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Plate Tectonics 2 Making oceans and continents

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1 Plate Tectonics 2 Making oceans and continents
Plate Tectonics 2 Making oceans and continents

2 Pangea* seen at about 225 mya
Collision of Laurasia and Gondwana Pangea* seen at about 225 mya Sir Francis Bacon 1620 Benjamin Franklin 1782 The crust of the earth must be a shell floating on a fluid interior. Thus the surface of the globe would be broken … by … movements of the fluids…. Wegener 1912: evidence * Breakup begins about 200 mya, floods about 190 mya

3 Alfred Wegener 1912 Continental drift hypothesis
Continents "drifted" to present positions Evidence used in support of continental drift hypothesis Fit of continents Fossil evidence Rock type and mountain belts Paleoclimatic evidence

4 Pangaea about 200 mya Evidence: Precise Matching of Continental Shelves of Circum-Atlantic Continents

5 Ranges of Triassic Reptiles

6 Similar Rocks on opposite shores
Example, NJ and Morocco

7 Why wasn’t Wegener’s idea accepted?
Objections to drift hypothesis Inability to provide a mechanism capable of moving continents across globe Wegener suggested that continents broke through the ocean crust, much like ice breakers cut through ice

8 Continental drift and paleomagnetism
In 1950’s there was renewed interest in Wegener’s continental drift idea. New data came from seafloor topography and paleomagnetics. Magnetized minerals in rocks Show direction to Earth’s magnetic poles Provide a means of determining their original latitude Horizontal Magnetite = at equator, vertical = at pole In between latitude can also be calculated Identical fossils show proximity

9 The Ocean-Floor Topography discovered
Beginning WWII Sonar revealed Trenches, Mid-Ocean Ridges, transform faults, sediments

10 The scientific revolution begins
Extensive mapping of the ocean floor revealed the mid-ocean ridges in great detail Recall that Seafloor spreading hypothesis was proposed by Harry Hess in the early 1960s

11 Geomagnetics tested Hess’ idea
Geomagnetic reversals are recorded in the ocean crust pillow lavas Data from towed magnetometers, record North or South pointing minerals Hess’s concept of seafloor spreading predicts matching bands of lava polarity on either side of mid-ocean ridges. In early 60’s Fred Vine and D. Matthews looked for symmetric magnetic stripes in the ocean crust data near ridges.

12 Maps of Magnetic Stripes in Oceanic Crust
Paleomagnetic data were the most convincing evidence to support the concept of seafloor spreading

13 Geomagnetic reversals
Recall the tests Geomagnetic reversals Magnetic North and South exchange places at irregular intervals, average ~100K years but with large variance Dates when polarity of Earth’s magnetism changed were determined from radiometric dating of lava.

14 Magnetic Anomalies (again)

15 Example from the past 4 million years
Pattern is irregular so useful for corellation

16 Hess’ seafloor spreading in detail
Seafloor spreading occurs along relatively narrow zones, called rift zones, located at the crests of ocean ridges called Mid-Ocean Ridges (MOR’s). These are above hot rising mantle. As plates pulled apart, cracks allow low pressure and water to hit mantle. Causes partial melting. Magma moves into fractures and makes new oceanic lithosphere

17 Hess’s Seafloor spreading (cont)
New lithosphere pulled from the ridge crest by moving conveyor-belt. Conveyor belt formed by convection currents in the asthenosphere below Newly created crust at the ridge is elevated because it is heated and therefore occupies more volume than the cooler rocks of the deep-ocean basin Area also seems to be pushed up by mantle upwelling

18 How fast do Plates Move? Hot Spots are magmas from rising plumes from the deep mantle, probably heated by the liquid outer core. Their lavas are datable. As plates move over them, new volcanic seamounts and islands are formed. Eventually any subaerial (exposed to the air) parts are eroded away, and as they move away from the Hot Spot, they cool, contract, and submerge. Called Guyots. Hot spots form chains.

19 The Big Island of Hawaii
The big Island of Hawaii is a composite of five volcanoes. Kohala is the oldest. Kilauea is very active because it is closest to the hot spot, which is to the southeast of the big island.

20 Hot Spots and Hawaii Worldwide, plate speeds vary from 1 to 10 centimeters per year Before satellites, we measured plate speeds as the distance between two islands divided by the age of the youngest basalts Flood Basalt was subducted Hey look, the direction changed!

21 Hot Spots & Plate Motions
Average 5 centimeters/year

22 Determining plate speeds for continents
LAGEOS and GPS satellites determine that plates move cm per year, avg 5 Just find position wrt distant stars, then watch fixed objects on earth move .

23 Latitude for ocean floor
Orientation of magnetic minerals gives latitude (north or south of equator) Radiometric dates of ocean floor basalts, plus distance from ridge, gives paleolongitude since 200 million years ago, when Pangaea began to break apart.

24 150 mya Atlantic is already open 110 mya Displaced (Exotic) Terranes from S. Am. hits W. N.Am. 60 mya another terrane forms Cuba, Hisp. About 50 mya Southern Ocean forms 20 mya Himalayas forms About mya Central America forms

25 Origin of Pangaea

26 Origin of Pangaea

27 Active Rifting of A Continental Plate
Note 3-D Triple Junction Discussion: eggshells

28 Active Rifting of A Continental Plate
Inactive Branch: Aulocogen;Subsided Passive Margins

29 East African Rift Zone Active: Red Sea and Gulf of Aden Failed Arm: Great Rift Valley (aulocogen) Discussion: Fault Block Mountains, HA normal fault, rain shadows, divergent margin. global cooling & grasslands Humans as tall savannah specialists, voice Story: The drunk and the lamp post

30 Mid-Ocean Ridge dimensions
Total kilometers (40,000 miles) long As wide as 1500 km (900 miles) Some more than 3 km high above ocean floor.

31 Mid-Ocean Ridge System Motion
Fracture Zones and Transform Faults Shallow weak earthquakes

32 Subduction-Zone Features
Note sequence from land to trench Note: over here are some ocean plate rocks that don’t get subducted in a collision We will see some on the field trip, as well as the volcanic arc If a continent converges from the left, what rocks will fold in the collision? Rocks in the Himalayas Reverse faults at convergent margin

33 Mélange from California Coast
Sea-floor and land-derived sediments, + some volcanics. When stuffed down trench into Low Temperature- High Pressure zone, result is Blueschist Facies Source: Betty Crowell/Faraway Places

34 Shield + Platform = Craton
High Angle Normal faults of Rift Escarpment Active and unstable continental margin Craton : the stable portion of the continental crust versus regions that are more geologically active and unstable

35 Anatomy of a Continent Canadian Shield, North America’s
Crystalline core exposed by glaciers

36 Exotic (Displaced) Terrains
Collisions with Volcanic Island Arcs and microcontinents Continental Crust buoyant hard to subduct. Erosion resistant parts Suture Zone Pieces are volcanic island arcs, and microcontinents Moved along transform faults, then accreted. Anecdote Western California

37 Ideas:Earth's Convection Cells
Aesthenosphere shallow convection model

38 Ideas: Earth's Convection Cells
Deep mantle/core convection model – Plumes cause MOR’s – Morgan

39 Ideas: Earth's Convection Cells
Combination

40 Mapping the ocean floor
Three major topographic units of the ocean floor Continental margins Deep-ocean basins Mid-ocean ridges

41 Passive continental margins
Found along coastal areas that surround oceans w central MOR Not near active plate boundaries because MOR is far offshore Little volcanism and few earthquakes East Coast of US an example

42 A passive continental margin

43 Active continental margins
Continental slope descends abruptly into a deep-oceanic trench Located primarily around the Pacific Ocean sediment and oceanic crust scraped off ocean crust to form accretionary wedges

44 An active continental margin

45 The world’s trenches and ridges
Trench an entrance to Subduction Zone, Ridges and Rises are Mid-Ocean Ridges

46 Volcanic Island Arc (Japan)
CONTINENT Back Arc Basin Volcanic Island Arc (Japan) Trench Abyssal Plain FAB Accretionary Wedge Seamounts

47 Features of the deep-ocean basin
Abyssal plains Can be sites of thick accumulations of sediment Found in all oceans Studded by old cold seamounts and ridges See previous slide

48 Seafloor sediment Ocean floor is mantled with sediment Sources
Turbidity currents on continent margins Sediment that slowly settles to the bottom from above – fine mud and plankton Thickness varies Thickest in trenches – accumulations may exceed 9 kilometers there

49 Biogenous sediment Types of sediment
Shells and skeletons of marine animals and plants Calcareous oozes from microscopic organisms (only in shallow water) Siliceous oozes composed of opaline skeletons of diatoms and radiolarians (only in deep water) Carbonate compensation depth - 4km

50 Foraminifera (a.k.a. Forams)
Form deepwater carbonate oozes, depths less than 4 km

51 Chert sample below carbonate line >4 km Diatoms (siliceous ooze)

52 Mid-ocean ridges Characterized by
Heating => elevated ridge w/ radial cracks Closely spaced normal faulting: HW down Mantle flow below pulls the crust apart – High Angle Normal Faults steeper than cartoon Newly formed basalt ocean floor fills in cracks

53 Bathymetry of the Atlantic Ocean
Abyssal Plain Abyssal Plain Passive Margin MOR Passive Margin

54 The structure of oceanic crust

55 Hydrothermal Metamorphism
Recall …

56 Black Smokers Circulation of hot water in cracks at mid-ocean ridge dissolves metals (Copper, Iron, Zinc, Lead, Barium) which are re-precipitated as (for example) sulphide ores. Hydrothermal waters are capable of metamorphism.

57 Ocean Floor layers:Ophiolite Suite
Structure of oceanic crust Three layers in crust Upper layer – consists of sediments over pillow lavas Middle layer – numerous interconnected dikes called sheeted dikes Lower layer – gabbro formed in basaltic magma chambers Layer in mantle also part of the Ophiolite complex - Magma that creates new ocean floor originates from partially melted mantle rock (peridotite) in the asthenosphere

58 Ophiolite Suite Some Serpentine is formed
due to hot water (called Hydrothermal) circulation

59 End Plate Tectonics 2 Outcrop of pillow basalt


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