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
