Oceanography Chapter 2

Magnetic polarity reversals etc...

- 184 major reversals in past 83 million years - Strength of magnetic field decreases by ~5% each year • May be precursor to reversal - Magnetic north pole moves over time; originally in Canadia Arctic (1831) • Moves ~50km/year • Will pass within 400km of north pole this year; in Siberia by 2050

Transform boundary

- 2 plates move past one another - Usually offset segments of ocean ridges (two divergent boundaries) - Can also join up different combination of divergent and convergent boundaries

Assume same direction and rate of plate motions as now

- Atlantic will enlarge, Pacific shrink (i.e. due to many subduction zones and encroaching continental plates) - New linear sea from enlarged East African rift valleys; Red Sea larger - India keep moving into Asia; further Himalayan uplift - North and South America move west and separate; land bridge to Antarctica with major impacts on oceanic circulation -Other changes may be caused by movement of terranes -Exotic terranes retain distinctive geologic history --- Differ from geology of surrounding area

Mid-ocean ridge - spreading centers

- Continuous underwater mountain range found in every ocean basin; Volcanic in origin; Rises >2.5km off ocean floor - New ocean floor forms at axis of midge-ocean ridge; Sea-floor spreading carries new ocean floor away from ridge

Ultra-slow (<2cm/year)

- Deep rift valley - Widely scattered volcanoes - Arctic and southwest India

Deep Ocean Trenches

- Deepest parts of ocean - Largest earthquakes occur near trenches; caused by plate bending down and plunging into interior of earth

Oceanic-Continental Convergence

- Denser oceanic plate is subducted under continent - Continental arcs formed by folding, uplift, and volcanism - Explosive andesitic volcanic eruptions - Mantle Wedge superheated by subducting plate and partially melted; molten rock is lower density; Rises to surface, feeds volcanoes

Oceanic-Continental Convergence etc..

- Distance from spreading center impacts trench development and depth - More distance gives more time for oceanic crust to cool and contract - Shorter distance does not provide enough time to cool, contract; trenches less well developed

Magnetic polarity reversals

- Earth's magnetic poles switch every ~450,000 years; last one 780,000 ya - Switch takes ~5000 years - Recorded in rocks • Magnetite particles frozen in position capturing strength and orientation of magnetic field, including reversals

Oceanic Rise

- Fast-spreading, produces more crust • Up to 16.5 cm/year - Gentle slopes (~1000m), smooth - Weaker seismic activity (MW=4.5) - small rift valley (e.g. EPR)

Age of Ocean Floor

- Late 1960s deep-sea drilling to test for sea-floor spreading - Radiometric dating of ocean rocks from ridges to trenches - Symmetric pattern of age distribution at mid-ocean ridges

Continental-Continental Convergence

- No subduction occurs - Tall mountains uplifted • Himalayas formed from India-Asia collision 45 mya

Divergent boundary

- Oceanic ridges - Mantle upwelling - New lithosphere created - Spreading centers

Three Types of convergent boundaries

- Oceanic-Continental • Subduction causes trench • Continental volcanic arc • E.g. Peru-Chile trench & Andes - Oceanic-Oceanic • Subduction causes trench (e.g. Mariana trench) • Creates volcanic island arc - Continental-Continental • Collision creates tall mountain range (e.g. Himalayas)

Convergent boundary

- Plates collide - One plate subducts under another - Three types

Convergent Boundary Features

- Plates move toward each other and collide - Oceanic crust destroyed • Ocean trench • Volcanic arc - Deep Ocean trench marks beginning of subduction zone - Volcanic Arc - highly active row of volcanoes above subduction zone - Deep focus earthquakes - Great forces involved - Mineral structural changes

History of Ocean Basins contained 2 key ideas

- Seafloor spreading - Convection cells in asnthenosphere as driving mechanism

East African Rift Valley

- Series of linear Rift Valleys occur throughout NE Africa • Series of volcanic peaks, down-dropped lakes - Linear Sea present in NE (i.e. Red Sea and Gulf of Aden); have fallen below sea level

oceanic ridge

- Slow-spreading • ~2.5 cm/year - Steep slopes (3000m), rugged - Stronger seismic activity (MW=6.0) - prominent central rift valley (e.g. MAR - 32km wide, 2km deep)

What did Combining sea floor stripes of Earth's reversing magnetic polarity with Hess' idea of sea floor spreading give way to?

- Strong evidence for continental drift - Ocean floor was acting as conveyor belt carrying continents

Tablemounts (or guyots)

- Tall underwater volcanoes with flattened tops • Subsidence of flanks of mid-ocean ridge or hotspot movement

Seamounts

- Tall underwater volcanoes with rounded/conical tops • Wave erosion may flatten seamount that reaches surface

Panthalassa

- one large ocean • Included the Tethys Sea

Lines of evidence for current theory of Plate Tectonics

- orientation of magnetic particles in the Earth's crust - magnetic dip (function of the latitude at which the rock cooled) - apparent polar wandering - magnetic polarity reversals - ocean floor magnetic anomalies - sea floor spreading - uneven heat flow in Earth's crust - worldwide earthquake distribution - use of satellite data to detect plate motion • Sea floor studies from World War II - Use of sonar • New technology enabled study of Earth's magnetic field

Theory of Plate Tectonics

- thin rigid plates comprising the Lithosphere move horizontally • Alfred Wegener first proposed idea in in 1912 - Called it "Continental Drift"

divergent plate boundaries have what?

-High volcanic activity -mid-Atlantic Ridge

Continental- continental divergent plate boundaries

-as continent splits apart, new sea floor is created -continental rifting sea floor features are rift valley, volcanoes, young lava flows -examples: east africa rift valleys, red sea, gulf of california

Continental-Continental convergent plate boundaries

-collision -sea floor features are tall mountains -examples: Himalaya Mountains, Alps

Oceanic-oceanic divergent plate boundaries

-new sea floor is created -sea floor is spreading -sea floor features are mid-ocean ridge, volcanoes,young lava flows -examples: mid-atlantic ridge, east pacific rise

Divergent boundaries have three types of spreading centers

-oceanic ridge -oceanic rise -ultra-slow

Oceanic- continental convergent plate boundaries

-old sea floor is destroyed -subduction -sea floor features are trench, volcanic arc on land -examples: peru-chile trench, andes mountains

Oceanic- Oceanic continental plate boundaries

-old sea floor is destroyed -subduction -sea floor features are trenches, volcanic arc as islands -examples: mariana trench, aleutian islands

continental transform plate boundaries

-transform faulting -sea floor feature is fault -examples: San Andras falt, alpine fault (new zealand)

Oceanic transform plate boundaries

-transform faulting -sea floor features are fault -examples: Mendocino fault, Eltanin fault (between mid-coean ridges)

Predictive plate tectonics model shows life cycle of ocean basin formation, growth, and destruction

1. Embryonic -heat source & upift 2. Juvenile -spreading & downdropping 3. Mature - mature basin with mid-ocean ridge 4. Declining -subduction zones; basin shrinks 5. Terminal - progressive narrowing of ocean 6. Suturing - ocean disappears; continents collide

How old is the oldest ocean floor?

180 million years old

When did deep water mapping/paleomagnetic studies begin?

1955, before that only terrestrial

Frederick Vine and Drummond Matthews (1963)

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

which basin has the most symmetric pattern of age distribution?

Atlantic ocean basin

Continental accretion

Continental material added to edges of larger continents through plate motion

Hotspot volcanic activity due to

Mantle Plumes - Tube-shaped areas of hot molten rock rising from the mantle - As plate moves over mantle plume, active volcano carried off -intraplate features-not at plate boundaries

Where is there very high heat flow?

Mid-Ocean ridges - 8X greater than average to other parts of Earth's crust

what rifted pangea apart?

Mid-atlantic ridge

what crust types do transform plate boundaries include?

Oceanic continental

what crust types are involved in divergent plate boundaries?

Oceanic- oceanic continental- continental

what crust types are involved in convergent plate boundaries?

Oceanic-continental oceanic-oceanic continental-continental

which basin has the least symmetric pattern of age distribution and why?

Pacific ocean basin, because of numerous subduction zones

Global Plate Boundaries -Seven Major plates:

Pacific, North American, South American, Eurasian, African, Antarctic, and Australian

Detecting Plate Movement with Satellites

Satellite-based measurements confirm regions of the Earth are moving in direction and rate predicted by plate tectonics; generally plates move 2-12cm/year on average

Where is there very low heat flow?

Subduction Zones - Older and colder oceanic crust - ~10% of average heat in crust (crust 2X as thick)

Where do most large earthquakes occur?

Subduction zones - Energy released during subduction

What does earthquake activity mirror?

Tectonic plate boundaries

Harry Hess

World War II submarine captain and geologist; depth recordings show sea floor features; "mountain ridges" and trenches

Where do transform faults always occur?

between mid-ocean ridge segments

Subduction can generate

deep ocean trenches

Fringing reefs

develop along margin of landmass where physical environment suitable for coral growth

Terranes

fragments of crustal material broken off one plate and accreted or sutured onto another; e.g. land west of San Andreas Fault (California terrane) will be accreted on to southern Alaska in ~50 my

Heat flow

heat from Earth's interior released to surface - Moves to surface with magma in convective motion

Magnetometer

instrument towed behind ocean vessel - Measures Earth's magnetic field; how it was affected by sea floor rocks

rise

less time to cool, contract, sink

What do the patterns of earthquake locations mirror?

locations of plate boundaries

Magnetic dip

magnetite particles in sedimentary rocks or igneous rocks such as basalt align with Earth's magnetic field; relates to latitude where rock formed

Where do earthquakes occur?

mid-ocean ridges, and along fault lines

ridge

more time for subsidence

Subduction zones

oceanic trench site of crust destruction - Slab pull of crust and Slab Suction in mantle drives plate motion

Wegener proposed Pangaea

one large supercontinent existed 200 million years ago

Magnetic anomalies

pattern of north-south magnetism "stripes" - Stripes were symmetrical about long underwater mountain range, but why?

What are two of the youngest seas on earth?

red sea and gulf of california (both in linear sea stage) • Will eventually become large ocean basins with well-developed mid-ocean ridge (like Indian Ocean)

Atolls

reefs continue to grow as volcanoes subside; corals build up toward surface; after volcano completely submerged atoll remains If coral growth rate (3-10m/1000 years) faster than rate of subsidence

Barrier reefs

separated from landmass by lagoon; linear or circular in shape; many found around tropical volcanic islands

Mid-ocean ridge

spreading center - Convection cells pull sea floor apart - New oceanic crust formed at ridges - Split apart at ridge and moved away toward subduction zones

Paleomagnetism

study of Earth's ancient magnetic field - Interprets where rocks first formed (i.e. as rock cools captures magnetic field information from where formed) - Earth's geographic and magnetic north pole do not coincide

Paleogeography

study of historical changes of continental shapes and positions (and of ocean basins)

Reef development relates to

subsidence of volcanic islands

Plate tectonics more encompassing theory (than continental drift) describing

the movement of the outermost portion of earth and creation of continental landforms and sea floor features

Wegener's mechanism for continental drift defied laws of physics

ultimately his hypothesis about continental drift was right, but his proposed mechanism, wrong. - Continental rock too weak to plow through oceanic rock - Tidal and gravitational forces are too small to move continents

• Sir Edward Bullard

used computer models to fit continents. - Used 2000m depth contour • Then he searched for matching sequences of rock units and ancient mountain chains • Similar rock types, ages, and structures are found on different continents

Pangaea formed ~240 mya

• 180 mya - Pangaea separates - N. and S. America rifted from Europe and Africa; Atlantic Ocean forms • 120 million years ago - S. America and Africa clearly separated • 45 million years ago - India begins collision with Asia; Australia moving north from Antarctica

Divergent Boundaries

• 2 Plates moving apart • Usually along Mid-ocean ridge - Rift valley: down-dropped linear depression • New ocean floor (lithosphere) created during rifting - plates continuously pulled apart; upwelling magma fills void • Faster spreading = less energy released in earthquakes • Shallow focus earthquakes - Intensity measured with seismic moment magnitude (not Richter scale) • Energy released to create long-period seismic waves

Great Barrier Reef

• 3000 reefs in barrier reef stage • 40km offshore, 150 km wide, and >2000 km long • Formed by Indian-Australian plate moving northward - Oldest and thickest portions (~30 my) of GBR in north, youngest in south - Southern boundary at Tropic of Capricorn (i.e. start of tropics)

Oceanic-Oceanic Convergence

• Denser plate is subducted - Usually older plate • Deep trenches generated • Oceanic island arcs generated by volcanism - E.g. West Indies, Aleutians

Evidence for Continental Drift

• Distribution of organisms • Same fossils found on continents that today are widely separated (e.g. mesosaurus, plants) • Modern organisms with similar ancestries (e.g. marsupials)

Bend in Nematath Track

• Due to change in motion of Pacific Plate and of Hawaiian mantle plume

Evidence for Plate Tectonics

• Earth's magnetic field and paleomagnetism • Earth has magnetic field and polarity - Generated by convective movement in outer core • North and South polarities - Magnetic objects align parallel to magnetic field • Magnetic polarity recorded in igneous rocks (from magma or lava) - Magnetite (magnetic Fe particle) in basalt

Glacial ages and other climate evidence

• Evidence of glaciation in now tropical regions; 2 possibilities - Glacial ice age ~300 million years ago (refuted by presence of coal in NA/Europe; evidence these regions were swamps 300 mya) - Some continents now in tropical regions were closer to the poles • Patterns and direction of glacial flow and rock scouring in tropical regions of globe; Evidence of glaciers flowing away from south pole • Plant and animal fossils indicate different climate than today. - E.g. fossil palm trees in arctic; coal deposits in Antarctica; fossil corals

Divergent Boundary - Creation of an Ocean Basin

• Heat Source leads to unwarping - Molten material rises to surface; upwarping, volcanism, crust thins • Movement and splitting apart of land (i.e. rifting) creates linear rift valley - Downdropping causes surface to fall below sea level • Increased movement and down-dropping forms low area; eventually flooded by seawater; forms a narrow, linear sea • Eventually an ocean basin with mid-ocean ridge is created

Hotspots

• Intraplate regions of intense volcanic activity that remain in one location over geologic time (e.g. Hawaii, Yellowstone) • More than 100 globally over past 100 million years

Hawaiian Island - Emperor Seamount Chain nematath stretches NW/SE

• More than 100 intraplate volcanoes • Stretches over 5800 km (3000 mi) • Nematath - Chain of extinct volcanoes records plate movement • Volcano Age relationships suggest northwest migration of Pacific Plate

Transform Faulting

• Movement of one plate past another • Shallow but strong earthquakes • Continental transform faults have stronger quakes; due to thicker crust

2 Types of Transform Faults

• Oceanic Transform Fault - ocean floor only - Most common • Continental Transform Fault - cuts across continent - Rare (e.g. San Andreas Fault)

Transform Boundary Features

• Offsets oriented perpendicular to mid-ocean ridge - Segments of plates slide past each other • Offsets permit mid-ocean ridge to move apart at different rates • Shallow but strong earthquakes at transform faults

Wegener's Evidence for Continental Drift

• Wegener first noted puzzle-like fit of modern continents - But gaps and areas of overlap • Puzzle-like fit corroborated in 1960s

Apparent polar wandering from magnetic dip data

• When viewing samples from different locations suggested two different different poles • Solution: the magnetic poles didn't move; the continents moved