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Seafloor Spreading and Continental Drift

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Title: Seafloor Spreading and Continental Drift


1
Seafloor Spreading and Continental Drift
2
Plate Tectonics
A. Plate Tectonics Summary
  • 1. This is the basic idea that Earths crust is
    divided into a few large, thick ____________
    which are large slabs of the lithosphere.
  • (a) Plates are part of the______________ move
    slowly and change in size.
  • (b) Plates may be
  • (1) entirely ______________ rock
  • (2) both ____________ and _____________ rock
  • (3) entirely ______________ rock

plates
lithosphere
continental
sea floor
continental
sea floor
3
Tectonic Plates
4
World Distribution of Earthquakes
  • Earthquakes with focal depths between 0 and 670
    km
  • Over a six-year period

5
Volcanoes
6
Plate boundaries are geologically active with
earthquakes
  • __________________
  • __________________
  • __________________

volcanoes
young mountain ranges
7
B. Plate Tectonics combines two preexisting ideas
Continental Drift
  • a. ____________________which is the idea that the
    continents move freely over Earths surface,
    changing their positions relative to one another
  • b. ____________________ which is the hypothesis
    that sea floor forms a mid-oceanic ridge crests
    and then moves horizontally away from the ridge
    towards oceanic trenches.

Sea-Floor Spreading
8
II. Sea Floor Spreading
A. This is the hypothesis that 1. Was
originally proposed by Harry Hess, a Princeton
University geologist
  • Sea floor forms at the___________
  • The sea floor moves _________________ from the
    ridge crest toward an oceanic trench where it
    ___________.
  • The two sides move in __________ directions

Mid-ocean ridge
horizontally
subducts
opposite
Convection
5. ________________ in the mantle is the driving
force.
9
B. The Mid-Ocean Ridge
  • Hot mantle rock rises beneath the ridge as a
    result of
  • convection
  • 2. This expains high heat flow and basaltic
    volcanic eruptions.
  • 3. a. ________ at the ridge crest results in
    cracking open of oceanic crust to form a
    ____________
  • b. Shallow focus earthquakes

Tension
Rift Valley
rift valley
10
c. ______________
Fracture Zones
  • Major lines of weakness in Earths crust.
  • Cross the mid-ocean ridge at nearly right angles.
  • Extend for 1000's kilometers across the ocean
    floor.
  • The mid-ocean ridge was once continuous across
    the fracture zones but is now offset.

11
d. _________________
Transform Faults
  • Mid-ocean ridges are offset along fracture zones
  • Transform motion of rocks on either side is not
    always in opposite directions.
  • Rocks move in opposite directions only in the
    section between two segments of ridge crest.
  • This is the only section that experiences
    earthquakes instead of along the entire section
    as would normally be expected.

12
4. Ocean _________
Trenches
  • Sea floor moving away from the ridge cools
  • It becomes denser and ___________, perhaps
    sinking back into the mantle.
  • Trenches are explained by the downward plunge of
    cooler rock and explains negative gravity
    anomalies.

subducts
13
5. Young Age of the Sea Floor
200 million years
  • Less than ______________________ old.
  • New sea floor continually is formed by basalt
    eruptions at the _________________.
  • Basalt is carried horizontally away from the
    ridge crest where the ____________ rock is found.
  • Sea floor is continually destroyed by subduction
    into the mantle at the oceanic trenches

ridge crests
youngest
14
Deep Ocean Sediments
  • Deep ocean (pelagic) sediment is thin or absent
    on the crest of the mid-oceanic ridges.
  • Sediment becomes thicker away from the ridge.

15
C. ______________ Data at the Ridges
Paleomagnetic
Airborne magnetometer on a U.S. Navy Orion P-3
New
  • _______ rock formed at the center of the ridge
    acquires Earths magnetic polarity at that time.
  • Parallel to the ocean ridges there are long
    strips with alternating magnetic polarity
    (magnetic anomalies that are symmetrical about
    the ridge crest).

16
Vine-Matthews Hypothesis
  • Developed by British geologists Fred Vine and
    Drummond Matthews
  • Proposes that the magnetic anomalies match the
    pattern of magnetic reversals of Earths magnetic
    field as measured in continental rocks.

17
Correlation of Magnetic Anomalies with Magnetic
Reversals
Same Age
18
D. _________ Models for mid-ocean
ridge processes
Ophiolites
  • Pieces of oceanic plate that have been thrusted
    (obducted) onto the edge of continental plates.

19
From seafloor drilling
and seismic studies
Coninental Sequence
  • an assemblage of mafic and ultramafic lavas and
  • hypabyssal rocks found in association with
    sedimentary rocks
  • They are found in areas that have complex
    structure

20
Ophiolites
In the French Alps
In Cyprus
21
III. Continental Drift
22
Alfred Wegener and the Continental Drift
Hypothesis
  • German meteorologist
  • Credited with hypothesis of continental drift

23
A. Wegeners Evidence For Continental Drift
Pangaea
  • 1. ___________
  • Wegener proposed the presence of a giant
    continent, Pangaea (also spelled Pangea), which
    literally translated means all lands.
  • When it split apart it separated into two parts
    with the proto-Atlantic between them (called
    Tethys Sea).
  • ____________was the northern supercontinent that
    contained present-day North America and Eurasia
    (not including India
  • _____________________ (also called Gondwana) was
    the southern supercontinent, composed of all
    present-day southern hemisphere continents and
    India.

Laurasia
Gondwanaland
24
2. _______ Fit of Continents
Jig-Saw
  • Shorelines of continents seem to fit together

25
3. ____________________ Between Continents
Matching Rock Types
Continental Shelf (light blue)
Broad belts of rocks that Correlate in type and
age
26
4. ____________________
Matching Mountain Ranges
When continents are brought together, their
mountain ranges form a single continuous range
of the same age and style of deformation.
27
5. _________ Evidence
Fossil
  • Almost identical late Paleozoic fossils in South
    America, Africa, India, Antarctica, and
    Australia.
  • Bones of land reptiles have been found in
    Antarctic rocks. Antarctica is now completely
    separate and isolated from other continents.

28
c. _____________
Glossopteris
  • (1) A Late Paleozoic plant found in rocks on all
    five continents.
  • (2) When the land areas are joined, similarity
    can be seen.

29
d. _______________
Extinct Reptiles
  • Areas where found reveal narrow, sharply defined
    habitats extending across
  • three continents and
  • the subcontinents of Madagascar and India.
  • The shape of the animals ranges can best be
    explained by assuming that these lands were once
    united as one landmass.

30
Mesosaurus
  • A freshwater carnivorous reptile cm long
  • Fossils found in Permian-aged rocks in Brazil and
    Africa

31
Lystrosaurus
  • Early Triassic terrestrial mammal-like reptile
  • About 1 meter long with two long teeth protruding
    from the upper jaw
  • Fossils found in Africa, India, and Antarctica

32
Cynogathus
  • Early Triassic terrestrial mammal-like reptile
  • About 1 meter in length
  • Fossils found in Brazil and Africa

33
Fossil Evidence
34
6. Late Paleozoic _________
Glaciation
Glacial evidence shows the Origin of the glaciers
in the Atlantic
Boulders in S. America traced to a source in
Africa
  • Distribution of Late Paleozoic continental
    glacial evidence on the Gondwanaland continents
    can only be explained by a supercontinent.

35
7. _____________ (Ancient Climates)
Paleoclimates
  • a. Inferring the Location of the Poles
  • If it is assumed that ancient climates had the
    same geographic distribution as present-day
    climates,
  • then the distribution of sedimentary rocks can be
    used to infer the locations of the ancient poles
    and the paleoequator.

36
Glacial Evidence
  • (1) Glacial ____ (soil) and __________
    (scratches) on bedrock are found in cold polar
    climates.

till
striations
37
Coral Reefs
  • Coral reefs are found in tropical regions (as far
    as 300 north or south of the equator)

38
Cross-Bedded Sandstones
  • Indicate the locations of ancient deserts
  • Latitudes of 30o

39
(2) ______________
Polar Wandering
  • Wegener inferred that the ancient poles were in
    different positions that the present-day poles.
  • This apparent change in positions of the poles is
    termed polar wandering.

40
Explaining Polar Wandering
  • (A) Continents remain stationary and the poles
    actual change position
  • (B) Poles remain stationary and the continents
    change position

41
Paleomagnetic Evidence for Continental Drift
  • Magnetic dip of magnetite crystals increases
    towards the North magnetic pole.
  • Aligned with magnetic lines of force

42
The Permian North Pole
  • Permian rocks in N. America point to a pole
    position in eastern Asia
  • Rocks in Europe point to a different position
  • Every continent shows a different Permian pole
    position

43
Polar Wandering
  • Paths of polar wandering have similar shapes
  • If North America is theoretically pushed back
    towards Europe, its polar wandering path lies
    exactly on the path for Europe
  • Suggests
  • one north magnetic pole
  • The continents were joined.

44
Rodinia The Late Proterozoic Supercontinent
  • Assembled between 1.3 and 1.0 billion years ago
  • Began fragmenting 750 million years ago
  • Pieces formed another supercontinent, Pannotia,
    about 650 million years ago

45
Late Cambrian Paleogeography
Russia east of Ural Mts. Asia north of
Kazkhsstan And south of Mongolia
Most of N. America, Greenland, NW Ireland,
Scotland
Africa, Antarctica, Australia, Florida, India,
Madagascar, and parts of Middle East
southern Europe
Russia west of Ural Mts., major part of N. Europe
46
Late Ordovician Paleogeography
47
Middle Silurian Paleogeography
48
Early Paleozoic Era Continental
Drift Summary(Cambrian Silurian)
  • Six major continents
  • Four were located near the paleoequator
  • Laurentia moved northward
  • Gondwana moved to a south polar location

49
Late Devonian Period Paleogeography
50
Early Carboniferous Period Paleogeography
51
Late Carboniferous Period Paleogeography
52
Late Permian Period Paleogeography
53
Late Paleozoic Continental Drift
Summary (Cambrian Silurian)
  • Baltica and Laurentia collided, forming Laurasia
  • Siberia and Kazakhastania . . .
  • Collided
  • Became sutured to Laurasia
  • Gondwana moved over the South Pole
  • During the Permian . . .
  • the formation of Pangaea was completed
  • Panthalassa, a global ocean, surrounded the
    supercontinent

54
Mesozoic Era Paleogeography The Triassic Period
55
Mesozoic Era Paleogeography The Jurassic Period
56
Mesozoic Era Paleogeography The Cretaceous Period
57
Allochthonous Terranes in Western North America
  • Microcontinents incorporated into the crumpled
    margin of a larger continent.
  • Called
  • Allochthonous terranes
  • Suspect terranes
  • Alien terranes

Green terranes are probably from continents
other than N. America
Pink terranes are probably from displaced parts
of North America
58
Early Mesozoic Evaporites
  • Rifting of Pangaea opened the Proto-Atlantic
    Ocean
  • Evaporites accumulated in shallow basins

59
Change in Oceanic Circulation
60
Mesozoic Era Continental Drift Summary
  • The main event was the breakup of Pangaea.
  • The breakup of Pangaea influenced global climatic
    and atmospheric circulation patterns

61
Pangaea Breakup and Continental Drift
62
Pangaea Ultima
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