2. Two Ways of Looking at EarthTwo Ways of Looking at Earth
SeismicSeismic
discontinuitiesdiscontinuities
reveal the crust,reveal the crust,
mantle, and coremantle, and core
and show that theyand show that they
have differenthave different
chemicalchemical
compositionscompositions..
Seismic studiesSeismic studies
reveal much aboutreveal much about
thethe physicalphysical
naturenature of theof the
interior, revealing ainterior, revealing a
solid inner core, asolid inner core, a
liquid outer core, aliquid outer core, a
soft asthenosphere,soft asthenosphere,
and a rigidand a rigid
lithosphere.lithosphere.
Figure 01.05: The internal structure of Earth
consists of layers of different composition and
layers of different physical properties.
19. Evidence for Continental DriftEvidence for Continental Drift
Jigsaw Puzzle fit ofJigsaw Puzzle fit of
continentscontinents
Alfred Wegener
during Greenland
expedition
20. More evidenceMore evidence
Matching fossils onMatching fossils on
continents nowcontinents now
located thousandslocated thousands
of miles apart.of miles apart.
Example =Example =
Mesosaurus, aMesosaurus, a
freshwater reptilefreshwater reptile
Many othersMany others
25. More evidenceMore evidence
Matching geologicMatching geologic
structuresstructures
including:including:
–Mountain chainsMountain chains
–Ore depositsOre deposits
–Same rocks ofSame rocks of
same agesame age
26. More evidenceMore evidence
Climate changeClimate change
evidenceevidence
– Glacial deposits atGlacial deposits at
current equatorcurrent equator
– Fossilized palmFossilized palm
trees in Greenlandtrees in Greenland
Map shows whyMap shows why
according to theaccording to the
placements ofplacements of
current continentscurrent continents
within Pangaeawithin Pangaea
27. Wegener not believedWegener not believed
Why? -Why? -
–What could possibly force theWhat could possibly force the
continents to move across thecontinents to move across the
ocean floor in this way. Theyocean floor in this way. They
would be crushed.would be crushed.
–He was a meteorologist,He was a meteorologist,
not a geologistnot a geologist
28. Developments 1950s and 1960sDevelopments 1950s and 1960s
World War II submarines using sonarWorld War II submarines using sonar
found mountains under the oceans –found mountains under the oceans –
the mid-ocean ridgesthe mid-ocean ridges
Sea floor drilling showed rocksSea floor drilling showed rocks
younger than expected and youngestyounger than expected and youngest
towards the center of the mid-oceantowards the center of the mid-ocean
ridgeridge
Theory of seafloor spreadingTheory of seafloor spreading
suggested by Princeton professor Dr.suggested by Princeton professor Dr.
Harry HessHarry Hess
29. Indias role in continental driftIndias role in continental drift
36. Structure of the EarthStructure of the Earth
The Earth isThe Earth is
made up of 3made up of 3
main layers:main layers:
– CoreCore
– MantleMantle
– CrustCrust
Inner core
Outer core
Mantle
Crust
37. The CrustThe Crust
This is where we live!This is where we live!
The Earth’s crust isThe Earth’s crust is
made of:made of:
Continental Crust
- thick (10-70km)
- buoyant (less dense
than oceanic crust)
- mostly old
Oceanic Crust
- thin (~7 km)
- dense (sinks under
continental crust)
- young
38. SonarSonar
A device thatA device that
bounces soundbounces sound
waves offwaves off
underwater objectsunderwater objects
and then recordsand then records
the echoes of thesethe echoes of these
sound waves.sound waves.
Sonar mappedSonar mapped
mid-ocean ridges.mid-ocean ridges.
39. Great Puzzles in 1960Great Puzzles in 1960
Rock in oceans always basalt,Rock in oceans always basalt,
younger than 225 million yearsyounger than 225 million years
old.old.
Ocean crust thin; continental crustOcean crust thin; continental crust
thick & graniticthick & granitic
Rocks on continent varied, oldestRocks on continent varied, oldest
3.8 billion years old3.8 billion years old
Volcanoes & Earthquakes limited inVolcanoes & Earthquakes limited in
locationslocations
Atlantic & Pacific differentAtlantic & Pacific different
topography from land and fromtopography from land and from
each othereach other
40. 1963 – Harry Hess1963 – Harry Hess
Observed rocks near ridge youngerObserved rocks near ridge younger
than rocks at edge of ocean basinthan rocks at edge of ocean basin
Observed higher heat escaping fromObserved higher heat escaping from
Earth’s interior at ridge crest (riftEarth’s interior at ridge crest (rift
valley) than on sidesvalley) than on sides
Ridge appeared to be under tensionRidge appeared to be under tension
– like rift valley was two normal– like rift valley was two normal
faultsfaults
41. Proposed answer: Sea FloorProposed answer: Sea Floor
SpreadingSpreading
New sea floor created by insertion of lava inNew sea floor created by insertion of lava in
rift valleyrift valley
Pushes ridge apart in centerPushes ridge apart in center
Center is hot & expands; edges cold &Center is hot & expands; edges cold &
contract, sinkscontract, sinks
Old
Cold
Old
Cold
Young
Hot
Magma source
Mantle, mafic
42. Vine & Matthews Test & ConfirmVine & Matthews Test & Confirm
Hypothesis:Hypothesis:
–If rocks are being put into placeIf rocks are being put into place
in rift valley and cooling throughin rift valley and cooling through
Curie Point, then should seeCurie Point, then should see
symmetrical magnetic polaritysymmetrical magnetic polarity
anomalies on either side of theanomalies on either side of the
ridgeridge
ConclusionConclusion
–Found symmetrical magneticFound symmetrical magnetic
polarity anomalies on either sidepolarity anomalies on either side
45. Calculate Spreading RatesCalculate Spreading Rates
Age of point on sea floorAge of point on sea floor
Distance from ridgeDistance from ridge
Simply divide distance by date & getSimply divide distance by date & get
spreading ratespreading rate
Half rate – how fast crust is beingHalf rate – how fast crust is being
formedformed
Double half rate – how fastDouble half rate – how fast
continents are moving apartcontinents are moving apart
Typical half rates – 2 to 4 cm/yrTypical half rates – 2 to 4 cm/yr
47. Harry HessHarry Hess
An AmericanAn American
geologist whogeologist who
studied mid-oceanstudied mid-ocean
ridges.ridges.
He suggested thatHe suggested that
the ocean floorsthe ocean floors
move like conveyormove like conveyor
belts, carrying thebelts, carrying the
continents alongcontinents along
with them.with them.
48. At the mid-ocean ridge, moltenAt the mid-ocean ridge, molten
material rises from the mantle andmaterial rises from the mantle and
erupts. The molten material thenerupts. The molten material then
spreads out, pushing older rock tospreads out, pushing older rock to
both sides of the ridge. As theboth sides of the ridge. As the
molten material cools, it forms amolten material cools, it forms a
strip of solid rock in the center of thestrip of solid rock in the center of the
ridge. Then more molten materialridge. Then more molten material
flows into the crack.flows into the crack.
49. Sea-Floor SpreadingSea-Floor Spreading
Harry Hess proposed that new ocean floor
is formed at the rift of mid-ocean ridges.
The ocean floor, and the rock beneath it,
are produced by magma that rises from
deeper levels. Hess suggested that the
ocean floor moved laterally away from the
ridge and plunged into an oceanic trench
along the continental margin.
50. Evidence #1 - Molten MaterialEvidence #1 - Molten Material
The submersible,The submersible,
Alvin, foundAlvin, found
strange rocksstrange rocks
shaped like pillowsshaped like pillows
or like toothpasteor like toothpaste
squeezed from asqueezed from a
tube. Such rockstube. Such rocks
can form onlycan form only
when moltenwhen molten
material hardensmaterial hardens
quickly afterquickly after
erupting undererupting under
51. Evidence #2 - Magnetic StripesEvidence #2 - Magnetic Stripes
ScientistsScientists
discovered that thediscovered that the
rock that makes uprock that makes up
the ocean floor liesthe ocean floor lies
in a pattern ofin a pattern of
magnetizedmagnetized
“stripes”. They“stripes”. They
hold a record ofhold a record of
reversals in Earth’sreversals in Earth’s
magnetic field.magnetic field.
52. Evidence #3 - Drilling SamplesEvidence #3 - Drilling Samples
The GlomarThe Glomar
Challenger did aChallenger did a
drilling sample anddrilling sample and
found rocks thatfound rocks that
the farther awaythe farther away
from the ridge thefrom the ridge the
older the rocksolder the rocks
were. The youngerwere. The younger
ones were in theones were in the
center of the ridge.center of the ridge.
53. Evidence for sea floor spreadingEvidence for sea floor spreading
Alvin and otherAlvin and other
submersibles found crackssubmersibles found cracks
and lava showingand lava showing
spreading and volcanismspreading and volcanism
at mid ocean ridges andat mid ocean ridges and
odd life formsodd life forms
Matthews and Vine’sMatthews and Vine’s
survey of the Indiansurvey of the Indian
Ocean sea floor showedOcean sea floor showed
matching stripes ofmatching stripes of
reversing polarities onreversing polarities on
either side of ridge – whateither side of ridge – what
caused these?caused these?
– Lava spreading duringLava spreading during
alternating magnetic periodsalternating magnetic periods
54. The Earth’s magnetic fieldThe Earth’s magnetic field
It is produced by theIt is produced by the
outer core of the earthouter core of the earth
which is made of liquidwhich is made of liquid
iron and nickeliron and nickel
This moving magneticThis moving magnetic
material produces amaterial produces a
moving magnetic field,moving magnetic field,
which in turn produceswhich in turn produces
a moving electric field.a moving electric field.
It is a dynamo!It is a dynamo!
Earth’s magnetic fieldEarth’s magnetic field
varies over time and itvaries over time and it
protects us fromprotects us from
cosmic radiationcosmic radiation
63. Age of volcano increases
linearly away from
current hot spot!
64. Consistent with the idea that hotspot remains fixed
relative to plate motion.
65. Hotspot tracks: Hawaii
Linear increase of ages
with distance along the
Hawaii-Emperor chain.
Gradual decrease in
elevation with increasing
distance from the active
volcano.
66.
67. Timeline of Hawaiian History
NOT to scale!
85 mya:
Earliest
Emperor
Seamounts
visible today
are formed
43 mya:
Catastrophic
event causes
bend in
Hawaiian
chain
300-750 AD
Polynesians
arrive from
Tahiti in
outrigger
canoes
1778: British
explorer Captain
James Cook
lands on Hawaii
1900:
Hawaii
becomes a
US territory
1959:
Hawaii
becomes a
US state and
the age of
tourism
begins
1983:
Kilauea
Volcano
begins to
erupt
65 mya: The
dinosaurs
(not native to
Hawaii)
become
extinct
5 mya:
Island
of
Kauai
forms
1 mya:
Island of
Hawaii
forms
70. Is the plate tectonics concept universally accepted ?
Not universally accepted by all geoscientists
It does not provide satisfactory explanations ie
1)The total length of the subduction zones where crust is consumed is
far shorter then the length of crust generating ridges.
2)While ocean floor spreading is recognised in all the ocean basins,
Zones of subduction are almost entirely around the pacific. No
explanation.. Some plates increased in area by 50% since the
mesozoic but have no subduction zones
3) The drift of the spreading ridges is not inconsistent with concept of
plate tectonics, but what is the motive force for the movement of the
ridge marked by volcanicity
4) Evidences in some cases show that some plates move in two
directions .
5) Benioff zone is not complete in some cases, in north America deep
and intermediate earthquakes are missing.
6) North American plate moves in no regard to the spreading sites,
plate margins and transform faults. Hard to explain…
7) In 1968, the concept was put forth , 6 major plates were
recognised. Presently the number of minor plated is about 20…
complications in te concept.
8) Mountain building not satisfactorily explained
71. Pangaea revisitedPangaea revisited
By piecing togetherBy piecing together
this information,this information,
we can see howwe can see how
the continentsthe continents
have moved overhave moved over
the past 200the past 200
million years, duemillion years, due
to seafloorto seafloor
spreadingspreading
72. Plate tectonicsPlate tectonics
Sea floor spreading provides theSea floor spreading provides the
driving mechanism for movementdriving mechanism for movement
However, it is not the continents thatHowever, it is not the continents that
are moving, but the “plates” ofare moving, but the “plates” of
lithosphere “floating” in effect on thelithosphere “floating” in effect on the
asthenosphereasthenosphere
The lithosphere is made up of aboutThe lithosphere is made up of about
20 plates which move relative to20 plates which move relative to
each other in several wayseach other in several ways
Let’s look at a generalized sketchLet’s look at a generalized sketch
73. Evidence supporting the
theory of plate tectonics:
Apparent Polar wandering:
plate movement causes the
apparent position of the
magnetic poles to have shifted.
The paleomagnetic fields in the
rocks would indicate a single
pole until the continents drift
apart.
Confirmation of the Theory ofConfirmation of the Theory of
Plate TectonicsPlate Tectonics
74. Paleomagnetism: strips of alternating magnetic polarity at
spreading regions.
Confirmation of Plate Tectonics TheoryConfirmation of Plate Tectonics Theory
The patterns of paleomagnetism support plate tectonic
theory. The molten rocks at the spreading center take on
the polarity of the planet while they are cooling. When
Earth’s polarity reverses, the polarity of newly formed
77. ““Plates” of lithosphere are moved around by the underlying hot mantlePlates” of lithosphere are moved around by the underlying hot mantle
convection cellsconvection cells
80. 3 types of boundaries3 types of boundaries
Convergent –Convergent –
where plates comewhere plates come
together. Seetogether. See
example next slideexample next slide
81. Age of Oceanic CrustAge of Oceanic Crust
Courtesy of www.ngdc.noaa.gov
83. Spreading ridgesSpreading ridges
– As plates move apart new material isAs plates move apart new material is
erupted to fill the gaperupted to fill the gap
Divergent BoundariesDivergent Boundaries
84.
85. Iceland has a divergentIceland has a divergent
plate boundary runningplate boundary running
through its middlethrough its middle
North American Plate andNorth American Plate and
Eurasia Plate divergingEurasia Plate diverging
Iceland:Iceland: An example of continental riftingAn example of continental rifting
91. Oceanic lithosphereOceanic lithosphere
subducts underneath thesubducts underneath the
continental lithospherecontinental lithosphere
Oceanic lithosphereOceanic lithosphere
heats and dehydrates asheats and dehydrates as
it subsidesit subsides
The melt rises formingThe melt rises forming
volcanismvolcanism
E.g. The AndesE.g. The Andes
SubductionSubduction
92. Called SUBDUCTIONCalled SUBDUCTION
Continent-Oceanic Crust CollisionContinent-Oceanic Crust Collision
97. Andes MountainsAndes Mountains
formed along the coastformed along the coast
of South Americaof South America
Nazca Plate divesNazca Plate dives
under the Southunder the South
American PlateAmerican Plate
SubductionSubduction
101. Japan formed as an island chain asJapan formed as an island chain as
the Pacific Plate dives beneath thethe Pacific Plate dives beneath the
Eurasian PlateEurasian Plate
Mt Fuji
103. Aleutian Islands formedAleutian Islands formed
as an island chain asas an island chain as
Pacific Plate divesPacific Plate dives
beneath Northbeneath North
American PlateAmerican Plate
104.
105.
106. Forms mountains,Forms mountains, e.g. European Alps,e.g. European Alps,
HimalayasHimalayas
Continent-Continent CollisionContinent-Continent Collision
111. Interesting plate collisionInteresting plate collision
This picture shows aThis picture shows a
place inplace in
Newfoundland whereNewfoundland where
a massive collisiona massive collision
actually forcedactually forced
mantle rock on topmantle rock on top
of the crust, duringof the crust, during
the collision thatthe collision that
formed Pangaea andformed Pangaea and
the Appalachianthe Appalachian
mountains. Thismountains. This
looks down the oldlooks down the old
plate boundary.plate boundary.
112. Mantle rocks are toxicMantle rocks are toxic
These rocks have veryThese rocks have very
different compositionsdifferent compositions
than crustal rocks.than crustal rocks.
They contain heavyThey contain heavy
metals, which do notmetals, which do not
support life forms onsupport life forms on
the earth’s surface, sothe earth’s surface, so
few organisms livefew organisms live
there.there.
However, in someHowever, in some
places their heavyplaces their heavy
metal concentrationsmetal concentrations
produce rich metalproduce rich metal
deposits and are mineddeposits and are mined
113. Divergent BoundariesDivergent Boundaries
Found at spreadingFound at spreading
centers – either midcenters – either mid
ocean ridges or midocean ridges or mid
continental rift zonescontinental rift zones
114. Transform fault boundaryTransform fault boundary
This shows the SanThis shows the San
Andreas Fault.Andreas Fault.
It is a transform faultIt is a transform fault
boundary, where theboundary, where the
plates move sidewaysplates move sideways
past each other, ratherpast each other, rather
than away from eachthan away from each
other (at divergentother (at divergent
boundaries), orboundaries), or
towards each other (attowards each other (at
convergent boundaries)convergent boundaries)
118. Review of different boundariesReview of different boundaries
Divergent –mid ocean ridge like Iceland orDivergent –mid ocean ridge like Iceland or
continental rift zone like the African Riftcontinental rift zone like the African Rift
ValleyValley
ConvergentConvergent
– Ocean/ocean like Japanese IslandsOcean/ocean like Japanese Islands
– Continent/ocean like Andes and CascadesContinent/ocean like Andes and Cascades
– Continent/continent like the HimalayasContinent/continent like the Himalayas
Transform fault like the San Andreas faultTransform fault like the San Andreas fault
Hot spots are not at plate boundaries, butHot spots are not at plate boundaries, but
give us information about plate motiongive us information about plate motion
ActivityActivity
119. What causes plate tectonics?What causes plate tectonics?
Convection in the mantle, asConvection in the mantle, as
the plastic asthenospherethe plastic asthenosphere
flows, carrying the plates withflows, carrying the plates with
it.it.
This is probably aided by slabThis is probably aided by slab
pull at subduction zones andpull at subduction zones and
ridge push at mid ocean ridgesridge push at mid ocean ridges
and rising plumes in the mantleand rising plumes in the mantle
This diagram shows severalThis diagram shows several
different model hypothesesdifferent model hypotheses
128. Formation of mountainsFormation of mountains
Two forces areTwo forces are
constantly atconstantly at
work on thework on the
earth.earth.
– WeatheringWeathering
and erosionand erosion
tear structurestear structures
down whiledown while
– Plate tectonicsPlate tectonics
builds them upbuilds them up
129. So we have mountains!So we have mountains!
But they will not last forever.
130. Mountains form in different waysMountains form in different ways
-Volcanic mountains--Volcanic mountains-
Volcanoes form by subduction andVolcanoes form by subduction and
melting of platesmelting of plates
Volcanic mountains form over hotVolcanic mountains form over hot
spotsspots
Volcanic mountains form at rift zonesVolcanic mountains form at rift zones
131. Other mountain typesOther mountain types
Folded mountains form From convergingFolded mountains form From converging
continents like the Himalayascontinents like the Himalayas
How do these look?How do these look? Direction?Direction?
Fault block mountains form where blocks ofFault block mountains form where blocks of
rock drop at faults – mostly near platerock drop at faults – mostly near plate
boundaries, but not alwaysboundaries, but not always
Uplifted mountains – where large sectionsUplifted mountains – where large sections
of the crust are pushed up, perhaps byof the crust are pushed up, perhaps by
magma, or other forcesmagma, or other forces
132. IsostasyIsostasy
Just as a boat sinks or rises with changes inJust as a boat sinks or rises with changes in
weight, so does the crust sink or rise withweight, so does the crust sink or rise with
changes in weight. Plate tectonics buildschanges in weight. Plate tectonics builds
mountains and the extra weight causes the crustmountains and the extra weight causes the crust
to sink. As erosion occurs the weight of theto sink. As erosion occurs the weight of the
mountains decreases and the crust rises again.mountains decreases and the crust rises again.
This process is called isostasy or isostatic changeThis process is called isostasy or isostatic change
133. IsostasyIsostasy
Isostasy is balance. A floating object
is balanced in the water, like an
iceberg. If some of the top melts, the
iceberg rises in the water to stay in
balance. If you get into a boat, the
boat sinks to maintain balance. The
same thing happens with mountains.
As plate motions push it higher the
mountain sinks into the mantle to stay
in balance. On the other hand, when
the mountain erodes, it will rise in the
mantle as the top erodes. So, a
mountain may lose 1 meter from
erosion, but regain 0.8 meters from
rising due to isostasy. It maintains
isostatic equilibrium.
•This is also a great
example of feedback within
a system. Is it positive or
negative? Figure it out.
Remember that maintaining
equilibrium is __ feedback
because it keeps the system
in balance. So, what is it,
positive or negative?
•This is a good example of
Newton’s 3rd
law. For every
force, there is an equal and
opposing force. In this case
the force of gravity is
opposed by the buoyant
force.
Right! It’s negative.
134. StressStress
Due mostly to plate movements,
the earth’s crust is under a lot of
stress. There are 3 types, shown
at the right
A occurs where plates pull apart,
divergent boundaries, and is
called tension
B occurs where plates converge,
and is called compression
C occurs where plates move past
each other, at transform fault
boundaries and is called shearing
135. StrainStrain
This stress leads to strainThis stress leads to strain
on the crust which bends it.on the crust which bends it.
––
If it is warm, underground,If it is warm, underground,
it can bend. This calledit can bend. This called
ductile deformation.ductile deformation.
Features are called folds.Features are called folds.
Upturned folds areUpturned folds are
anticlines while downturnedanticlines while downturned
folds are synclines.folds are synclines.
Or, the rock may break, ifOr, the rock may break, if
it is brittle. This causesit is brittle. This causes
faults –breaks of the earth.faults –breaks of the earth.
A fold above and a fault below
anticline syncline
Stress and strain video
Economic value of folds/faults
136. Different faultsDifferent faults
Faults move inFaults move in
different ways,different ways,
depending on thedepending on the
type of stress ontype of stress on
them. Here are 3them. Here are 3
types that form.types that form.
1 is a normal fault1 is a normal fault
2 is a strike slip2 is a strike slip
faultfault
3 is a reverse fault3 is a reverse fault
2
3
1
137. MatchingMatching
These form atThese form at
different platedifferent plate
boundaries. Canboundaries. Can
you figure outyou figure out
which formswhich forms
where? Matchwhere? Match
themthem
A. Convergent
C. Transform Fault
B. Divergent
2
3
3
2
1
1
138. Can you match the stress and strain?Can you match the stress and strain?
A is 1st
, B is 3rd
, C is 2nd
2
3
1
Editor's Notes
The interior of the Earth is divided into layers based on chemical and physical properties.
The Earth has an outer silica-rich, solid crust, a highly viscous mantle, and a core comprising a liquid outer core that is much less viscous than the mantle, and a solid inner core.
Working from the centre of the Earth out we have:
The inner core is a primarily solid sphere about 1220 km in radius situated at Earth's center.
Based on the abundance of chemical elements in the solar system, their physical properties, and other chemical constraints regarding the remainder of Earth's volume, the inner core is believed to be composed primarily of a nickel-iron alloy, with small amounts of some unknown elements.
The temperature is estimated at 5,000-6,000 degrees Celsius and the pressure to be about 330 to 360 GPa (which is over 3,000,000 times that of the atmosphere!)
The liquid outer core is 2300 km thick and like the inner core composed of a nickel-iron alloy (but with less iron than the solid inner core).
Iseismic and other geophysical evidence indicates that the outer core is so hot that the metals are in a liquid state.
The mantle is approximately 2,900 km thick and comprises 70% of Earth's volume. (the core makes up about 30% of Earth's volume, with the outer crust [where we live] <1%!!).
The mantle is divided into sections based upon changes in its elastic properties with depth.
In the mantle, temperatures range between 500-900 degrees Celsius at the upper boundary with the crust to over 4,000 degrees Celsius at the boundary with the core.
Due to the temperature difference between the Earth's surface and outer core, and the ability of the crystalline rocks at high pressure and temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a convective material circulation in the mantle (mantle convection cells). Hot material rises up as mantle plumes (like a lava lamp!), while cooler (and heavier) material sinks downward to be reheated and rise up again.
- We shall see that this process is very important for plate tectonic motion…
The outer most layer is the crust - this is the most familiar to us as it is where we live.
The distinction between crust and mantle is based on chemistry, rock types and seismic characteristics.
Presenter: Ask the students to guess what the most abundant element in the earth’s crust is…..they may be surprised to learn that it is actually Oxygen (46.6% Oxygen; 27.7% Silica; 8.1% Aluminum; 5.0% Iron; 3.6% Calcium; 2.8% Sodium; 2.6% Potassium; 2.1% Magnesium; plus trace elements)
Click to next slide for more on the Crust….
The Earth has two different types of crust: Continental crust and Oceanic crust. Each has different properties and therefore behaves in different ways.
Continental crust:
Continental crust forms the land (the continents, as the name suggests) that we see today.
Continental crust averages about 35 km thick. Under some mountain chains, crustal thickness is approximately twice that thickness (about 70 km thick).
- The mountains we see on earth have deep roots in the crust that we can’t see. The crust “floats” on the more dense mantle and, like how only the tip of an iceberg sticks up out of the water, we see only the tip of the continental crust - the mountain ranges.
Continental crust is less dense and therefore more buoyant than oceanic crust
Continental crust contains some of the oldest rocks on Earth.
- Ancient rocks exceeding 3.5 billion years in age are found on all of Earth's continents. The oldest rocks on Earth found so far are the Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 Ga) [Ga = billion years ago] and the Isua Supracrustal rocks in West Greenland (3.7 to 3.8 Ga), but well-studied rocks nearly as old are also found in the Minnesota River Valley in the USA (3.5-3.7 billion years), in Swaziland (3.4-3.5 billion years), and in Western Australia (3.4-3.6 billion years).
Oceanic crust:
As the name already suggests, this crust is below the oceans.
Compared to continental crust, Oceanic crust is thin (6-11 km).
It is more dense than continental crust and therefore when the two types of crust meet, oceanic crust will sink underneath continental crust.
The rocks of the oceanic crust are very young compared with most of the rocks of the continental crust. They are not older than 200 million years.
This diagram shows the major Tectonic Plates.
Presenter: Point out the UK, sitting on the Eurasian Plate. Also the plate boundary between Africa and South America (note that it has the same shape as the coastlines in these countries).
How and Why do tectonic Plates move around?
The question of how tectonic plates are moved around the globe is answered by understanding mantle convection cells.
In the mantle hot material rises towards the lithosphere (like hot air rising out of an open oven - ever opened an oven door and felt the blast of hot air coming past your face?). The hot material reaches the base of the lithosphere where it cools and sinks back down through the mantle. The cool material is replaced by more hot material, and so on forming a large “convection cell” (as pictured in the diagram).
This slow but incessant movement in the mantle causes the rigid tectonic plates to move (float) around the earth surface (at an equally slow rate).
To view this animation, click “View” and then “Slide Show” on the top navigation bar.
Ocean Ridges: This map shows the age of the oceanic crust. The red colouring shows the youngest ages, whilst the dark blue shows the oldest ages (around 200 million years old).
Presenter ask: Where are the Ocean Ridges located? I.e. where are the divergent boundaries?
Answer: The divergent boundaries are where the plates are pulling apart and new material is being produced. Therefore the Ocean ridges are in the middle of the red areas (the boundaries are in fact shown on the map). We can see a progression of the oceanic crust getting older away from the ocean ridges (like a conveyer belt).
Presenter: Before moving on to the next slide, point out Iceland. The divergent boundary runs straight through Iceland….
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.
The Andes mountain range along the western edge of the South American continent is an example of a mountain belt formed by subduction.
The continental crust of the South American plate has buckled under the compressional strain of converging with the Nasca and Antarctic plates. Additionally there are many volcanoes, the result of melting of the subducting slab and the production of new material that has risen through the crust to the surface.
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.
When continental crust pushes against continental crust both sides of the convergent boundary have the same properties (think back to the description of continental crust: thick and buoyant). Neither side of the boundary wants to sink beneath the other side, and as a result the two plates push against each other and the crust buckles and cracks, pushing up (and down into the mantle) high mountain ranges. For example, the European Alps and Himalayas formed this way.
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!!
To view this animation, click “View” and then “Slide Show” on the top navigation bar.
To view this animation, click “View” and then “Slide Show” on the top navigation bar.
From here is ESCP only since I am combining plates and mountains chapters for them
