Ch 13-Volcanoes
• What causes most volcanoes?
– Movement of tectonic plates
• Magma-liquid rock produced under Earth’s
surface
– Temp of rock rises above melting pt of the minerals
the rock is composed of, rock melts
– Enough pressure is removed from the rock, melting pt
will decrease and rock will melt
– Addition of fluids, such as water, may decrease the
melting pt of some minerals in the rock and cause
rock to melt
Volcanism
• Any activity that includes the movement of
magma toward or onto Earth’s surface
• Magma rises upward through the crust cuz the
magma is less dense than surrounding rock
• Lava-magma that flows onto Earth’s surface
• Vent-opening that lava flows through
• Volcano-vent or fissure in Earth’s surface
through which magma and gases are expelled
Major Volcanic Zones
• Where do they occur?
– Convergent and divergent boundaries of tectonic plates
– Pacific Ring of Fire
• Subduction Zones-one tectonic plate moves under
another
– Oceanic to oceanic, continental to oceanic
• Mid Ocean Ridges-largest amount of magma comes to
surface
– Plates pull apart magma flows upward along rift zone
– Fissures-cracks through which lava flows to Earth’s surface
• Hot spots-volcanically active area of Earth’s surface
– Areas of volcanism w/in interiors of lithosphere plates
Intrusive Activity
• Magma intrudes overlying rock, magma
that doesn’t reach Earth’s surface cools
and solidifies inside crust
• Plutons-igneous rock
• Dikes-small tabular plutons
• Batholithes-large plutons
Sec 2-Volcanic Eruptions
• Types of magma
– Mafic-magma or rock that is rich in
magnesium and iron, dark
– Felsic-magma or rock rich in light colored
silicate materials
• Mafic-oceanic crust
• Felsic and mafic-continental crust
Types of Eruptions
• Viscosity-resistance to flow, of magma affects force w/
which particular volcano will erupt
• Mafic-runny lava, low viscosity, quiet eruptions
• Felsic-sticky lava, high viscosity, explosive eruptions
• Quiet eruptions
– Oceanic eruptions, mafic magma. Low viscosity=gases escape
so volcanoes are quiet
• Lava flows
– Lava cools rapidly, crust forms and lava continues to flow, it
forms rock=pahoehoe
– Crust deforms rapidly, forms wrinkles, surface breaks into jagged
chunks=aa
– Blocky lava-high viscosity, produces large chunks, and lava
continues to flow
• Explosive Eruptions
– Felsic lavas of continental volcanoes-cooler
and stickier
– Large amounts of trapped gases-CO2 and
water vapor
– Explodes and throws pyroclastic material into
air
– Pyroclastic material-fragments of rock that
form during volcanic eruption
Types of Pyroclastic Material
• Volcanic ash-less than 2 mm in diameter
• Volcanic dust-volcanic ash less than .25 mm in
diameter
• Lapilli-large pyroclastic particles, less than 64
mm
• Volcanic bombs-large clots of lava, red hot when
thrown out
• Volcanic blocks-largest pyroclastic material, form
from solid rock blasted from vent
Types of Volcanoes
• Crater-funnel shaped pit at top of volcanic
vent, forms when materials are blown out
• Types of cones
– Shield Volcanoes-broad at base, sloping
sides, wide area, quiet eruptions
– Cinder Cones-steep slopes, explosion
eruptions, made up of pyroclastic material
– Composite volcanoes-alternating layers of
hardened lava flows and pyroclastic material
Volcanic Eruptions
• Calderas-large circular depression, forms when
magma chamber below volcano partially
empties, causes ground to sink
– Krakatau in Indonesia
• Predicting Volcanic Eruptions
– Earthquake activity-most important sign is changes in
earthquake activity around volcano
– Patterns in activity-upward movement of magma
beneath the surface, causes surface of volcano to
bulge outward before eruption