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Section 17.1 and 17.2
Plate Tectonics, Earthquakes, and
Volcanoes
Background – History of the Earth
This chapter is based on the idea that Earth is
about 4.5 billion years ago.
• Began as a ball of small particles pulled together
by gravity. As pressure built, the material heated
up.
• Possible explanations for the heat:
- collisions of particles
- compression of the interior of the planet
- radioactive decay of some natural
elements
• The Earth began to cool and the heavier
(more dense) elements sank while the
lighter (less dense) materials floated to the
surface.
17.1 Earth’s Interior and Plate
Tectonics
The Earth’s Interior
is divided into 4
basic layers
• Crust
- Outermost and thinnest layer of Earth =
4 - 40 km thick
- Made of hard, solid rock
- Can be either continental crust (thicker)
or oceanic crust (thinner)
- The crust (and part of the mantle) is
divided into 7 large plates
• Mantle
- Thickest layer of Earth = 2900 km thick
- 80 % of Earth’s volume
- Scientists have never drilled to the mantle, so
they can only guess what it is made of based on
what we learn from earthquakes and volcanoes.
- outer mantle is made mostly of solid rock
- inner mantle is made of hot and melting rock
- rich in iron, magnesium, silicon, and oxygen
• Outer Core
- surrounds the inner core with liquid metal
boiling under pressure: nickel and iron
• Inner Core
- made of solid metal: nickel and iron
Temperatures increase as one gets closer
to the center of the earth.
Plate Tectonics = the theory that the Earth’s
surface is made up of large moving plates.
• Alfred Wegner studied world maps and
hypothesized about
Pangea – a supercontinent that he
believed existed in history and split into
the current continents.
Continental drift = the mechanism for the
movement of continents caused by
interactions between:
• Lithosphere
- the earth’s stiff outer shell made of the
crust and hard upper mantle
- divided into 7 large pieces and several
smaller pieces called tectonic plates.
• Asthenosphere
- the liquid inner mantle which is in
constant slow movement because of
the heating from the center of the earth
- convection currents result from the
liquid rock being heated near the core
and rising toward the outer mantle then
cooling
• Interaction = currents in asthenosphere push the
lithosphere plates like waves push around floats
Evidence supporting continental
drift
• The shapes match
Ex: South America fits with Africa like
puzzle pieces
• The plants and animals match
Ex: identical fossils along the coastal parts
of Africa and South America
• The rocks match
Ex: the same type of broad belts of rocks
in Africa and South America; these bands
have alternating magnetic polarities
Shapes match
Plants and Animals Match
• The ice marks match
Ex: Glacial striations on rocks show that
glaciers moved from Africa onto South
America without drifting away in the
Atlantic Ocean
• The current positions don’t match
Ex: coal that is mined in Pennsylvania was
actually formed from tropical plant life that
lives near the equator.
Basic Types of Plate movement
Plate movement occurs at faults, which are
any cracks in the earth where movement
occurs. There is are forces at each fault
(convection currents) that cause
movement in a certain direction. We use
the direction to categorize the fault into a
boundary. Plates move about 2 cm to 10
cm a year.
• Three types of boundaries: divergent,
convergent, transform.
• Three types of faults: normal, reverse,
strike slip
• Three types of forces: tension,
compression, shear
Divergent Boundaries
• Two plates are moving apart leaving gaps;
the gaps are filled with magma rising from
the mantle which cools into new crust
• Occur at normal faults
• Caused by tension force
• Movement results in volcanoes and
oceanic rift valleys surrounded by high
mountains.
- Mid-Atlantic Ridge = a submerged
mountain range under the Atlantic Ocean
Mid-Atlantic Ridge
Iceland on a Divergent Boundary
Convergent Boundaries
• Two plates are moving towards each other
• Occur at reverse faults
• Caused by compression forces
• Movement results in subduction zones which
create ocean trenches, mountains, and
volcanoes.
- subduction = when one plate dives beneath
another; the lower plate is often melted by the
heat of the asthenosphere
• - Andes Mountains in South America where a
continental plate and an oceanic plate meet
- Mariana Trench in Pacific Ocean near Japan
where two oceanic plates meet is more than
11km deep (6.8 mi)
- Himalayan mountains are due to the collision
of two continental plates
Transform Boundaries
• Two plates are moving horizontally past
each other like two cars on a road going in
opposite directions
• Occur at strike slip faults
• Caused by shear forces
• Movement results in
earthquakes
- San Andreas fault
runs from Mexico
through California
17.2 Earthquakes and Volcanoes
What are Earthquakes?
• Vibrations from rocks sliding past one
another
• Occur mostly at plate boundaries; all three
types of the boundaries.
What happens?
• As plates move, pressure builds and the
rock eventually breaks at the focus. This
releases seismic waves in all directions
around the focus.
• There are three types of energy waves:
- longitudinal waves (primary waves or P
waves) move like a compressing spring or
a slinky. They are the fastest and first
waves that run through rock
- transverse waves (secondary waves or S
waves) move like a shaking rope and are
slower than P waves
- surface waves only move across the
Earth’s surface in a rolling motion; cause
more destruction
• The epicenter is
the point on the
earth’s surface
directly above the
focus.
Seismology – the study of
earthquakes
• Seismographs are machines that record
info about the three types of waves sent
out by an earthquake.
• Measure ground motion in three directions
(North to South, East to West, up and
down) using inertia.
• 1000 seismograph stations across the
world; three are needed to triangulate an
earthquake’s movement.
Richter Scale
• a measure of the energy released at the
focus of an earthquake
• the magnitude is recorded from 2.0 to 10
Volcanoes
• Any opening, or vent, through which
magma reaches the Earth’s surface.
• Magma = molten rock under the surface of
the earth
• Lava = magma has reached the surface
• Tephra = materials of all types that erupt
from a vent; ash, cinder
The makeup of the magma and type of
eruption determines the type of volcano
Shield Volcanoes
• Magma is rich in iron and magnesium which
flows easily and far
• Several mild eruptions
• Creates many layers of lava into a gently
sloping mountain
• Ex: Mauna Lao in Hawaii
Composite Volcanoes
• Magma is rich in silica; thicker; filled with
gas bubbles
• Gases cause alternating lava flows then
an explosion of cinders and ash
• Creates alternating layers of ash, cinders,
and lava
• Ex: Mount St. Helens
• Seamount volcanoes are found
underwater and resemble composites
Cinder Cones
• Smallest and most abundant volcanoes
• Large amounts of gas in the magma which
causes violent eruptions
• Made of layers of cinders
• Tend to be active for a short time than
become dormant
Volcanoes occur mainly at plate
boundaries
Convergent – as plates sink under during
subduction, the crust is melted and
magma rises to the surface
- Ring of Fire – circle
of volcanoes around
the Pacific Ocean
• Divergent – magma rises between two
divergent plates
- Iceland is a volcanic island that is
continually growing at its center
Volcanoes occur at hot spots
• Mantle plumes rise from deep in the
mantle and can break through weaker
spots of the oceanic crusts called hot
spots.
• Lava and ash build up where magma
broke through can build up to an island
like the Hawaiian Islands which form a trail
or island chain
Hawaiian Islands – Hot spot