Chapter 10- Volcanoes

Lecture notes

• Broad, gently sloping sidesBroad, gently sloping sides• Basaltic lavaBasaltic lava• Small amounts of gases and Small amounts of gases and

silicasilica

Shield VolcanoesShield Volcanoes

• Material ejected Material ejected high into the air falls high into the air falls back to Earthback to Earth

• Steep sidesSteep sides• More water and More water and

silica than shieldsilica than shield• Large volume of Large volume of

gasesgases• More explosive than More explosive than

shieldshield

Cinder Cone Cinder Cone VolcanoVolcano

• Volcanic fragments Volcanic fragments alternate with lavaalternate with lava

• Large amounts of Large amounts of silica, water, and silica, water, and gasesgases

• Dangerous to Dangerous to humans and humans and environmentenvironment

• Violently explosiveViolently explosive

Composite Composite VolcanoVolcano

Vesuvius – Sorrento (Italy)Vesuvius – Sorrento (Italy)

•Intrusive igneous rock bodies – BatholithsBatholiths- largest plutons, irregularly shaped masses, course grained

– StocksStocks- smaller than batholiths– LaccolithsLaccoliths- mushroom shaped– SillSill- forms parallel to layers of rock– DikeDike- cuts across layers of rocks

PlutonsPlutons

• 3 Factors Effect Magma Formation3 Factors Effect Magma Formation– Temperature Temperature

•800 – 1200c° for rock to melt and form 800 – 1200c° for rock to melt and form magmamagma

•Temperature increases with depthTemperature increases with depth

– PressurePressure •Pressure increases with depthPressure increases with depth•Increased pressure raises a rock’s Increased pressure raises a rock’s melting pointmelting point

•Raised melting point prevents all rock Raised melting point prevents all rock from meltingfrom melting

Magma Magma FormationFormation

-Water-Water – Water molecules occupy pore Water molecules occupy pore

space within rocksspace within rocks– Water lowers the melting point of Water lowers the melting point of

a rocka rock– The more water, the lower the The more water, the lower the

melting pointmelting point

Magma Magma FormationFormation

• BasalticBasaltic– rocks in upper mantle rocks in upper mantle

meltmelt– low silicalow silica– low viscositylow viscosity– quiet eruptionsquiet eruptions– very hot (1000-1250very hot (1000-1250°C)°C)– fast moving lavafast moving lava– melted basaltmelted basalt

Magma TypesMagma Types

• AndesiticAndesitic– oceanic crust or oceanic crust or

oceanic sedimentoceanic sediment– 60% silica (high)60% silica (high)– intermediate viscosityintermediate viscosity– intermediate eruptionsintermediate eruptions– melted andesitemelted andesite

Magma TypesMagma Types

•RhyoliticRhyolitic– molten material rises and molten material rises and

mixes with overlying silica mixes with overlying silica and water rich continental and water rich continental crustcrust

– high viscosityhigh viscosity– very explosive eruptionsvery explosive eruptions– highest silica contenthighest silica content– thick, slow movingthick, slow moving lavalava– melted granite melted granite

Magma TypesMagma Types

Volcanoes

•80% of volcanoes found along 80% of volcanoes found along convergent boundariesconvergent boundaries– Circum-Pacific Belt- along Circum-Pacific Belt- along western coastlines of N. and S. western coastlines of N. and S. Am.Am.

•Divergent plates move apart, Divergent plates move apart, magma forced upwardmagma forced upward– Ocean ridges (mid-Atlantic)Ocean ridges (mid-Atlantic)– IcelandIceland

Hot Spot Hot Spot VolcanoesVolcanoes

• Located away from Located away from plate boundaryplate boundary• Magma plume rises Magma plume rises from corefrom core• Plume melts the Plume melts the crustcrust•Islands are formed Islands are formed by escaping magmaby escaping magma•Usually forms arcs Usually forms arcs as a result of moving as a result of moving platesplates

•Volcanoes and Volcanoes and earthquakes are generally earthquakes are generally located togetherlocated together

•Both take place at Both take place at convergent boundariesconvergent boundaries