Plate Tectonics

1968

Year the theory of plate tectonics was formulated

Continental drift theory

Alfred Wegener believed landmasses of Earth once fit together to form single landmass called "Pangaea"; landmasses "drifted" to where they are today; theory disregarded due to lack of evidence.

Fossil match. Continents fit like a jigsaw puzzle. Similar rocks types and geologic features. Ancient climates.

Evidences supporting the continental drift theory

Mesosaurus and Glossopteris

Organisms whose fossils match on opposite sides of different continents.

Mesosaurus

Aquatic fish-catching reptile. Fossil found in South America and Africa. Remains found in Permian black shales.

Glossopteris

Fossil seed-fern; tongue-shaped leaves. Seeds too heavy to be carried by wind. Fossils found in Africa, Australia, India, and South America. Grows in sup-polar climates.

Paleoclimate

A climate prevalent at a particular time in the geological past.

Ocean ridge systems exist. Tectonic activity, through earthquake studies, was occurring at great depths (trenches). Seafloor is no older than 180 MYO.

Findings found by post-WW2 research of the ocean floor.

Divergent plate boundaries Convergent plate boundaries Transform plate boundaries

Types of plate boundaries

Spreading center

Manifestations of divergent plate boundaries. Can be on ocean basins or continents.

Oceanic ridges

Underwater mountain ranges where the crust is spreading apart creating new ocean floor. 70,000km long, 20% of the Earth's surface. Crest = 2-3km high (gradual slope) Ridge = 1000-4000km wide

Rift valley

Deep, downfaulted structures that are evidence of tensional forces.

Seafloor speading

Mechanism that operates along the oceanic ridge system.

5 cm/year

Average rate of seafloor spreading.

80 million years

Amount of time needed for temperature of crust from oceanic ridges to stabilize / stop contracting.

The hotter the rock, the denser it is.

False.

Mantle upwelling. Stretching of lithosphere. Formation of continental rift.

Stages on forming continental rifts.

Subduction zone

Site where old, dense oceanic lithosphere descends to the mantle.

Deep-ocean trench

Surface manifestation of subduction of oceanic lithosphere to the mantle. Long and deep. Happens on an angle.

Warm and buoyant oceanic lithosphere produces a steep, nearly 90 degree angle when subducting.

False.

Cold and dense oceanic lithosphere produces a shallow and gentle-sloped subduction.

False.

~100km

Depth where melting will begin.

Hydrous minerals and pore spaces between rocks and sediments.

Source of water that lowers the melting of mantle rock during partial melting.

Island arc

A chain of volcanic islands formed at an ocean-ocean convergent boundary.

Continental volcanic arc

Mountains formed in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent.

~80km

Interval between volcanoes in a volcanic island arc.

100-300km

Distance of formed volcanic island arc from the trench.

Lesser Antilles and Sandwich Islands

The only volcanic island arcs in the Atlantic Ocean.

Orogeny

A process in which a section of the earth's crust is folded and deformed by lateral compression to form a mountain range.

Active zone

Part of the fracture zone. Lie only between two offset ridge segments. Produces weak and shallow earthquakes.

Inactive zone

Part of a fracture zone. Beyond the ridge crest. Fractures preserved as linear topographic depressions. Parallel to plate motion; useful in mapping.

Fracture zone

A linear zone of irregular topography on the deep-ocean floor that follows transform faults and their inactive extensions. Prominent linear breaks in the seafloor.

Ocean drilling. Hotspots. Paleomagnetism.

Ways to prove the plate tectonics theory.

Deep Sea Drilling Project (DSDP)

1968-1983 Made use of the Glomar Challenger which used fossil remains to determine age.

Sediments increase in age with increasing distance from ridge. No seafloor older than 180 MYO. Sediment thickness increase with increasing distance from ridge.

Major findings of the Deep Sea Drilling Project (DSDP)

JOIDES Resolution

A drill ship that replaced the Glomar Challenger in 1985. It has a tall metal drilling rig to conduct rotary drilling. Became part of the Integrated Ocean Drilling Program (IODP) in October 2003.

Hotspot

A weak spot in the middle of a tectonic plate where magma surfaces; forms a volcano. Surface manifestation of a mantle plume.

Mantle plume

Cylindrically-shaped upwelling of hotter than usual mantle material.

Hotspot track

Linear chain of extinct volcanoes formed as the plate moves over the hotspot. Example is the Hawaiian Island-Emperor Seamount chain.

Paleomagnetism

The study of changes in Earth's magnetic field, as shown by patterns of magnetism in rocks that have formed over time.

585 degrees Celsius

Curie point. Threshold temperature for magnetism.

Fossil magnetism

Record of the direction of magnetic poles at the time of formation.

Apparent pole wandering

Due to the observed magnetic alignment of Fe-rich minerals in rocks indicating the position of paleomagnetic poles. Evident for the past 500 million years.

Normal polarity

Same as today. Magnetic north is almost at geographic north and magnetic south is almost at geographic south.

Reverse polarity.

Different compared today. Magnetic north is at geographic south and magnetic south is at geographic north.

Magnetic time scale

The detailed history of Earth's magnetic field reversals as determined by measuring the thermoremanent magnetization of rock samples whose ages are known.

Chron

Divisions of the magnetic time scale. Each last ~ 1,000,000 years.

Magnetometer

Device used to map the ocean floor that detects small changes in magnetic fields.

High intensity magnetometer reading

Signifies normal polarity and enhanced magnetic field.

Low intensity magnetometer reading

Signifies reverse polarity and weakened magnetic field.

50 MYO

Time when the India subcontinent collided with the Eurasian plate to form the Himalayas.

20 MYO

Time when the Arabian plate diverged from the African plate to form the Red Sea.

Hotspot tracks. From space using GPS.

Ways to measure plate motion

Slab pull

As slabs sink, they pull the trailing plate along due to the density of the descending oceanic slab.

Ridge push

Causes lithosphere slabs to slide down to flanks of ridge thus, pushing the plate. Gravity-driven. Has less contribution than slab pull.

Mantle drag

The dragging force between the asthenosphere and the overlying lithosphere.

Plate movement is enhanced when flow of asthenosphere > plate.

True.

Plate movement is impeded when flow of asthenosphere < plate due to friction.

True.

Layering at 660km depth. Whole mantle convection.

Models for plate-mantle convectio