Geologi Dasar - 3

5/20/2018 Geologi Dasar - 3

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What is a volcano?

A conical hill or mountain

produced by accumulation of

lava and tephra emitted from a

vent in Earth's surface.


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Seventy-five percent of the Earthsactive volcanoes (yellow dots) lie in

the ring of fire, a chain of subduction zones at convergent plate

boundaries (heavy red lines with teeth) that encircles the Pacific Ocean.


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Magma Sources and Types

Magma sources tend to be 50 to 250 km deep

into the crust and upper mantle

Temperatures increase as depth increases

Volcanoes are generated at (see figure): Divergent Plate Boundaries

Convergent Plate Boundaries

Hot Spots


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Sites where volcanoes are generated


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Basalt and Granite Basalt is oceanic crust

Rich in iron and magnesium, low in silica Dark color

Called Mafic

Basaltic magma flows readily and easily

Granite is continental crust

Rich in silica, low in iron and magnesium

light color

Called Felsic

Granitic magma doesnt flow readily, very viscous


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Magma Sources and Types

Mafic magmas produce basalt lavas

Felsic magmas produce rhyolite lavas

Intermediate magmas produce andesite lavas


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Magma at Divergent Plate Boundaries

Magma produced at a Divergent PlateBoundary is typically melted asthenosphere

material

Asthenosphere is extremely rich inferromagnesian (ultramafic) and a melt from it

is mafic (or ultramafic)

Basalt is emplaced as new seafloor at the

spreading ridge or a rift


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Magma at Convergent Plate Boundaries

Magmatic activity at convergent boundaries iscomplex

The composition of the subducted platedetermines the composition of the lava

Subducted continental crust may melt andproduce rhyolite lava

Subducted oceanic crust may melt and producebasalt or andesite lava

Subduction of sediments derived from the top ofthe subducted slab may produce a variety of lavas


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Magma at Hot Spots

Magmas associated with a hot spot volcano inan ocean basin will produce a basalt lava

Magmas associated with a hot spot volcano

under continental crust generally will producea felsic lava (and often an explosive one)


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ACTIVE VOLCANIC ZONE

IN SUMATRA-JAVA

Lowland

area along

eastern

Sumatra


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Talang Eruption in West Sumatra.This volcanic activity has been triggered byIndian plate subducting beneath the island.

People leave

their homes to

stay in the

temporarily

safe region


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Merapi Eruption in CentralJava, due to Indian platesubducting beneath the

island

extremely hot volcanic

clouds, locally known as

wedus gembel

Very hot pyroclastic

materials flowing

down the slope of

Merapi


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Geologi Dasar - 3