Transcript Plate Tectonics, Earthquakes and Volcanoes

Plate Tectonics, Earthquakes and
Volcanoes
J. Quigley
Plate Tectonics
• Plate tectonics is a discovery that revolutionized
the field of geology!
• Plate tectonics is the theory that pieces of the
earth’s lithosphere, called plates, move about
slowly on top of the asthenosphere
Plate Tectonics
• Plate tectonics explains the formation and
movement of earth’s plates.
• Until plate tectonics were understood,
geologists couldn’t explain how fossils from
ocean creatures were found on mountain
tops, how mountains and valleys…
• Plate tectonics explain earthquakes and
volcanoes… it is amazing!!
Continental Drift
• A German scientist
hypothesized in 1912
that the continents
were once joined
together in a single
supercontinent,
which then broke
into pieces and
moved apart… called
Pangaea.
Continental Drift
• Fossils of land based
plants and animals were
discovered on
continents separated by
large oceans… these
fossils puzzled
geologists until
continental drift and
plate tectonics were
understood
Continental Drift
• Continental drift
explains how
continents move
slowly across earth’s
surface. It explains
why continents seem
to fit together and it
explains why there are
fossils all over the
globe that are similar
Sea-Floor Spreading
• Geologist Harry Hess proposed the theory of sea floor
spreading.
• Sea-floor spreading is the process by which new oceanic
crust is created at mid-ocean ridges as older crust moves
away
Mid-Oceanic Ridge
• The mid-ocean
ridge is a huge
crack in the crust
where magma
pushes upward.
• Pieces of the ocean
floor on either side
of the mid-ocean
ridge are moving
slowly apart.
Formation of Oceanic Crust
• As the ocean floor moves magma from the mantle
wells up and solidifies to form new oceanic crust.
Subduction of Oceanic Plates
• As sea-floor spreading occurs old oceanic plates
sink into the mantle in the process of subduction.
• As a plate sinks through a subduction zone, it
bends, forming a depression in the ocean floor
called a trench
Subduction of Oceanic Plates
• Sea floor spreading creates new oceanic crust at
mid ocean ridges. Subduction destroys old oceanic
crust at subduction zones
Evidence for Sea-Floor Spreading
• Rocks were sampled on both sides of the midocean ridge and they found patterns of
parallel magnetic “stripes” that were identical
on the two sides
• The stripes exist because Earth’s magnetic
field has reversed itself many times in the past
• The stripes showed that new ocean floor was
being added to both sides of the mid-ocean
ridge at roughly the same rate.
Theory of Plate Tectonics
• According to the theory: Earth’s plates are
constantly moving, each with a different rate and
direction.
• Convection currents form in the mantle as hot
rick rises at mid-ocean ridges, cools and spreads
out horizontally as ocean lithosphere, and then
sinks back into the mantle at subduction zones.
• These sinking slabs of dense lithosphere and heat
from within Earth drive the circulation of
convection currents in the mantle.
• Plate motions are the visible part of the process
of mantle convection.
Plate Boundaries
• There are thee types of plate
boundaries
–Divergent boundaries
–Convergent boundaries
–Transform boundaries
Divergent Boundaries
• Plates move away from each other at a divergent
boundary
• When plates diverge (move away from each other)
magma rises from the mantle to fill the gap. Magma
cools to form new rock at the edge of each plate.
Convergent Boundaries
• When plates come together or collide they are
convergent.
• The most common convergent boundary is one where
an oceanic plate is subducted beneath a trench. When
oceanic crust collides with continental crust, the denser
oceanic crust slides under the continental crust
Transform Boundaries
• Plates slide past each other, moving in opposite
directions.
• Rock is neither created or destroyed at a transform
boundary
Mountain Building
• Geologists have found that most mountains
form along plate boundaries.
Earthquakes
EARTHQUAKES
• An earthquake is
movement of Earth’s
lithosphere that occurs
when rocks in the
lithosphere suddenly shift,
releasing stored energy.
• When energy is released
during an earthquake it is
carried by vibrations called
seismic waves.
Earthquakes
• As tectonic plates move, • http://www.youtube.co
they cause stress in the
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crust, which in turn
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produces faults and
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folds
Earthquakes
• A fault is a break in a
mass of rock along
with movement
occurs.
• Two slabs of rock on
either side of a fault
move in relation to
each other
• Many faults occur
along plate
boundaries
Earthquakes
• A fold is a bend in layers of rock
• Forms form where rocks are squeezed together, but do
not break
• Rocks tend to fold rather than break when they are
under high temperature or pressure
• Folds can vary greatly in size
Seismic Waves
• Earthquakes occur
because stress forces
have exceeded the
strength of rock
• The location beneath
Earth’s surface when
an earthquake begins
is called the focus
• The location on
earth’s surface directly
above the focus is the
epicenter
Seismic Waves
• As an earthquake occurs, seismic waves move out in
all directions from the focus
Types of seismic waves
• P waves- primary waves are longitudinal waves similar
to sound waves. They compress and expand the earth
like an accordion. They are also the fastest seismic
waves and travel large distances quickly. They travel
through solids and liquids.
Types of seismic waves
• S waves- secondary
waves are transverse
waves. They cause the
particles in the
materials they pass
through to vibrate at
right angles to the
direction the waves
move. They move like a
waving flag. They travel
through solids only.
Types of Seismic Waves
• Surface Waves- waves
that develop when
seismic waves reach
earth’s surface. They
move more slowly than
the other waves. They
usually produce large
ground movements and
greater damage.
Measuring Earthquakes
• To measure
earthquakes and
pinpoint their
epicenters, geologist
record seismic waves
using a seismograph
• Seismograph- a device
that can detect and
record seismic waves.
Seismographic Data
• Most earthquakes are
concentrated along plate
boundaries, where many
faults are found.
• Some earthquakes are in
the interior of plates
• Scientists have mapped
earth’s interior, analyzing
how seismic waves move
through its layers
Seismographic Data
• The speeds of seismic waves and the paths
they take are affected by the temperature,
composition and density of the rocks they
pass though.
• Geologists infer that Earth’s outer core is
liquid because S waves cannot pass through it
• They can also tell the core is mostly iron
because P waves travel through it at a speed
that matches laboratory experiments on iron.
Volcanoes
Formation of a Volcano
• Under certain
conditions, small
amounts of mantle
rock can melt,
forming liquid
magma. The magma
rises upward through
the crust, erupting at
the surface as a
volcano.
How a Volcano Erupts
• The process is similar to if you quickly open a bottle of soda
that has been shaken.
• Magma is under pressure and contains dissolved gasses, as
magma approaches the surface, lower pressure allows the
gases to expand rapidly.
• An eruption occurs when the gases bubble out through a crack
in the crust, propelling magma to the surface
Structure of a Volcano
• Before an eruption,
magma usually collects
in a pocket called a
magma chamber
• Magma slowly
accumulates in the
magma chamber until
enough pressure builds
up to start an eruption
Structure of a Volcano
• Then, magma rises to the surface in a narrow, vertical
channel called a pipe
Structure of a Volcano
• An opening in the ground where magma escapes to the
surface is called a vent
• There is often one central vent at the top of a volcano, but
sometimes there are vents that open along the side of the
volcano
Structure of a Volcano
• At the top of the central vent of most volcanoes is a
bowl shaped pit called a crater
Structure of a Volcano
• After an eruption, a
volcano’s magma
chamber and main
vent may be empty
of magma creating a
hollow shell. If the
shell collapses
inward, it creates a
huge depression
called a caldera, at
the top of the
volcano
Eruptions
• Volcanoes erupt
explosively or quietly,
depending on the
characteristics of the
magma
• Magma varies in
viscosity (thickness)
based on temperature,
water content and silica
content
Quiet Eruptions
• Volcanoes that
have very hot,
low silica magma
general erupt
quietly. In a quiet
eruption lava is
of low viscosity
and flow great
distances.
Quiet Eruptions
• They produce two
types of lava
– Hot, fast moving
with rope like
surface called
pahoehoe (pah
HOH ee hoh ee)
Quiet Eruptions
• Cooler, slow moving lava with a chunky
crumbly appearance is called aa (Ah ah)
Explosive Eruptions
• High silica magma
produces explosive
eruptions. Thick magma
can clog a volcanic pipe,
causing enormous
pressure to build up.
When the volcano
finally explodes, lava
and hot gasses are
hurled outward.
Explosive Eruptions
• Explosive eruptions
have particles that get
thrown through the air.
Some particles are small
like ash, some are
pebble sized… and
occasionally, chunks of
lava the size of a small
car can be thrown!
Location and Types of Volcanoes
• Most volcanoes occur along plate boundaries or
at hot spots in the crust
• Volcanoes often form along converging plate
boundaries where an oceanic plate is subducted
into the mantle. The plate sinks through the
mantle and is melted. Magma forms and rises to
the surface
• Volcanoes can also form along diverging plate
boundaries where magma rises to fill the gap
between two separating plates.
Ring of Fire
• Volcanoes forming along the trenches that rim the
pacific ocean
Hot Spots
• Some volcanoes form in
hot spots… a region
where hot rock extends
from deep within the
mantle to the surface.
• Hot spots are
responsible for building
the Hawaiian islands.
Types of Volcanoes
• Shield Volcano- a quiet
eruption of low
viscosity lava produces
a wide, flat volcano.
Types of volcanoes
• Cinder Cone- if an
eruption is entirely ash
and cinders, the result
will be a small, steepsided volcano
Types of Volcanoes
• Composite Volcano- A
volcano that forms from
explosive eruptions that
produce a combination
of lava and ash.
Other Igneous Features
• Igneous features formed by magna include
batholiths, sills, dikes and volcanic necks.
THE END