Continental drift was first proposed by Alfred Lothar Wegener in 1912,
he suggested that all of the continents were once joined together in one super continent which
he called Pangaea. He cited evidence of various types to support his theory,
the similarity of the coastlines,
fossil evidence,
rock sequences
and
glacial evidence.
Although his theory is now accepted world-wide by most geologists, at that time northern
hemisphere scientists found it
difficult to believe,
as Wegener could not offer a sensible mechanism for continental drift to take place. In 1928
Scottish Geologist Arthur Holmes suggested a mechanism, convection in the earth's interior.
Wegener died before he could adapt the ideas of Holmes but with some modification these ideas
now make a major contribution to Plate Tectonic Theory.
In 1962 A Geologist called Harry Hess suggested that new crust was continually being created at
rift valleys in the mid ocean ridges, which were found in all the major oceans and seas. He
suggested that the crust moved away from the ridges and eventually was destroyed in deep ocean
trenches at continental margins. Robert Dietz proposed an improved theory where not just the
crust but the entire
asthenosphere
slid over the mantle. He called this process
sea-floor spreading.
Evidence to support Dietz was found by seismologists, when studies of these ridges showed them
to be major centers of seismic activity.
The evidence for sea-floor spreading was all the more convincing when
magnetic anomalies
were found in oceanic crust. These anomalies were found to be in patterns repeated on either
side of the mid ocean ridges and represented reversals of the earths magnetic field 'frozen' into
rocks formed at the time. This led geologists to believe that the sea floor patterns were created
at the same time as the coresponding patterns on the other side of the ridge, also the age of the
sea floor was found to increase away from the ridges with the oldest rock found at the ocean
edges. This was conclusive proof for most geologists and sea-floor spreading was accepted as a
reality.
Seismic activity and volcanoes were found to be almost synonymous with
plate margins
, with basaltic
volcanics and shallow earthquakes found at ocean ridges and andesitic volcanics and deep earthquakes
found in association with subduction zones. The most famous example of plate margins and volcanics
is the Pacific ring of Fire, an almost continuous ring of volcanoes around the edge of the
pacific ocean.
The mid ocean ridges were termed constructive margins or divergent plate boundaries.
Other Plate boundaries were identified as destructive margins or convergent plate boundaries and
conservative margins or transformational plate boundaries.
Constructive Margins
are where plates are moving apart (Normally Oceanic) at the mid ocean ridges and new crust
is being formed from basaltic lava rising from the mantle below.
Lava Fountains in Iceland as New Crust is Formed
The most famous of these is the mid Atlantic ridge but there are medial ridges in every
ocean. Associated with mid ocean ridges are basaltic volcanics and shallow earthquakes. The
newest ocean basin to form will be the great Rift Valley in east Africa which is a continental
plate rifting apart. This basin is not yet wide enough for the sea to invade but will
ultimatly become an ocean basin with a mid ocean ridge.
East African Volcano associated with the Great Rift Valley
destructive margins
are where one oceanic plate has been forced under another plate, oceanic or continental, and
formed a deep ocean trench. These margins are called
subduction zones
and are areas where crust is being destroyed. Associated with subduction zones are andesitic
volcanics
Mount St Helens, a subduction Volcano
and deep earthquakes. The most famous example of a destructive margin is the west
coast of South America.
conservative margins
are where two plates are moving parallel to each other but in opposite directions, as crust is
not being created or destroyed there are normally no volcanics but earthquakes are common. The
most famous example of a Conservative margin is the San Andrea's fault in California.
