Constructive Plate Margins

1
Constructive Plate Margins
2
The features

• A central rift because the crust is being pulled
  apart by the diverging convection currents.
• A mountainous mid ocean ridge because the hot
  currents below are making the land rise.
• Transform Faults form at 90 to the spreading
  ridge.

3
Chert

• Minute skeletons of radiolaria that are Si rich.
• These gradually accumulate to form fine mud/ooze.
• When this sediment is compacted it eventually
  becomes a chert.

4
Magma!

• What is it that melts?
• Upper mantle/peridotite.
• What is its composition?
• Ultrabasic.
• Why does it melt?
• Reduced P as crust above is stretched and
  thinned.
• Increase in T due to hot rising convection
  currents.

5
Basalt

• What kind of magma is produced by melting the
  ultrabasic mantle?
• Basic.
• What does the magma do?
• Rises up through the thin crust to the surface.
• It may erupt underwater to produce what?
• Basalt pillow lavas.

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Basalt

• Fine-grained.
• Forms pillow lava (explain formation).
• Most activity occurs along the central part of
  the M.O.R. called the "Central Rift Valley",
  eruptions often occur along fault lines produced
  by the tension.
• Basalt is not very viscous, therefore it flows
  quite a way before solidifying.
• Therefore these volcanoes have fairly low angled
  sides.

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Basaltic Volcanoes

• Eventually the volcanoes may break the surface
  islands e.g. Iceland.
• Therefore the volcanic hazard is obvious.
• These islands have the least dangerous form of
  volcanic activity eruptions are usually just of
  lava, so this presents little problem.

8
Dykes

• You know these.
• They are small scale intrusions (no more than a
  few 100 m wide).
• They form close to the surface Hypabyssal.
• Dykes are discordant cut across the beds.
• They often force their way along lines of
  weakness such as faults.
• In cross section or a map they will cross cut
  beds.

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Sills

• You also know these.
• They are also small scale intrusions (no more
  than a few 100 m wide).
• They also form close to the surface Hypabyssal.
• Sills are concordant run parallel to the beds.
• They often force their way along lines of
  weakness such as bedding planes.
• In cross section and map views they run parallel
  to the beds.

10
Dolerite and Gabbro

• Some of this basic magma will cool in the
  fractures feeding up to the surface.
• What rock and structure will form?
• Sheeted dolerite dykes.
• Some magma will cool in the magma chamber to form
  what rock?
• Gabbro.

11
Magnetic Stripes

•   After the Second World War geologists had the
  technology to study the sea floor.
•  They used ships and planes with magnetometers to
  measure the magnetic field above the ocean floor.
•  During the history of the Earth the direction of
  magnetic north has frequently changed/reversed.
•  Thus sometimes rocks are magnetised to the North
  and sometimes to the South.

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Magnetic Stripes

•  It was found that rocks could be mapped as
  magnetic stripes parallel to the M.O.R. with
  alternating North and South magnetised rocks.
• These stripes were the same on either side of the
  M.O.R. i.e. they were mirror images.
• The rocks also got older as you moved away from
  the ridge.