Download Ch. 21 - Tri-City

5/18/15 22.1 Earth’s Interior and Plate Tectonics
Chapter 21: Planet Earth
PHYSICAL SCIENCE - DOERFLER
—  Key Ideas:
¡  How is Earth’s interior structured?
¡  How has the appearance of Earth changed over time?
¡  What geologic features are common near tectonic plate
boundaries
22.1 Key Terms
—  Crust
What is Earth’s Interior Like
—  Earth’s interior is made up of several distinct
compositional layers
—  Mantle
—  Crust – topmost layer of Earth
¡  Made up of hard, solid rock
¡  Relatively thin and makes up only 1% of Earth’s total mass
¡  Crust beneath ocean is called oceanic crust (3.1 to 4.9 mi. thick
¡  Continental crust is thicker and less dense than oceanic crust
—  Core
—  Lithosphere
—  Plate tectonics
—  Subduction
—  Fault
÷  Avg.
thickness of 12 to 25 mi (deepest beneath high mountains,
can be 70 km deep)
Earth’s Interior
—  Mantle – layer of rock beneath crust
¡  Denser than crust
¡  Nearly 1,800 mi thick
¡  Makes up nearly 80% of Earth’s total volume
—  Scientists have never drilled to mantle
—  Can only infer consistency from geologic events, like
earthquakes and volcanoes
Earth’s Interior
—  Gets warmer with depth
—  Caves are cool because the air and rock are shielded
from the sun
—  Travel deeper in to mine, temperatures rise
—  South African gold mines
¡  Can be up to 2 mi deep
¡  Temperatures reach 120 degrees F
—  Caused by energy from Earth’s interior
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—  Mantle believed to be much thicker than crust
Earth’s Core
—  Composition is solid
—  Nearly as hot as surface of the sun
¡  Believed to be anyways
—  Outermost layer is rigid
—  Core – center of the Earth, made of mainly iron and
—  Going deeper a few hundred kilometers
¡  Mantle is extremely hot and under incredible pressure
—  Geologists think the mantle is plastic
¡  Oft, and easily deformed like gum
nickel
—  Has two layers
¡  Inner core – solid metal
¡  Outer core – liquid metal surrounding inner core
Earth’s Core
—  Metals contained in core would boil on Earth’s
surface
—  Iron – boils at 4,982 degrees F
—  Nickel – boils at 4,950 degrees F
—  Pressure is so great on outer core form mantle and
crust, substances in outer core can’t turn to gas form
—  Similarly, atoms in inner core are pushed together
due to pressure, remains solid at temps above boiling
point
Radioactivity and Internal Temperatures
Plate Tectonics
—  Interior contains radioactive isotopes
—  1900 – German scientists, Alfred Wegener, saw that
—  Nuclei decay and release more energy
¡  Mainly uranium, thorium, and potassium
eastern South America and western African coasts
appeared to fit together
—  Wegener hypothesized that all of the
continents might have been part of one
landmass in the past before they drifted apart
—  Isotopes are quite rare
¡  Earth is so big, there is enough to provide huge amounts of
energy as heat
¡  This energy is major factor in Earth’s high internal
temperature
¡ 
Called this mass of land Pangaea
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—  Wegener supported his idea with evidence
¡  Said 200 million years ago same kinds of animals lived on
continents that are now oceans apart
—  He argued that the animals could not have evolved
on separate continents
—  Fossil of Mesosaurus found in Brazil has also been
found in western Africa
Evidence of Wegener’s
—  Wegener’s idea was called continental drift
¡  Theory that Earth’s surface is made up of large moving plates
—  Idea was largely ignored until 1960s
¡  Discovered structures on ocean floors
—  New technology provided images of “bands”
¡  Bands of rock have alternating magnetic polarities
¡  Bands differ from another in the magnetic alignment of the
rocks that they contain
Alignment of Oceanic Rocks
—  Supports theory of moving plates
Alignment of Oceanic Rock
—  Molten rock pouring onto ocean floor
¡  Creates new rock and hydrothermal vents
—  Evidence based on these aligned rocks (bands)
¡  Earth’s magnetic field has reversed many times in history
¡  North pole becomes south pole
—  Iron minerals (magnetite) in magma align with
—  On average, occurs once every 200,000 years
Earth’s magnetic field
Magnetic regions become fixed as it cools
¡  Creates permanent record of Earth’s magnetic field
¡ 
—  Bands are symmetrical on either side of Mid-Atlantic
ridge
—  Rocks are youngest near the ridge, older away from it
¡  Suggests the crust was moving away from plate boundary in
both directions
Earth Has Plates
—  These plates move over mantle
—  Lithosphere – approximately 100 km (60 mi) thick
and made up of the crust and upper portion of the
mantle
¡ 
Made up of 7 large pieces (and several smaller ones) called
tectonic plates
—  Continents are embedded into these plates
¡  Plates move in relation to one another
¡  This theory is called plate tectonics
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How Do They Move?
—  These plates move at a rate of 1 to 10 cm per year
—  Scientists really don’t know how they do it
—  May seem slow
¡  But they have moved considerable distances
¡  Have been doing this for millions of years
—  Boundaries do not always coincide with continental
boundaries
—  Some plates move toward each other
¡  Some move away from each other
How Do They Move?
Plate Boundaries
—  One hypothesis
¡  Suggests plate movement results from convection current in
asthenosphere – hot, plastic portion of the mantle
—  Plate tectonics helps scientists
¡  Study and predict volcanic eruptions
¡  Has provided information on earthquakes
—  Plates of lithosphere float on asthenosphere
¡  Rock is “bubbling” like hot oatmeal and moves the plates
slowly
—  Volcanoes and earthquakes most often occur
—  Others believe this method does not provide enough
energy to move the plates
¡ 
They say that plates are driven by the force of gravity acting on
their own massive weight
where tectonic plates come together
—  At plate boundaries, many other geologic
features, such as mountains and rift valleys,
can also occur
Mid-Oceanic Ridges
—  They are the result of divergent boundaries
¡  Occurs where two plates move apart and create a gap between
them
¡  Hot rocks rise from the asthenosphere
¡  Cools to form lithosphere rock
¡  Two diverging plates pull the newly formed lithosphere from
the gap
¡  Drop in pressure causes asthenosphere to partially melt,
forming magma
¡  This then forms new oceanic crust
Mid-Oceanic Ridges
—  These are mountain ranges at divergent boundaries
in oceanic crust
¡ 
Formed by magma rising to Earth’s surface and cooling
—  Along center of mid-ocean ridge is rift valley
¡  Narrow valley that forms where plates separate
—  Most studied is Mid-Atlantic Ridge
¡  Most of this ridge is under water
¡  A portion of it is at sea level in Iceland
¡  Run roughly down center of Atlantic Ocean, from Arctic Ocean
to southern tip of South America
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—  Oceanic plates dive beneath continental plates at
convergent boundaries
—  Newer lithosphere is created at divergent boundaries
¡  Older lithosphere is destroyed at convergent boundaries
¡  Example: Andes mountains in South America
—  Subduction – denser lithosphere of oceanic plate
sinks into the mantle, dragging oceanic crust with it
Subduction Zones
Subduction and Volcanoes
—  Ocean trenches, mountains, and volcanoes are
—  Chains of volcanic mountains form along overriding
formed at subduction zones
—  Ocean trenches: form along boundary between
oceanic plates or oceanic and continental plates
—  Water-bearing rock and sediment are heated by
Can be very deep
¡  Marianas trench is deepest (deepest point is 6.8 mi beneath
ocean surface)
¡ 
plate at subduction zones
surrounding mantle
Releases water into overlying mantle
Water lowers melting point of rock at high pressures
¡  Magma forms and rises into crust
¡ 
¡ 
—  Magma cools and low-density rocks build up over
millions of years, creating high mountain chains and
plateaus
Islands
—  Sometimes, convergent boundary occurs when two
plates of oceanic lithospheres collide
—  Denser of two plates sinks beneath other
¡ 
Forms deep oceanic trenches
—  Fluids released from subducted plate causes mantle
rock to melt, forming magma
—  Magma rises, forming island arc
¡ 
Mountains
—  Continental plates that collide produce mountain
ranges
¡ 
Himalayas – tallest range in the world
—  Grow in both width and height
—  Since they move so slowly, they only grow several
centimeters a year
Chain of volcanic islands
5 5/18/15 Transform Fault Boundaries
—  Plate movement can cause cracks in lithosphere
Transform Fault Boundaries
—  Transform fault boundary – when rock moves
—  Once break occurs, rock continues to move and
horizontally at faults along plate boundaries
scrapes nearby rock
—  Fault – crack where rock moves in this manner
—  Plate movement at these areas is one cause of
¡ 
Can occur in any area where forces are great enough to break
rock
earthquakes
¡ 
San Andreas fault in California
21.2 What are Earthquakes
—  You can see that earthquakes occur mostly at
the boundaries of tectonic plates where the
plates move with respect to one another
—  Earthquakes occur at plate boundaries
¡ 
Energy is released as seismic waves
Earthquakes and Waves
—  Energy is transferred by waves
¡  Measured as shock waves
—  Three types of waves are generated
¡  Longitudinal – originate from focus
÷  Move
faster through rock than others
waves to reach recording stations
÷  For these reasons, they are known as primary waves, or P waves
÷  Travel by compressing Earth’s crust in front of it and stretching
the crust back of it
—  Focus – exact point inside Earth where earthquake
÷  First
originates
—  Epicenter – point on surface directly above the
focus
¡ 
¡ 
Seismic waves move in all directions
Damage here is usually greatest
Transverse Wave
—  Move more slowly than longitudinal waves
—  These are called secondary or S waves
—  Motion is similar to when is shaken up and down
Surface Wave
—  P and S waves spread out from focus like light
leaving a light bulb
—  Surface wave – moves only across Earth’s surface
—  The surface bends and reshapes as it shakes
—  Surface waves cause more destruction than P or S
waves
¡ 
Act in a circular motion
—  P and S shake buildings up and down and back and
forth in high frequency
¡ 
Surface waves have rolling motion and longer frequncy
6 5/18/15 Measuring Earthquakes
—  Because energy from earthquakes is
transferred by waves, scientists can measure
the waves to learn about earthquakes and
about the interior of Earth through which the
waves travel
—  Scientists measure the power of each earthquake
¡ 
And the amount of damage the earthquake causes
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