Presentation on theme: "Plate Tectonics 2 Making oceans and continents"— Presentation transcript:
1Plate Tectonics 2 Making oceans and continents Plate Tectonics 2 Making oceans and continents
2Pangea* seen at about 225 mya Collision of Laurasia and GondwanaPangea* seen at about 225 myaSir Francis Bacon 1620Benjamin Franklin 1782The crust of the earth mustbe a shell floating on a fluidinterior. Thus the surface ofthe globe would be broken… by … movements of thefluids….Wegener 1912: evidence* Breakup begins about 200 mya, floods about 190 mya
3Alfred Wegener 1912 Continental drift hypothesis Continents "drifted" to present positionsEvidence used in support of continental drift hypothesisFit of continentsFossil evidenceRock type and mountain beltsPaleoclimatic evidence
4Pangaea about 200 myaEvidence: Precise Matching of Continental Shelves of Circum-Atlantic Continents
6Similar Rocks on opposite shores Example, NJ and Morocco
7Why wasn’t Wegener’s idea accepted? Objections to drift hypothesisInability to provide a mechanism capable of moving continents across globeWegener suggested that continents broke through the ocean crust, much like ice breakers cut through ice
8Continental 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 rocksShow direction to Earth’s magnetic polesProvide a means of determining their original latitudeHorizontal Magnetite = at equator,vertical = at poleIn between latitude can also be calculatedIdentical fossils show proximity
10The scientific revolution begins Extensive mapping of the ocean floor revealed the mid-ocean ridges in great detailRecall that Seafloor spreading hypothesis was proposed by Harry Hess in the early 1960s
11Geomagnetics tested Hess’ idea Geomagnetic reversals are recorded in the ocean crust pillow lavasData from towed magnetometers,record North or South pointing mineralsHess’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.
12Maps of Magnetic Stripes in Oceanic Crust Paleomagnetic data were the most convincing evidence to support the concept of seafloor spreading
13Geomagnetic reversals Recall the testsGeomagnetic reversalsMagnetic North and South exchange places at irregular intervals, average ~100K years but with large varianceDates when polarity of Earth’s magnetism changed were determined from radiometric dating of lava.
15Example from the past 4 million years Pattern is irregular so useful for corellation
16Hess’ 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
17Hess’s Seafloor spreading (cont) New lithosphere pulled from the ridge crest by moving conveyor-belt. Conveyor belt formed by convection currents in the asthenosphere belowNewly created crust at the ridge is elevated because it is heated and therefore occupies more volume than the cooler rocks of the deep-ocean basinArea also seems to be pushed up by mantle upwelling
18How 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.
19The 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.
20Hot Spots and HawaiiWorldwide, plate speeds vary from 1 to 10 centimeters per yearBefore satellites, we measured plate speeds as the distance between two islands divided by the age of the youngest basaltsFlood Basalt was subductedHey look, the direction changed!
21Hot Spots & Plate Motions Average 5 centimeters/year
22Determining plate speeds for continents LAGEOS and GPS satellites determine that plates move cm per year, avg 5Just find position wrt distant stars, then watch fixed objects on earth move .
23Latitude 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.
24150 mya Atlantic is already open110 mya Displaced (Exotic) Terranes from S. Am. hits W. N.Am.60 mya another terrane forms Cuba, Hisp.About 50 mya Southern Ocean forms20 mya Himalayas formsAbout mya Central America forms
27Active Rifting of A Continental Plate Note 3-D Triple JunctionDiscussion: eggshells
28Active Rifting of A Continental Plate Inactive Branch: Aulocogen;Subsided Passive Margins
29East African Rift ZoneActive: 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 & grasslandsHumans as tall savannah specialists, voiceStory: The drunk and the lamp post
30Mid-Ocean Ridge dimensions Total kilometers (40,000 miles) longAs wide as 1500 km (900 miles)Some more than 3 km high above ocean floor.
31Mid-Ocean Ridge System Motion Fracture Zones andTransform FaultsShallow weak earthquakes
32Subduction-Zone Features Note sequence from land to trenchNote: over here are some ocean plate rocks that don’t get subducted in a collisionWe will see some on the field trip, as well as the volcanic arcIf a continent converges fromthe left, what rocks will foldin the collision?Rocks in the HimalayasReverse faults at convergent margin
33Mélange from California Coast Sea-floor andland-derivedsediments,+ some volcanics.When stuffed down trench intoLow Temperature-High Pressurezone, result isBlueschist FaciesSource: Betty Crowell/Faraway Places
34Shield + Platform = Craton High Angle Normal faults of Rift EscarpmentActive and unstable continental marginCraton : the stable portion of the continental crust versus regions that are more geologically active and unstable
35Anatomy of a Continent Canadian Shield, North America’s Crystalline coreexposed by glaciers
36Exotic (Displaced) Terrains Collisions with Volcanic Island Arcs and microcontinentsContinentalCrust buoyanthard to subduct. Erosion resistant partsSuture ZonePieces are volcanic island arcs, and microcontinentsMoved along transform faults, then accreted.Anecdote Western California
37Ideas:Earth's Convection Cells Aesthenosphere shallow convection model
38Ideas: Earth's Convection Cells Deep mantle/core convection model – Plumes cause MOR’s – Morgan
40Mapping the ocean floor Three major topographic units of the ocean floorContinental marginsDeep-ocean basinsMid-ocean ridges
41Passive continental margins Found along coastal areas that surround oceans w central MORNot near active plate boundaries because MOR is far offshoreLittle volcanism and few earthquakesEast Coast of US an example
43Active continental margins Continental slope descends abruptly into a deep-oceanic trenchLocated primarily around the Pacific Oceansediment and oceanic crust scraped off ocean crust to form accretionary wedges
45The world’s trenches and ridges Trench an entrance to Subduction Zone, Ridges and Rises are Mid-Ocean Ridges
46Volcanic Island Arc (Japan) CONTINENTBack Arc BasinVolcanic Island Arc (Japan)TrenchAbyssal PlainFABAccretionary WedgeSeamounts
47Features of the deep-ocean basin Abyssal plainsCan be sites of thick accumulations of sedimentFound in all oceansStudded by old cold seamounts and ridgesSee previous slide
48Seafloor sediment Ocean floor is mantled with sediment Sources Turbidity currents on continent marginsSediment that slowly settles to the bottom from above – fine mud and planktonThickness variesThickest in trenches – accumulations may exceed 9 kilometers there
49Biogenous sediment Types of sediment Shells and skeletons of marine animals and plantsCalcareous 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
50Foraminifera (a.k.a. Forams) Form deepwater carbonate oozes, depths less than 4 km
52Mid-ocean ridges Characterized by Heating => elevated ridge w/ radial cracksClosely spaced normal faulting: HW downMantle flow below pulls the crust apart – High Angle Normal Faults steeper than cartoonNewly formed basalt ocean floor fills in cracks
53Bathymetry of the Atlantic Ocean Abyssal PlainAbyssal PlainPassive Margin MOR Passive Margin
56Black SmokersCirculation 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.
57Ocean Floor layers:Ophiolite Suite Structure of oceanic crustThree layers in crustUpper layer – consists of sediments over pillow lavasMiddle layer – numerous interconnected dikes called sheeted dikesLower layer – gabbro formed in basaltic magma chambersLayer in mantle also part of the Ophiolite complex- Magma that creates new ocean floor originates from partially melted mantle rock (peridotite) in the asthenosphere
58Ophiolite Suite Some Serpentine is formed due to hot water (called Hydrothermal)circulation