Lecture 6
IGNEOUS ACTIVITY
Factors Affecting Volcanic
Eruptions
The primary factors affecting volcanic eruptions
are:
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The magma's composition;
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The magma's temperature;
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The magma's amount of dissolved gases.
The more viscous a material the more resistant it
is to flow. Viscosity is affected by the silica content. Below is a description
of viscosity and silica content.
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If silica content is about 50%, the result is basaltic magma (viscosity
is thin).
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If silica content is about 60%, the result is andesitic magma (viscosity
is intermediate).
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If silica content is about 70%, the result is rhyolitic magma (viscosity
is thick).
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The higher the silica content the more viscous the magma and violent the
volcanic eruption.
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The greater the viscosity the more gases are trapped in the magma, and
the more violent the eruption will be; thus, rhyolitic magmas produce the most
violent eruptions.
Materials Extruded During Eruptions
Lava flows have low silica content (basaltic
lavas) and are usually very fluid.
Magma will flow as sheets or stream like ribbons.
Pahoehoe flows are smooth flows, no wrinkles;
resemble the twisted braids in ropes.
Aa (pronounced ahah) has a rough surface; jagged
blocks with sharp edges.
Magmas contain dissolved gases, which make up
about 1-5% of the magma material.
Pyroclastics are ejected lava fragments ranging
in size from fine dust to large boulders; associated with gaseous and viscous
magmas Some examples of pyroclastic material are:
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Ash;
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Welded tuff;
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Lapilli;
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Cinders;
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Blocks;
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Bombs.
Shield
volcanoes have the following characteristics:
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They resemble shields.
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They are domed.
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They are built from basaltic lava flows.
Cinder
cones have the following characteristics:
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They have very steep slopes.
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They are rather small.
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They are built from pyroclastic material.
Composite
cones have the following characteristics:
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They are composed of interbedded lava flows and pyroclastic deposits.
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They are nearly symmetrical.
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They have a central vent.
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They can create nuee ardentes, which are glowing avalanches of turbulent
steam and ash flows.
Volcanic Landforms
Intrusive Igneous Structures
Dikes are
sheet like bodies produced when magma is injected into fractures that cut
across rock layers.
Sills are
tabular plutons formed when magma is injected along sedimentary bedding
surfaces (horizontal structures).
Laccoliths
from the same way as sills, but form from more viscous magma, which collects as
a lens shaped mass that arches the overlying strata upward.
Batholiths
are the largest intrusive igneous bodies. They usually form the cores of
mountains.
Origin of Magma
Magma originates from solid rock in the crust and
mantle.
Temperature is one way to melt rock material. The
source of heat comes from the decay of radioactive minerals inside the Earth.
An increase in the confining pressure increases
the rock’s melting temperature; reducing confining pressure decreases a rocks
melting temperatures.
Most often melting of rock is not complete; the
consequence is the production of a melt or magma with higher silica content
than the parent rock material. This is called partial melting.