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Volcanic and Tectonic Landforms
Chapter
13
Landforms
Landforms are the natural physical features found on the
Earth’s surface, such as mountains, canyons, beaches,
and sand dunes.
Their creation, subsequent transformation, and ultimate
removal from the landscape attest to the dynamic nature of
the planet.
The scientific study of the processes that shape
landforms is covered in geomorphology.
Landforms
Landforms can be generally divided into two basic groups:
initial landforms and sequential landforms.
Initial landforms are directly produced by volcanic and
tectonic activity (resulting landforms include volcanoes and
lava flows,
flows as well as rift valleys and elevated mountain
blocks in zones of recent crustal deformation).
Landforms, such as river valleys which are shaped by
processes and agents of denudation, belong to the group
of sequential landforms, which develop after the initial
landforms have been created.
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Volcanic Activity
Magma extruded by volcanic activity (volcanism) can form
imposing mountain ranges comprised of volcanic peaks
and accumulated lava flows.
A volcano is a conical or dome-shaped initial landform built
of lava and ash emitted from a constricted vent in the
Earth’s surface. The magma rises in a narrow, pipe-like
conduit from a magma reservoir.
Volcanic Activity
Upon reaching the surface, magma may pour out in
tongue-like lava flows, or it may be violently ejected in the
form of solid fragments driven skyward under the pressure
of confined gases.
Ejected fragments
fragments, ranging in size from huge boulders to
fine dust, are collectively called tephra.
The type of lava and the amount of tephra ejected during
eruptions determines the size and shape of a volcano.
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Volcanic Activity
Stratovolcanoes
The nature of volcanic eruptions, whether explosive or
subdued, depends on the viscosity of the magma.
Felsic lava (rhyolite and andesite) is highly viscous; it is
thick and sticky, and resists flow.
Consequently, volcanoes of felsic composition typically
have steep slopes as the lava does not usually flow far
from the vent.
Volcanic Activity
Stratovolcanoes
When the volcano erupts, tephra falls on the area
surrounding the crater and contributes to the structure of
the cone.
Included in the tephra are volcanic bombs, which are
solidified masses of lava that can be the size of large
boulders.
Volcanic Activity
Stratovolcanoes
The inter-layering of sluggish streams of felsic lava and
eruptions of tephra produces stratovolcanoes (sometimes
p
volcanoes,, or composite
p
cones,,
referred to as composite
since they are formed from layers of ash and lava).
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4
Volcanic Activity
Stratovolcanoes
Another important form of emission from explosive
stratovolcanoes is a cloud of white-hot gases and fine ash.
These intensely hot clouds, called nuée ardentes or ash
flows (sometimes pyroclastic flows), travel rapidly down the
flank of a volcanic cone, searing everything in its path.
Volcanic Activity
Calderas
One of the most catastrophic of natural phenomena is a
volcanic explosion so violent that it destroys the entire
portion of the volcano.
central p
Vast quantities of ash and dust are emitted and fill the
atmosphere for many hundreds of square kilometres
around the volcano).
A great central depression, named a caldera, remains after
the explosion.
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Volcanic Activity
Stratovolcanoes and Subduction Arcs
Most of the world’s active stratovolcanoes lie within the
circum - Pacific mountain belt.
Andesitic magmas rise beneath volcanic arcs of active
continental margins and island arcs.
Volcanic Activity
Shield Volcanoes
In contrast to thick, gassy felsic lava, mafic lava (basalt) is
usually fluid, has a low viscosity, and holds little gas.
As a result, eruptions of basaltic lava are much less violent,
with the lava often travelling long distances, spreading out
in thin layers.
Large basaltic volcanoes are typically broad rounded
domes with gentle slopes; they are called shield volcanoes
(Hawaiian volcanoes).
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Volcanic Activity
Hotspots, Sea-Floor Spreading, and Shield
Volcanoes
The chain of Hawaiian volcanoes was created by the
motion of the Pacific Plate over a hotspot—a plume of
upwelling basaltic magma from very deep within the Earth’s
mantle (also the Galapagos Islands).
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Volcanic Activity
Hotspots, Sea-Floor Spreading, and Shield
Volcanoes
As the hot mantle rock rises, magma forms in bodies that
melt their way through the lithosphere and reach the sea
flfloor.
Each major pulse of the plume sets off a cycle of volcano
formation.
However, the motion of the oceanic lithosphere eventually
carries the volcano away from the location of the deep
plume, and so it becomes extinct.
Volcanic Activity
Hotspots, Sea-Floor Spreading, and Shield
Volcanoes
Erosion processes wear the volcano away, and ultimately it
becomes a low island.
Continued attack by waves and slow settling of the island
reduces it to a coral-covered platform.
Eventually only a sunken seamount, or guyot, exists.
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Volcanic Activity
Hotspots, Sea-Floor Spreading, and Shield
Volcanoes
Where a mantle plume lies beneath a continental
lithospheric plate, the hotspot may generate an enormous
volume
l
off b
basaltic
lti llava th
thatt emerges ffrom numerous vents
t
and fissures and accumulates layer upon layer over an
extensive area.
The basalt may ultimately become thousands of metres
thick; such accumulations are called flood basalts.
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Volcanic Activity
Hotspots, Sea-Floor Spreading, and Shield
Volcanoes
Small volcanoes, known as cinder cones, are often found
with shield volcanoes and basaltic flows.
Cinder cones form when frothy basalt magma is ejected
under high pressure from a narrow vent, producing tephra.
The rain of tephra accumulates around the vent to form a
roughly circular hill with a central crater (typically grow to
heights of a few hundred metres).
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Volcanic Activity
Hot Springs and Geysers
Where hot rock material is near the Earth’s surface, it can
heat nearby groundwater to high temperatures.
When it reaches the surface, the heated groundwater
provides hot springs.
At some places, jet-like emissions of steam and hot water,
at temperatures not far below the boiling point, occur at
intervals from small vents producing geysers.
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Volcanic Activity
Hot Springs and Geysers
The heat from masses of igneous rock close to the surface
in areas of hot springs and geysers provides a source of
energy for electric power generation, and numerous
geothermal
th
l power stations
t ti
are iin operation.
ti
Volcanic Activity
Hot Springs and Geysers
As new oceanic crust is formed along the mid-oceanic
ridges, it is strongly fractured through deformation, thermal
contraction, or earthquakes.
Sea water seeps into these fractures, becomes heated,
and begins to leach minerals from the rock.
As water temperature rises, dark-coloured, mineralenriched water and steam are expelled as black smokers
through chimney-like vents built up of sulphide minerals.
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Volcanic Activity
Global Pattern of Volcanic Activity
Many volcanoes are located along subduction boundaries
where lithospheric plates are converging (the “ring of fire”
around the Pacific Rim).
Other volcanoes are located on or near mid-oceanic rifts.
Hotspot activity is represented in Hawaii and in several
other islands in the Pacific, such as the Galapagos Islands,
Easter Island, Tahiti and the Society Island group.
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Volcanic Activity
Volcanic Eruptions and Environmental Hazards
Volcanic eruptions and lava flows are severe environmental
hazards, often taking a heavy toll on plant and animal life and
devastating human habitations.
Loss of life and destruction of towns and cities are frequent in
the history of peoples who live near active volcanoes.
Devastation might occur in several ways: advance of lava flows
engulfing whole cities; showers of ash, cinders, and volcanic
bombs; from clouds of incandescent gases that descend the
volcano slopes; and violent earthquakes associated with
volcanic activity.
Earthquakes
An earthquake is a motion of the ground surface, ranging
from a faint tremor to a wild movement capable of shaking
buildings apart.
This type of movement can be produced by volcanic
activity or when magma rises or recedes within a volcanic
chamber, but most result from movements along the
boundaries of lithospheric plates.
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Earthquakes
Typically, tectonic forces slowly bend the rock at the plate
boundaries over many years.
When a critical point is reached, the rocks on opposite
sides move in different directions to relieve the strain.
A large quantity of energy is instantaneously released in
the form of seismic waves, which shake the ground.
The waves move outward in widening circles from a point
of sudden energy release, called the focus, and gradually
lose energy as they travel outward in all directions.
Earthquakes
Earthquakes produce four basic types of waves, two of
which — P waves and S waves—travel deep within the
Earth’s interior, and two of which — Rayleigh waves and
Love waves — travel near the surface.
These different wave types travel out from the centre of the
earthquake in all directions.
The centre of an earthquake is determined by the time of
arrival of the different wave types at three locations on the
surface.
The point on the Earth’s surface directly above the focus of
an earthquake is referred to as the epicentre.
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Earthquakes
The faster of the two deep-seated wave types is called the
primary or P waves.
The P waves propagate by alternately pushing and pulling the
rock, and can travel through both solid and molten rock material,
as well as the water of the oceans.
S waves create a transverse motion perpendicular to the
direction of
propagation.
This can be a vertical or horizontal movement, either of which
shears the rock at right angles to the direction of propagation.
Earthquakes
S waves cannot propagate through liquids and so do not develop
in Earth’s outer core; hence the inference that this region is
essentially molten iron.
The speed of P and S seismic waves depends on the density
and properties of the rocks through which they pass.
In most earthquakes, the P waves travel through the crust at 5 to
7 km s-1 and at about 8 km s-1 in the mantle and core.
These are followed by the shaking and twisting motion of the S
waves, which propagate at about 3 to 4 km s-1 through the
crustal rocks, 4.5 km s-1 in the mantle, and 2.5 to 3.0 km s-1 in
the solid inner core.
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Earthquakes
Seismographs do not detect S waves on the opposite side
of the Earth from an earthquake’s point of origin, it has
been concluded that the core is liquid.
The core’s size has been determined by the distance at
which S waves are detected.
As P waves move through the Earth’s interior, they refract
or bend, with abrupt changes in direction occurring at the
boundary between different layers.
Earthquakes
P waves entering the outer core are bent toward the
Earth’s centre so they reach a region opposite the
earthquake’s point of origin. Thus a shadow zone
separates the P waves that only pass through the mantle
from the P waves that pass through the mantle and the
core.
The fact that very weak P waves are felt in the shadow
zone suggests that the inner core is solid.
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Earthquakes
The Richter scale was designed in 1935 to assess the local
magnitude (ML) of earthquakes.
The numbers on the Richter scale range from 0 to 9, but
there is really no upper limit.
For each whole unit increase (e.g., 5.0 to 6.0), the
amplitude of the earthquake wave increases by a factor of
10 and the quantity of energy released increases by a
factor of 32.
The Richter scale is based on the amount of ground
shaking, as measured on a seismograph. It is one of three
common measures of earthquake magnitude.
Earthquakes
The oldest measure, the Mercalli scale devised in 1902, is
based on the amount of damage caused by an earthquake
and human response to it.
The Mercalli scale is descriptive and originally included 10
categories; it was subsequently modified to a 12-point
scale that ranges from I (most people do not notice
notice,
animals may be uneasy) to XII (all structures are
destroyed).
The Moment magnitude scale (MMS), introduced in 1979,
is based on the movement that occurs on a fault, not on
how much the ground shakes during an earthquake.
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Earthquakes
Earthquakes and Plate Tectonics
Most seismic activity occurs primarily near lithospheric
plate boundaries.
Th greatest
The
t t intensity
i t
it off seismic
i i activity
ti it is
i ffound
d along
l
converging plate boundaries where oceanic plates are
undergoing subduction.
Strong pressures build up at the downward-slanting contact
of the two plates.
They are relieved by sudden fault slippages that generate
earthquakes of large magnitude.
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Earthquakes
Seismic Sea Waves
An important environmental hazard often associated with a
major earthquake centred on a subduction plate boundary
is the seismic sea wave, or tsunami.
A succession of these waves is often generated in the
ocean by a sudden movement of the sea floor at a point
near the earthquake source.
The waves travel over the ocean in ever-widening circles,
but they are not perceptible at sea in deep water.
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Tsunami Travel Time of the
March 11, 2011, Japan Earthquake
Earthquakes
Seismic Sea Waves
The need to provide early warnings of tsunamis led to the
establishment of the Pacific Tsunami Warning Center
(PTWC) in Hawaii in 1949.
Currently the PTWC
Currently,
PTWC, representing 26 countries
countries, is
responsible for monitoring seismological and tidal stations
throughout the Pacific Basin.
The present global monitoring system is based on the
deployment of Deep-ocean Assessment and Reporting of
Tsunami (DART) buoys.
The program was initiated in 2001 and expanded to a full
network of 39 stations in March 2008
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A Look Ahead
Following this survey of the Earth’s crust and the geologic
processes that shape it, attention now turns to other
landform creating processes.
The p
processes of weathering
g and mass wasting
g break up
p
rock and move Earth materials downslope under the
influence of gravity.
An examination of the importance of running water in
shaping landforms concludes with an examination of
landforms created by wind, waves, and glacial ice.
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