Transcript 6a. volcanic landform Prabha.ppsx

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Volcanic Eruption and
landform Creation
Volcanic Landforms
• What are volcanic landforms?
• The term volcano refers not only to a volcanic
vent, but also to the landform that develops as
the materials from a volcanic eruption harden.
• The shape and structure of a volcano are
determined by the nature of the eruption and the
materials that are ejected during the volcanic
activity.
What is a volcano?
vent
•
cone
conduit
•
A volcano is a vent or
'chimney' that connects
molten rock (magma)
from within the Earth’s
crust t or mantle to the
Earth's surface.
The volcano includes the
surrounding cone of
erupted material.
magma chamber
How and why do volcanoes erupt?
•
Hot, molten rock (magma) is buoyant (has a lower density
than the surrounding rocks) and will rise up through the crust
to erupt on the surface.
– Same principle as hot air rising, e.g. how a hot air balloon
works
•
When magma reaches the surface it depends on how easily it
flows (viscosity) and the amount of gas (H2O, CO2, S) it has in
it as to how it erupts.
Large amounts of gas and a high viscosity (sticky) magma will
form an explosive eruption!
– Think about shaking a carbonated drink and then releasing
the cap.
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Small amounts of gas and (or) low viscosity (runny) magma
will form an effusive eruption
– Where the magma just trickles out of the volcano (lava
flow).
How and why do volcanoes erupt?
•
As depth increases below the surface of the earth pressure
also increases and this prevents actual melting of the very
hot material. The material below the crust is therefore, is
invariably at very high temperature and under high
pressure likely to acquire a molten condition by a slight
decrease in pressure. This creates the possibility of a
Volcano.
•
As, when and where in the body of the earth there is some
cause to release the pressure, a local pocket of molten
material comes into existence.
Materials of Volcano
•Material that falls following a volcanic eruption
•Lava:
•Solid material- the pyroclast
•Tephra- is fragmental material produced by a volcanic eruption regardless
of composition, fragment size or emplacement mechanism
• “Ash” – less than 2 mm in diameter
•Lapilli – between 2 mm and 64 mm in diameter
•Volcanic bombs/blocks – greater than 64 mm in diameter
• Gases: Gases impart mobility to the magma and supply pressure for its ascent
Image credit: http://home.flash.net/~alanm52/Pinatubo_ASHFALL.JPG
Types of Volcanic Products: Effusive
• Lava Flow
– Dominantly basalt (low viscosity and gas)
– Thin and laterally extensive sheets
• Pahoehoe flows: smooth, ropey flows
• Aa or block flows: rough and irregular flows
• Baked zones: oxidized zones due to contact with
high temperature lava flow
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• Lava Dome
– Dacite or rhyolite (high viscosity, low gas
content)
– Thick, steep-sided flows
Types of Volcanic Products: Explosive
• Pyroclastic particles
– Fragmental volcanic
material (TEPHRA)
• Vitric (glass shards)
• Crystals
• Lithic (volcanic rock
fragments)
Bombs
Tephra
– Broken during eruption
of magma
– Typically higher silica,
high gas content
– Categorized by size:
• Ash (< 2.0 mm)
• Lapilli (2-64 mm)
• Blocks and bombs (>64
mm)
Ash12
Types of eruptions
Central eruption or Mountain like landforms
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There are Several Kinds of Volcanoes:
Shield Volcanoes
Cinder Cones
Composite Volcanoes
Lava Plateaus
•Beneath continental plates hotspot generates large volume of
basaltic lava and forms flood basalts.
Volcanic Landforms
Shield Volcanoes
Shield volcanoes are associated with
mafic lava (basalt)
• Lava thin, not viscous
• Holds little gas
• Usually quiet eruptions
• Lava travels long distances, spreads out
in thin layers
• Shield volcanoes are rounded domes,
with gentle slopes
Shield Volcano: low, often large, dome-like
accumulation of basalt lava flows emerging from
long , radial fissures on flanks
Cinder Cones
• A cinder cone is the
simplest type of volcano;
it forms when molten
lava is thrown into the air
from a vent. As it falls,
the lava breaks into
fragments that solidify in
the air, before hitting the
ground. These fragments
accumulate in a cone
shaped mound.
• Cinder cones tend to be
smaller than other types
of volcanoes.
• They form in groups, and
on the sides of larger
volcanoes.
Volcanic Landforms
Stratovolcanoes
The nature of an eruption depends
on the type of magma involved.
Felsic lavas associated with
stratovolcanoes
• Rhyolite, andesite
• Thick, resistant to flow
• Builds steep slopes around volcanic
vents
• Tall, steep cone, with crater
• Most of these volcanoes associated
with subduction zones.
• Large amount of gas under high
pressure may emit glowing
avalanche
Glowing avalanche from a stratovolcano
Composite volcanoes
Composite volcanoes are tall coneshaped mountains that are typically
steep-sided, symmetrical cones of
large dimensions. The essential
feature of a composite volcano is a
conduit system through which
magma from a reservoir deep in the
earth's crust rises to the surface. The
volcano is built up by the
accumulation of material erupted
through the conduit and increases in
size as lava, cinders, ash etc. are
added to its slopes. These volcanoes
are built in layers by multiple
eruptions, sometimes recurring over
hundreds of thousands of years.
Most composite volcanoes have a
crater at the summit which contains
a central vent or a clustered group of
vents.
St. Augustine volcano-Alaska is a composite volcano
Craters and Caldera
A Crater is an oval opening in the middle
of a volcanic cone and may be from a
few meter to as much as a kilometer in
diameter.
A caldera is a very large (more than 1.5
km in dia.) crater shaped basin, which
forms when the top of the volcano
collapses inward. This happens when
the eruption empties the underground
magma chamber, and leaves an “empty”
space underneath.
Volcanic Necks: consists of pipe like
masses of lava and frequently material
cooled within the former conduits.
Removal of material from the sides due
to erosive processes leaves the neck
standing as stamps, towers or exposed
plugs.
Chocosuela caldera-Costa Rica
Fissure eruptions
Sometimes at plate tectonics
boundaries, there are long,
narrow cracks (fissures) in the
crust. Basaltic lava pouring from
the fissure spreads across the
land, forming a lava plateau.
Flood of lava continue for very
long periods building up new
landforms from the lava flows.
The Deccan Trap in the peninsular
India and the Panjal Traps in the
Himalayas are best examples of
fissure eruptions in the past.
Fumaroles: These are cracks,
openings in the crust of the earth
through which only hot gases
come out intermittently or
regularly.
Continental flood basalts,
Columbia Plateau, U.S.