easc lec 4

Plate tectonic theory explains

• Distribution of volcanic chains & collisional mountain belts

• Distribution of seafloor features

• Age & characteristics

• Distribution of earthquakes

Distribution of volcanic chains & collisional mountain belts (plate tectonic theory)

• Hot spots

• Island arcs

• Volcanic arcs

Distribution of seafloor features (plate tectonic theory)

• Deep-ocean trenches

• MOR (Mid-ocean rifts)

• Transform fracture systems

Age & characteristics of (plate tectonic theory)

• Oceanic crust (+ paleomagnetic ‘stripes’)

• Pelagic sediments

Deep-sea trench

A convergent plate margin where oceanic lithosphere is carred down into the mantle at the subduction zone beneath another oceanic or continental plate

• Long, narrow regions of the seafloor 

• Where old, cold oceanic lithosphere subducts

Mid-ocean ridge (MOR)

Occurs at a divergent plate margin where new oceanic lithosphere is formed by the seafloor spreading

• Portions of the ___ are offset by long fracture zones

  • The ridge axis is segmented along its length at fracture zones

• Expansive uplifted areas of warm, buoyant oceanic lithosphere

Examples

• East Pacific Rise

• Mid-Atlantic Ridge

East Pacific Rise (MOR)

Fast spreading (6-16 cm/yr), moving ocean

• Moves the Pacific & Nazca plates apart

Mid-Atlantic Ridge

Slow spreading (2-6 sm/yr, moving continents)

• Moves the south american & african plates apart

Harry Hess

Use sonar to map the ocean floor ⇒ Determines water depth using soundwaves

• Velocity of waves: Distance = Travel time x Velocity

Princeton geologist (During WW2)

Mid-Ocean Ridge mapping

• Warm

• Buoyant

• Young

• Slow spreading: 2-6 cm/yr

Abyssal Plain mapping

• Cold

• Dense

• Old

Mantle Convention

Mechanism for plate motion

• Motion enhanced by the gravity effects of ridge push & slab pull

Divergent Plate Margins

Convection brings mantle rock towards the surface. Moving to shallower depths, undergoing decompression melting that moves magma upward through fractured lithosphere

Decompression Melting

Some minerals melt

Deep Mantle Plume

• Heats the lithosphere causing doming

• Gravity pulls on the flanks of the upwarped crust, results in stretching & thinning

Rift Valley

• The stretched & thinned lithosphere

• Magma goes upwards through fractures, cooling & creating new lithosphere

Narrow Seaway

• Forms when the rifted lithosphere reaches the continent’s edge 

• Widening by ~15-20 cm/yr

Wide Ocean Basin

• Forms with an expansive MOR

• Crossed by transform features

East African Rift System

• Driven by the African superplume

• 3 arms of rifting split the African Plate into the Nubian (left), Somalian (right) & Arabian (top) plates

• 3 arms ⇒ Red Sea rift (left), Gulf of Aden rift (right) & East African rift (downwards)  meet at a triple junction, the Afar Triangle

Sampling from the Deep Sea Drilling Program 

• Progressively older oceanic crust with distance from the central rift

• Pelagic Sediments above the crust increase in age & thickness

Pelagic Sediments

Accumulate from the slow rain of materials settling through the water column down to the deep seafloor

Pelagic Sediments Materials

Plankton, wind brown dust, volcanic ash

Magnetometer

Towed across a MOR records a mirror image of oceanic crust with a striking pattern of ‘magnetic stripes’.

Stronger magnetism

Magnetic signature matches the magnetic field today = Magnetic normal

Weaker magnetism

Magnetic signature is opposite to the magnetic field today = Magnetic reversal

Magnetite (magnetometer)

• Accessory mineral within the newly formed oceanic lithosphere

• Grains align with the magnetic field at the time of crystallization

  • Earthrocks: Paleomagnetism (8.5 min)

Convergent Plate Margins

Lithospheric plates collide along continental suture zones & deep sea trenches

• Japan trench

• Peru Chile trench

• Aleutian trench

Continental Suture Zones

Subduction brings islands & continents to a subduction zone

• Landmasses are too thick to subduct, thus are added to the edge of the overlying plate

• Suture & Accretion

India’s Suture Zone

Rifted from Gondwana during the Cretaceous & was brought by seafloor spreading and subduction to the Eurasian Plate

• Suturing began 10M yrs ago & the collision still continues

  • The Himalayan Mountains & Tibetan Plateau are still rising

Island & Volcanic Arcs

Oceanic lithosphere is subducted at a deep sea trench. In the case of 2 oceanic plates, the older (& thus denser) plate subducts

• Seawater carried within sediments & fractured oceanic crust is heated 

• Becomes water vapour that rises through the overlying mantle rock. Lowers melting temperature & partial melting occurs

Pacific Ring of Fire

Near-continuous chain of island arcs & volcanic arcs along the perimeter of the Pacific Ocean

Transform Plate Margins 

Lithospheric plates slide past each other along transform faults

• Includes continental transforms & offset MORs

  • San Andreas Fault

Hotspot

Lithospheric plates carried over a deep mantle plume form a chain of hot spot volcanoes