at the asthenosphere
• Magma also forms at plate boundaries, where
intense heat and pressure develops from
friction between the plates.
I. Volcanoes
• Melted rock rises as it
heats up, becoming less
dense than surrounding
rock.
• Magma moves upwards through fractures in
the plates;
• If magma reaches the surface, it erupts
through an opening called a volcano.
II. 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
III. 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 and,buildings, clog
rivers, damage machines.
2. Lava
– Molten rock may flow over large areas, destroying
everything in its path.
3. Pyroclastic flow
• Destructive mix of superheated gas, ash and
debris, which can move > 100 km/h
4. Lahar
– Flow of mud, water, ash and debris that can result
when snow-covered volcanoes erupt.
IV. 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
(or less dense oceanic) 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.
• 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 very thick and traps gas
producing explosive eruptions
• i.e, Mt. St. Helen’s, 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,
where there are high concentrations of
radioactive elements.
• melting of crustal rock
produces magma that
rises up to form a volcano.
• As the plate moves, it carries the hot spot with
it, so that chains of volcanic islands are formed
• Magma is usually thin, so eruptions are not
explosive and volano’s sides have a gentle
slope.
• i.e., Hawaii