2. SEISMIC WAVES
BODY WAVES:
• P-waves: faster, compressional,
can travel through any type of material,
including fluids
• S-waves: slower, transverse, can travel
only through solids
SURFACE WAVES: Rayleigh
and Love waves
density temperaturemelting point
3. Geochemistry model
Compositional layers
The Earth is made up of 3
main layers:
Crust:
- Continental (10-70Km) less
dense and older. Granitic
- Oceanic (5-10 km) more
dense and younger. Basaltic.
Covers two-thirds of Earth's
solid surface
Mantle (2900 km):
- Silicate rocks. Peridotite
- Flows
- D" layer.
Core: Fe-Ni
- Outer core, liquid (2900- 5100 km)
- Inner core, solid (5100- 6370 km)
-
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t
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4. Geochemistry model
Compositional layers
Schematic view of the interior of
Earth.
Layers:
1.Continental crust
2.Oceanic crust
3.Upper mantle
4.Lower mantle
5.Outer core
6.Inner core
Discontinuities
A: Mohorovičić discontinuity
B: Gutenberg discontinuity
C: Lehmann discontinuity.
5. Dynamic model
Mechanical layers
Lithosphere:
- Crust and the uppermost mantle.
It can be identified on the basis of its
mechanical properties.
- The lithosphere is subdivided into tectonic
plates.
6. Continental drift & Plates tectonics
- Alfred Wegener (1880-1930) was a German
meteorologist and geologist who proposed the theory
of continental drift as a unified theory.
- In his book, "Origin of Continents and
Oceans," he calculated that 200 million years
ago the continents were originally joined
together, forming a large supercontinent.
- He named this supercontinent Pangaea.
7. 1. Continents seemed
to fit together, not at
the continuously
changing shoreline,
but at the edge to
their continental
shelves.
CONTINENTAL DRIFT
EVIDENCES
GEOGRAPHICAL
8. 2. Fossil evidence, similar plant and animal fossils are
found around the shores of different continents,
suggesting that they were once joined.
P
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O
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10. 4. Geological fits:
When the geology of
eastern South
America and
West Africa was
mapped it
revealed that
ancient rock
outcrops (cratons)
over 2,000 million
years old were
continuous from
one continent to
GEOLOGICAL
11. 5. Tectonic fit. Fragments of an old fold mountain
belt between 450 and 400 million years ago are
found on widely separated continents today.
Pieces of the Caledonian fold mountain belt are
found in Greenland, Canada, Ireland, England,
Scotland and Scandinavia.
13. TOWARD PLATE TECTONICS
The Seafloor
• Mid ocean ridge
New ocean floor is created on ridges
Mid-ocean ridge is an underwater mountain system:
- Having a valley known as a rift, running along its
spine, as wide as 10–20 km.
- It wraps around the globe for more than 65,000 km.
- The average depth to the crest of the ridge is 2,5 km
- Oceanic ridges are split up by transform faults and
fracture zones.
14. • Ocean trenches
Ocean trenches are the deepest parts of the ocean.
An ocean trench is a long, deep depression in the ocean floor near continental
shelves. Others are found near chains of volcanic islands, called volcanic arcs.
15. Harry Hess published ‘The History of Ocean
Basins' in 1962, in which he outlined a
theory that could explain how the continents
could actually drift. This theory later became
known as ‘Sea Floor Spreading'.
7. Seafloor spreading.
He suggest that oceans grew from their centres, with
molten material (basalt) up from the Earth’s mantle along
the mid ocean ridges. This created new seafloor which
then spread away from the ridge in both directions.
17. Evidence of seafloor spreading
1. Frederick Vine and Drummond Matthews (1963) noticed there
was a symmetrical pattern of magnetic stripes on either
side of the mid ocean ridges.
This suggested that the ocean floor was created at the mid ocean
ridges, then was split in half by later activity and pushed sideways.
2. Basalts stripes were found to be the same age at similar
distances away from the ridge on each side.
A. Magnetic stripes
18. Lavas (igneous rocks)
are progressively
buried by sediments as
the seafloor spreads
away from the ridges.
So the thickness of
sediment on the
oceanic crust increases
with the age of the
crust. Oceanic crust
adjacent to the
continents can be
deeply buried by
several kilometres of
sediment.
B. Sediment thickness
19. 7.Earthquackes
The map above shows the distribution of
earthquakes with magnitudes greater than 5.0
that occurred between 1965 and 1995.
20. 7.Volcanoes
In the map below, each triangle represents the location of
a recently active (on a geologic time scale) volcanoes.
22. TECTONIC PLATES
The surface of the Earth (lithosphere) is broken up into large plates:
tectonic plates o lithospheric plate.
Earth surface is divided into 7 major and 8 minor plates
23. • Divergent
Plates move away from each other
Mid-Ocean ridges
TYPES OF
PLATE
BOUNDARY
• Convergent
Plates move towards each other
Trenches
• Transform
Plates slide past each other
Transform faults
24. • Spreading ridges
– Volcanic activity.
– Submarine earthquakes (related with transform faults)
DIVERGENT BOUNDARIES
– Where a divergent boundary forms:
• on a continent it is called a RIFT.
• under the ocean it is called an OCEAN RIDGE
25. • Iceland has a divergent plate
boundary running through its
middle
Iceland: An example of continental rifting
26. Rift valley: An example of continental rifting
East African Rift System
27. The Rift Valley and
Associated
Features
• Eventually a new plate
will form
• The sea will flood the
valley and connect to
the Red Sea
28. CONVERGENT BOUNDARIES
SUBDUCTIONThere are three styles of convergent
plate boundaries
– Oceanic-continent crust collision
– Ocean-ocean collision
– Continent-continent collision: forms
mountains
The Earth's unchanging size implies that the crust must be
destroyed at about the same rate as it is being created
Subduction:
- Trench
- Volcanic arc
- Volcanoes
- Mountain belt
29. SUBDUCTION:
1. The oceanic crust which is thinner and more dense than the
continental crust, sinks below the continental crust.
2. It dehydrates and releases water into the overlying
mantle wedge, inducing partial melting in the
overlying mantle wedge, new melt which rises up into
the overlying continental crust forming volcanoes.
Subduction is a way of recycling
the oceanic crust
33. TRANSFORM BOUNDARIES
The zone between two plates
sliding horizontally past one
another.
Most transform faults are found
on the ocean floor. However, a
few occur on land, for example
the San Andreas.
They commonly offset the active
spreading ridges, producing zig-
zag plate margins.
They are generally defined by
shallow earthquakes.
38. Magmas generated by
mantle rise up below t
erupt on Earth's surfac
systems, the longest m
the world. When the m
basalt, the planet's mo
the basis for oceanic c
39.
40. Look next links about structure and
composition of Earth
• The Geological Society: Plate tectonics
• Smithsonian National Museum of Natural
History: The dynamic Earth.
Notas do Editor
In plate tectonics, a divergent boundary is a linear feature that exists between two tectonic plates that are moving away from each other. These areas can form in the middle of continents or on the ocean floor.
As the plates pull apart, hot molten material can rise up this newly formed pathway to the surface - causing volcanic activity.
Presenter: Reiterate the process by going through the diagram, including the presence of mantle convection cells causing the plates to break apart and also as a source for new molten material.
Where a divergent boundary forms on a continent it is called a RIFT or CONTINENTAL RIFT, e.g. African Rift Valley.
Where a divergent boundary forms under the ocean it is called an OCEAN RIDGE.
Iceland is located right on top of a divergent boundary. In fact, the island exists because of this feature.
As the North American and Eurasian plates were pulled apart (see map) volcanic activity occurred along the cracks and fissures (see photographs).
With many eruptions over time the island grew out of the sea!
Question: Why don’t we have islands like Iceland where ever we get an Ocean Ridge?
Answer: Scientists believe that there is a large mantle plume (an upwelling of hot mantle material) located right underneath where Iceland has formed. This would mean that more material would be erupted in the Iceland area compared with if there was just the divergent boundary without the plume underneath it.
Convergent boundaries are where the plates move towards each other.
There are three types of convergent boundary, each defined by what type of crust (continental or oceanic) is coming together.
Therefore we can have: continent-continent collision, continent-oceanic crust collision or ocean-ocean collision….
At a convergent boundary where continental crust pushes against oceanic crust, the oceanic crust which is thinner and more dense than the continental crust, sinks below the continental crust.
This is called a Subduction Zone.
The oceanic crust descends into the mantle at a rate of centimetres per year. This oceanic crust is called the “Subducting Slab” (see diagram).
When the subducting slab reaches a depth of around 100 kilometres, it dehydrates and releases water into the overlying mantle wedge (Presenter: explain all of this using the diagram).
The addition of water into the mantle wedge changes the melting point of the molten material there forming new melt which rises up into the overlying continental crust forming volcanoes.
Subduction is a way of recycling the oceanic crust. Eventually the subducting slab sinks down into the mantle to be recycled. It is for this reason that the oceanic crust is much younger than the continental crust which is not recycled.
Example:
India used to be an island, but about 15 million years ago it crashed into Asia (see map).
As continental crust was pushing against continental crust the Himalayan mountain belt was pushed up.
“Mountains” were also pushed down into the mantle as the normally 35 km thick crust is approximately 70 km thick in this region.
Mt Everest is the highest altitude mountain on our planet standing 8,840 metres high. This means that below the surface at the foot of the mountain the crust is a further 61 km deep!!
