OCE3008: Chapter 2: Plate Tectonics and Ocean Floor

Most divergent plate boundaries occur?

Along the crest of the mid-ocean ridge, where sea floor spreading creates new oceanic crust

Who proposed theory of plate tectonics?

Alfred Wegener (1912) •Called it "continental drift"

Coral Reef Development Stages

1. (fringing reef) on an active volcano, coral growth builds up very close to the shore 2. (Barrier reef) as the volcano becomes inactive and sinks, coral builds up over time 3. (Atoll) eventually the island sinks low sea level but a thick section of coral builds up to stay close to the surface

Future Predictions from Plate Tectonics

1. Atlantic will enlarge, Pacific will shirnk 2. New sea from East Africa rift valleys 3. Further Himalaya uplift 4. Separation of North and South America 5. Part of California in Alaska

Three types of plate boundaries?

1. Divergent boundary 2. Convergent boundary 3. Transform boundary

Wilson Cycle

1. Embryonic stage, uplift (rift valleys) East Africa rift valleys 2. Juvenile, divergence (spreading) Red Sea 3. Mature, divergence (spreading) ocean basin w/ continental margins, Atlantic/Arctic Oceans 4. Declining, Convergence (subduction) Pacific Ocean 5. Terminal, Convergence/collision/uplift, Mediterranean sea 6. Suturing, Convergence/uplift, Himalaya mountains

What are the different types of coral reefs?

1. Fringing 2. Barrier 3. Atolls

Types of Spreading Centers

1. Oceanic Rise 2. Oceanic Ridge 3. Ultra-Slow

Three Types of Convergent Boundaries

1. Oceanic-Contiental 2. Oceanic-Oceanic 3. Continental-Continetal

What evidence supports plate tectonics?

1. Orientation of magnetic particles in Earth's crust 2. Magnetic dip 3. Apparent polar wandering 4. Magnetic polarity reversals 5. Ocean floor magnetic anomalies 6. Sea floor spreading 7. Uneven heat flow in Earth's crust 8. Worldwide earthquake distribution 9. Use of satellite data to detect plate motion

Hotspot Track

1. Plume of hot buoyant material detaches from deep mantle/core-mantle 2. Plume rises more rapidly than the plume head can push through viscous mantle which inflates head and elevates earth surface 3. Decompression near surface partially melts plume head which comes to surface and creates a hotspot volcano 4. Volcano carried away by plate motion, plume continues to feed subsequent volcanoes

Wegener's Evidence for Continental Drift

1. Puzzle-like fit of modern continents 2. Searched for matching sequences of rock units/ ancient mountain chains (similar rock types/ages/structures found on dif. continents) 3. Glacial ages/patterns & direction of glacial flow 4. Distribution of organisms

History of ocean basins

1. Seafloor spreading 2. Mantle convection cells (driving mechanism)

Divergent Boundary - Creation of an Ocean Basin

1. Shallow heat source under continent causes upwarping/volcanic activity 2. Movement apart creates linear rift valley 3. Increased spreading/downdropping = a linear sea 4. After millions of years, a full-fledged ocean basin is created, separating continental pieces

Transform faults occur?

Between mid-ocean ridge segments

Continental Accretion

Continental material added to edges of continents through plate motion

Wegner (The Origins of Continents and Oceans 1915) Proposed Mechanism

Continents plow through ocean basins due to gravitational attraction of equatorial bulge and tidal forces from the sun and moon

Continental Transform Fault

Cuts across continent (San Andreas Fault)

Fringing Reefs

Develop among margin of landmass

Glaciation

Evidence of glaciation in now tropical regions •patterns and direction of glacial flow and rock scouring •Plant/animal fossils indicate different climate than today

Tablemounts/Guyots

Flattened tops •If seamounts are tall enough to reach the surface and become islands, their tops are eroded flat by wave activity and become tablemounts

Heat flow

Heat from Earth's interior released to surface •Very high at mid-ocean ridges •Low at subduction zones

Nematath

Hotspot track

Earth's magnetic polarity is recorded in?

Igneous rocks (from magma or lava) •Magnetite in basalt

Magnetic dip

Magnetite particles in sedimentary rocks or igneous rocks (basalt) align with Earth's magnetic field

Oceanic Transform Fault

Ocean Floor Only

Subduction Zones

Oceanic trench site of crust destruction (slap pull/slab suction) •Subduction can generate deep ocean trenches

Detecting Plate Movement with Satellites

Plate tectonics describes the movement of the outermost portion of earth and creation of continental and sea floor features; plates part of lithosphere floating on asthenosphere

Atolls

Reefs continue to grow after volcanoes are submerged

Seamounts

Rounded tops •Tall volcanoes formed at volcanic centers •Subsidence of flanks of mid-ocean ridge/hotspot movement

Distribution of organisms

Same fossils found on continents today that are widely separated •modern organisms with similar ancestries

Barrier Reefs

Separated from landmass by lagoon

Mid-Ocean Ridge

Spreading center

Paleomagnetism

Study of Earth's ancient magnetic filed •Interprets where rocks first formed

Sir Edward Bullard

Used computer models to fit continents

Continental Drift

Wegener proposed Pangaea (one large supercontinent existed 200 mya) •Panthalassa = one large ocean (+ Tethys Sea)

Theory of Plate Tectonics

thin rigid plates comprising the lithosphere move horizontally

Breakup of Pangaea

•180 mya Pangaea separated (N & S America rifted from Europe & Africa, Atlantic Ocean forms) •120 mya S. America & Africa clearly separated •45 mya India starts Asia collision

Seafloor spreading evidence: Frederick Vine and Drummond Matthews

•Analysis of igneous rock stripes around mid-ocean ridge •Sea floor stripes record Earth's magnetic polarity

Ultra-Slow

•Deep rift valley •(<2 cm/year) •Widely scattered volcanoes •Arctic & SW INdia

Oceanic-Oceanic Convergence

•Denser (older) plate is subducted •Deep trenches generated •Volcanic island arcs generated

Oceanic Rise

•Fast spreading (16.5cm/yr) •Gentle Slopes •East pacific

Mantle Plumes/Hotspots

•Intraplate features •Volcanic islands within a plate •Island chains

Age of Ocean Floor

•Late 1960s deep sea drilling •Radiometric dating of ocean rocks •Symmetric pattern of age distribution at mid-ocean ridges •Oldest ocean floor only 180 myo

Divergent Boundary

•Mantle upwelling •New lithosphere created •Spreading centers •Plates are moving apart (mid-ocean ridge)

Divergent Boundary Features

•Mid-Ocean Ridge (Rift Valley) •New ocean floor (Rifting) •Faster Spreading: less energy released in earthquakes •Shallow focus earthquakes

Earthquakes

•Most large earthquakes occur at subduction zones •Earthquake activity mirrors tectonic plate boundaries

Continental-Continental Convergence

•No subduction •Tall Mountains uplifted Ex: Himalayas from India-Asia Collision

Oceanic-Continetal Convergence

•Ocean plate is subducted (denser oceanic crust subducts) •Continental arcs generated •Explosive andesitic volcanic eruptions ex: Mt. St. Helens

Convergent Boundary Features

•Oceanic crust destroyed (ocean trench, volcanic arc) •Deep focus earthquakes •Great forces invovled •Mineral structure changes

Transform Boundary Features

•Offsets oriented perpendicular to mid-ocean ridge •Offsets permit mid-ocean ridge to move apart at different rates •Shallow strong earthquakes 1. Oceanic Transform fault 2. Continental Transform Fault

Convergent Plate Boundaries: Plate movement, tectonic process

•Plate movement: Together •Tectonic process: Subduction(oceanic-oceanic crust), Collision (continental-continental) •Peru-Chile Trench, Andes Mountains, Marianna Trench, Aleutian Islands

Divergent Plate Boundaries: Plate movement, tectonic process

•Plate movement: apart •Tectonic process: sea floor spreading (oceanic-oceanic crust), continental rifting (continental-continetnal crust) •Mid-Atlantic Ridge, East Pacific Rise, East Africa Rift Valleys, Red Sea, Gulf of California

Transform Plate Boundaries: Plate movement, tectonic process

•Plate movement: past each other •Tectonic process: Transform faulting •Mendocino Fault, Eltanin Fault, San Andreas Fault, Alpine Fault

Convergent Boundary

•Plates collide (plates moving together) •One plate subjects under another ex: deep-ocean trench

Oceanic Ridge

•Slow spreading (2.5cm/year) •Steep slopes •Mid-Atlantic

Objections to Early Continental Drift Model

•Tidal gravitational attractions too small to move continents •Wegener's hypothesis about continental drift was right but proposed mechanism was wrong

Transform boundary

•Usually offset segments of ocean ridges (two divergent boundaries) •Can also join up different combination of divergent and convergent boundaries •plate slide past each other

Harry Hess

•World War II submarine captain and geologist •HIs depth recordings show sea floor features