Divergent plate boundaries

Page 1

Divergent Boundary Features

Surface features

Volcanic activity

Earthquake distribution

Magnetic features

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Oceanic Ridges

linear mountain chains, all major oceans have them

longest mountain chain- total length over 65 000 km

charecterised by shallow earthquakes of low magnitude, not the deep or inclined seismic belt of a convergent boundary

Page 3

Surface features and hydrothermal activity at ridges

Away from ridges most of the sea is flat 4km below sea level, referred to as abyssal plains, covered in sediment

This sea floor is dark and cold - average temp being 0-3 degrees C, pressure is high due to depth of water - a hostile environment

The ridges were first visited in the 1970s by submersibles

Black smokers (chimneys) pump out super heated mineral water at velocity of 1-5 m per second and a temp of 350 degrees C, at these pressures waters boiling point is in excess of 380 degrees C. The grey smoke like material forms as the hot water meets the nearly freezing sea water, the minerals are mainly iron and sulphide, they form particles as they cool and form these chimneys

The hot water in these hydrothermal vents does not boil due to the pressure, chimneys indicate a nearby heat source- the magma

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Hydrothermal circulation

Sea water filters down in fractures in the crust and is heated by proximity to the magma chamber, the hot water is bouyant and returns to the surface packed with minerals, the rapid upward movement leaves a vaccuum which is then filled by more sea water

bizarre lifeforms never seen before are seen in this environement (image of smoker mound covered in tubeworms. Extremophiles as they are able to survive in this hostile environment.

microscopic life also abundant in the mineral based ecosystem

mid ocean rocks are not covered in mud and have a low bait form, lava flow that comprises basaalt, they look like toothpaste squeezed from a tube - called pillow lava and only formed when lava errupts in water

75% of all lava is errupted along these ridges - most significant volcanic feature on the planet.

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Standard features of a dovergent boundary

a ridge or bathymetric high

localised shallow low magnitude earthquakes

active volcanism

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Ridge

usually a bout 1km higher than surrounding abyssal plain

shallow slop angle with a central graben (depression) at the ridge crest

faults and fissures due the ocean crust being pulled apart by divergence

the ridge is offset by fractures

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Earthquake distribution

run along the crest of the ridge , also along the offset fracture

Charlie-Gibbs fracture zone

on mid altantic ridge, the fracture zone is a transform fault and this is where earthquakes will form

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Magnetic features and seafloor spreading

iron rich basaalt lava is strongly magnatised

the ocean floor has a sequence of magnetic stripes (like a barcode)

the stripes are parrellel to the ridge in bands of high and low magnetisation

During the past Ma the earths magnetic field has reversed and back again, rocks created as lave during this time are fixed in the magnetic polarity of the time

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Continental Rifting

Francis Bacon in the 16th C recognised the fit of Africa and South America, no way to prove or know why

Sea floor spreading causes the continents to drift apart, but where did they start?

Rift valleys on the continents are divergent plates, East African Rift

The crust thins and the surface is lowered each time causing a rift valley

Continental rifts are volcanically active

As they lower eventually the sea invades and a narrow ocean basin forms like the red sea

Eventaully the continental crust is breached and lave errupts forming oceanic lithosphere

Earth is constantly changing, they drift becoming closer and nearer

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Transform faults/conservative boundaries

The San Andreas fault (California) is a major transform fault

Over 1200 km long

Boundary of Pacific and North American plates

ACtive for over 30 Ma years

Two points are now 560 km apart over that time