17.1
Structure of Earth
• Distinct properties define Earth’s three main layers:
crust, mantle, and core.
• The crust is the rigid outer layer that makes up the
continents and sea floors.
• The lithosphere is a region formed by the crust and
the rigid outer layer of the mantle.
• The asthenosphere is the fluid-like layer of mantle
beneath the lithosphere.
• Earth’s centre has two layers:
a liquid outer core and
a solid inner core.
VOCABULARY
crust
mantle
lithosphere
asthenosphere
core
outer core
inner core
17.1
Structure of Earth
• Scientists obtain information about the structure of
Earth through direct and indirect observations.
• Direct observations include analyzing rock directly
from Earth’s interior.
e.g. Taking rock samples, collected
from holes drilled into Earth during
e.g. Sampling lava flows, to
oil/mining exploration
compare minerals in magma to
other minerals in rock at or near
This
the Earth’s surface
sucks!
Continental rock
Ocean floor rock
17.1
Structure of Earth
• Scientists obtain information about the structure of
Earth through direct and indirect observations.
• Indirect observations include measuring
gravitational force differences around the globe to
infer the density of material beneath Earth’s surface.
 Measuring Earth’s magnetic field
 Measuring the local strength of gravity
17.2
Evidence of a Dynamic Earth
• The surface of Earth moves constantly.
• The lithosphere is divided into massive tectonic plates
that are pushed and pulled over the asthenosphere.
• Continental drift theory argues that the continents
have moved slowly since Earth formed.
• Fossil and geological evidence support
the continental drift theory.
• At one time, the continents formed
Pangaea, a single huge landmass.
VOCABULARY
tectonic plate
continental drift theory
paleoglaciation
17.2 Continental Drift Theory
 Wegener’s continental drift hypothesis
stated that the continents had once been
joined to form a single supercontinent.
• Wegener proposed that the supercontinent,
Pangaea, began to break apart 200 million years
ago and form the present landmasses.
17.2
Breakup
of
Pangaea
17.2 Continental Drift Theory
 Evidence
• The Continental Puzzle (see previous slides)
• Matching Fossils
- Fossil evidence for continental drift includes several similar
fossils of the same organisms found on different
landmasses.
17.2 Continental Drift Theory
 Evidence
• Rock Types and Structures
- Rock evidence for continental exists in the
form of several mountain belts that end at one coastline,
only to reappear on a landmass across the ocean.
17.2 Continental Drift Theory
 Evidence
• Ancient Glacial Evidence
- Glaciers scratch grooves into Earth’s crust as they move,
indicating their direction of movement. Groove patterns
match up with Wegener’s theory of how Pangaea broke
apart.
17.2
Glacier
Evidence
17.2 Continental Drift Theory
The idea of continental drift was met with a
lot of opposition.
The main problems with it were:
The age of the Earth would have to be billions
of years, not just thousands of years. At the
time, scientists believed it was only 6000 to
400 million years old.
More importantly, Wegner could not explain
HOW the plates moved.
17.3
New Evidence of a Dynamic Earth
• Sea-floor spreading occurs at mid-ocean ridges and
subduction occurs at deep ocean trenches.
VOCABULARY
mid-ocean ridge
sea-floor spreading
ocean trench
• Heat within Earth creates convection currents in the mantle that help
move the plates.
• Radioactive dating of core samples confirmed evidence that the sea
floor is older the further it is from the ridges.
• Magnetic striping patterns in sea floor rock are similar on both sides
of an oceanic ridge, suggesting new ocean floor is being created at
the ridge.
17.3
New Evidence of a Dynamic Earth
• During World War II, Henry Hess was in charge of a submarine that
discovered mid-ocean ridges. A mid-ocean ridge is a massive
undersea mountain range that runs along the bottom of the ocean.
Since this initial discovery, a total length of 75,000 km of mid-ocean
ridges has been discovered.
17.3
New Evidence of a Dynamic Earth
• Hess proposed a process called sea-floor spreading to
explain why the sea floor was widening at mid-ocean ridges.
Molten magma from the Earth’s core rises and fills the ridges.
• Future work suggested the sea floor was cycled back into the
Earth to create ocean trenches.
17.3
New Evidence of a Dynamic Earth
• Radioactive dating (using half-life!) suggests age of the rock
further from the mid-ocean ridge is older.
• Magnetic lava that has cooled to form rock has taken on the
alignment of the magnetic field of the Earth. Bands that show
opposite magnetization along the ocean floor suggest the rock
has formed at different geological times.
17.3
Polarity of the Ocean Crust
17.3
New Evidence of a Dynamic Earth
Some Videos:
Lava sampling = http://www.youtube.com/watch?v=rxJeY4C6SL4
Magnetic evidence = http://www.youtube.com/watch?v=BCzCmldiaWQ
Most incredible volcano expedition EVER! =
http://www.youtube.com/watch?v=egEGaBXG3Kg
17.3
New Evidence of a Dynamic Earth
• All of this evidence provided a convincing case that sea-floor
spreading is a real phenomenon – the Earth’s lithosphere is in
motion!
17.4
Theory of Plate Tectonics
• The theory of plate tectonics states that the
lithosphere is divided into 12 large sections (plates)
and about 20 smaller ones.
• These plates ‘float’ on the more dense, fluid-like
asthenosphere
VOCABULARY
theory of plate
tectonics
divergent boundary
rift valley
convergent boundary
subduction zone
oceanic–oceanic
convergent boundary
oceanic–continental
convergent boundary
continental–continental
convergent boundary
transform boundary
strike–slip fault
17.4
Theory of Plate Tectonics
• Three types of boundaries exist where tectonic
plates meet. These create Earth’s geological
features and events.
• Ridges, rifts, volcanoes and
earthquakes are created at divergent
boundaries, where plates are moving
away from each other.
• At convergent boundaries (where
plates move toward each other), we
find mountains, trenches, subduction
zones, volcanoes, and earthquakes,
depending on the types of plates
involved.
• Earthquakes and strike-slip faults are
created along transform boundaries,
where plates move past each other in
opposite directions.
VOCABULARY
theory of plate
tectonics
divergent boundary
rift valley
convergent boundary
subduction zone
oceanic–oceanic
convergent boundary
oceanic–continental
convergent boundary
continental–continental
convergent boundary
transform boundary
strike–slip fault
17.4
Theory of Plate Tectonics
Three Types of
Plate Boundaries
17.4
Divergent Boundaries
 Continental Rifts
• When spreading centers develop within a continent, the
landmass may split into two or more smaller segments,
forming a rift.
East African
Rift Valley
17.4
review
Convergent Boundaries
 A subduction zone occurs when one oceanic
plate is forced down into the mantle beneath a
second plate.
 Oceanic-Continental
• Denser oceanic slab sinks into the asthenosphere.
• Pockets of magma develop and rise.
• Continental volcanic arcs form in part by volcanic activity
caused by the subduction of oceanic lithosphere beneath a
continent.
• Examples include the Andes, Cascades, and
the Sierra Nevadas.
17.4
Convergent Boundaries
Oceanic-Continental
Convergent Boundary
17.4
Convergent Boundaries
 Oceanic-Oceanic
• Two oceanic slabs converge and one descends
beneath the other.
• This kind of boundary often forms volcanoes on the
ocean floor.
• Volcanic island arcs form as volcanoes emerge
from the sea.
• Examples include the Aleutian, Mariana, and
Tonga islands.
17.4
Convergent Boundaries
Oceanic-Oceanic
Convergent Boundary
17.4
Convergent Boundaries
 Continental-Continental
• When subducting plates contain continental
material, two continents collide.
• This kind of boundary can produce new
mountain ranges, such as the Himalayas.
17.4
Convergent Boundaries
Continental-Continental
Convergent Boundary
17.4
Convergent Boundaries
‘Collision of India and Asia’
17.4 Transform Boundaries
 At a transform fault boundary, plates grind past
each other without ‘destroying’ the lithosphere.
 Transform faults
• When the faults ‘overcome’ the
friction holding them in place, the
‘slip’ causes earthquakes
17.4 Transform Boundaries
Transform Fault
Boundaries
18.1
Causes and Effects of Plate Movement
• Scientists now believe that three forces work together
to move the tectonic plates:
1. mantle convection
2. ridge push
3. slab pull
VOCABULARY
mantle convection
ridge push
slab pull
•Forces at plate boundaries produce landscape features such as mountains,
trenches, and island arches.
18.1
Causes and Effects of Plate Movement
• Uplifted mountains, volcanic belts, and island arcs
are found parallel to convergent boundaries.
• Ocean trenches lie along convergent boundaries.
• Mid-ocean ridges and rift valleys mark divergent
boundaries.
VOCABULARY
mantle convection
ridge push
slab pull
18.1
Causes and Effects of Plate Movement
 Scientists believe tectonic plates are moved by three forces:
o Mantle convection is the idea that there is a treadmill-like
current in the asthenosphere that moves the crust that sits
above it. (hot magma rises in one area and cool magma sinks
in another)
• Convective
flow is the
motion of matter
resulting from
changes in
temperature.
18.1
Causes and Effects of Plate Movement
Convection causes Ridge Push and Slab Pull...
 Scientists generally agree that convection
occurring in the mantle is the basic driving
force for plate movement.
Ridge
Push
Slab
Pull
Ridge
Push
Slab
Pull
Ridge
Push
18.1
Causes and Effects of Plate Movement
 Scientists believe tectonic plates are moved by three forces:
o Ridge Push is the idea that the hot magma pushing up the
mid-ocean ridges is also pushing the ocean plates apart.
18.1
Causes and Effects of Plate Movement
 Scientists believe tectonic plates are moved by three forces:
o Slab Pull is when one plate slides underneath the other. The
subduction zone causes the bottom plate to be pushed back
into the mantle and this pulls the plate away from the midocean ridge.
18.1
Causes and Effects of Plate Movement
 Mountains form at convergent boundaries.
o Ocean – continent boundary causes mountains to form along
the coast. These often push land that was once underwater up
high causing ocean fossils to be found in the mountains.
18.1
Causes and Effects of Plate Movement
 Mountains form at convergent boundaries.
o Continent – continent boundary causes mountains to form
from the uplifting of the two continent edges. This can also
cause former coast lines and marine fossils to be found in the
mountains.
18.1
Causes and Effects of Plate Movement
• Rifts form at
divergent
boundaries.
• If this happens on land
a rift valley forms.
• Often volcanoes also
form along these rifts
since the crust is
thinner.
18.1
Causes and Effects of Plate Movement
 Trenches can form at a convergent boundary.
o They occur where one plate is subducted under the other. If
there are mountain chains along a coast then you know that a
few hundred kilometers out into the ocean that there must be a
trench.
18.1
Causes and Effects of Plate Movement
 Trenches can form at a convergent boundary.
o When trenches form at oceanic-oceanic convergent
boundaries, a deep ocean trench is also formed
o The Mariana trench is Earth’s deepest trench, still sinking, and
is ~ 11 km below the surface of the ocean!
18.2
Geological Events
• Volcanoes are produced over lithosphere cracks and
mantle hot spots.
• Hot spots are rising plumes of hot mantle magma. As
tectonic plates move over the hot spot, a chain of
progressively younger volcanoes is formed opposite
to the direction of plate movement.
• Volcanic island chains and volcanic belts are created
on the overriding plate, parallel to the direction of the
moving oceanic plate.
VOCABULARY
volcano
hot spots
volcanic belt
volcanic island arc
earthquakes
fault
focus
epicentre
shallow-focus
intermediate-focus
deep-focus
seismic wave
body wave
primary wave (P–wave)
secondary wave
(S–wave)
surface wave
18.2
Geological Events
• Earthquakes result when tectonic
forces overcome the friction
between plates. Earthquakes are
categorized based on how far
beneath the surface the focus is
located.
• The sudden movement of the lithosphere during
an earthquake sends seismic waves (vibrations)
through Earth.
• Primary and secondary body
waves travel through Earth,
starting at the focus, the site
of the earthquake within Earth.
• Surface waves travel along
the outside of Earth. They
cause the most destruction.
VOCABULARY
volcano
hot spots
volcanic belt
volcanic island arc
earthquakes
fault
focus
epicentre
shallow-focus
intermediate-focus
deep-focus
seismic wave
body wave
primary wave (P–wave)
secondary wave
(S–wave)
surface wave
18.2
Geological Events
 A Volcano marks a spot where there is a crack in the lithosphere.
a. They can form at mid-ocean ridges where plates are being pulled
apart, and magma rises up.
18.2
Geological Events
 A Volcano marks a spot where there is a crack in the lithosphere.
b. They can also form about 200 km from a trench, when trapped
water becomes steam and melts through the crust. This forms
volcano belts on continents and volcanic island arcs in the
ocean.
18.2
Geological Events
 A Volcano marks a spot where there is a crack in the lithosphere.
b. They can also form about 200 km from a trench, when trapped
water becomes steam and melts through the crust. This forms
volcano belts on continents and volcanic island arcs in the
ocean.
18.2
Geological Events
 A Volcano marks a spot where there is a crack in the lithosphere.
c. They can form above a hot spot in the mantle, caused by heated
concentrations of radioactive substances near the Earth’s core,
causing hot columns of rising mantle that melt through the
crust.
18.2
Geological Events
 Hot Spots
• A hot spot is a concentration of heat in the
mantle capable of producing magma, which rises
to Earth’s surface; The Pacific plate moves over
a hot spot, producing the Hawaiian Islands.
• Hot spot evidence supports that the plates move
over the Earth’s surface.
18.2
Geological Events
Hot Spot
18.2
Geological Events
 Earthquakes happen when two tectonic
plates move past each other. The plates
do not have smooth edges so there is
tremendous friction that keeps them from
sliding...until enough pressure builds up,
and then the plates move suddenly. This
sudden movement is what we call an
earthquake.
 The boundary where the two plates move
past each other is called a fault.
18.2
Geological Events
 The location of the earthquake is called the focus. The focus can be
deep in the earth or relatively shallow down. It depends which part
of the two plates slide past each other.
 The epicenter is the spot above the focus, on the surface of the
Earth. Earthquakes are categorized according to the depth of their
focus.
18.2
Geological Events
 Plotting epicenters on a map (this data: 1980-1996) clearly outline
some of the plate boundaries.
18.2
Geological Events
 Earthquakes travel through the Earth in wave form. These are called body waves
and there are two kinds:
a. Primary waves (P-waves) are waves that travel parallel to the direction of the
wave. They are a ‘compression’ wave. They are like pulling on a spring and
letting the end go. It stretches and compresses. They can travel through all
parts of the Earth.
b. Secondary waves (S-waves) are waves that travel sideways (shear) to the
direction of the wave. They are like a slithery snake. They can only travel
through the mantle and the crust. They cannot travel through the core.
18.2
Geological Events
 The most destructive kind of earthquake wave is called a surface
wave. They travel along the surface of the crust like a wave on the
ocean. They are what destroy buildings, bridges, etc.
 Scientists use instruments called seismographs to measure the
strength of earthquakes. They use the Richter scale. Each increase of
1 on the scale means that the earthquake is 10 times as strong. An
earthquake of 5 is 10 times stronger than a 4.