Earth Sciences 2240F/G Lecture 3: Chapter 3
13 Jun 2018
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Chapter 3: Plate Tectonics
1.0 Introduction
Earth is different from other planets because we have ‘plate tectonics.’ To Geological engineers, plate
tectonics is important because it explains why and where significant deformation of Earth’s surface
occurs (thus where not to build bridges, for instance). To economic earth scientists it explains the type
and location of many metallic mineral deposits. It further explains the global distribution of earthquake
and volcanic hazards. Earth is the only planet in the solar system whose surface is totally determined by
the processes called plate tectonics. Everything was formed by their movements (continents, ocean
basins, and mountain ranges).
2.0 Early Development of the Theory
2.1 Continental Drift
The first step toward formulation of a theory was made by Alfred Wegener; he grouped his ideas under
the heading ‘continental drift’ and published them in 1912. Nothing exceptional happened toward the
development of the plate tectonics theory until after the war when there was a need to detect both sunken
ships and lurking submarines. Very sensitive magnetometers were deployed and amazed scientists by
detecting curious, repetitive magnetic records in ocean floor rocks, but here was not much in place to
process the data until the war was over. In the 40’s and 50’s a definition of a huge volcanic ridge in the
Atlantic Ocean was discovered and was found to be active for the whole ridge, extending from the far
north to the far south where it connected with other similar ridges.
Harry Hess guessed that the ridges found represented spreading centers where Earth’s crust was moving
in opposite directions like conveyer belts, allowing new ocean floor to be built from volcanic rock at the
ridges. He calculated that, because of this activity, the Atlantic Ocean was widening about 2.5cm/year.
3.0 Paleomagnetism and Earth’s Magnetic Field
Sceptics of plate tectonics were convinced of the theory of plate tectonics when evidence of
paleomagnetism (the study of Earth’s magnetic field through the analysis of rock magnetism) was made –
the second step in the development of the theory of plate tectonics.
3.1 Earth’s Magnetic Field
Earth’s magnetic field is something that regenerates continuously and protects us from space radiation
and solar systems. We know that the Earth doesn’t have a bar magnet inside of it, however the best
hypothesis that best fits the facts is something called the dynamo model. Electric currents are generated by
enormous ‘dynamos’ driven by circulating hot currents in the liquid metal outer core, and magnetic fields
surround those electric currents. In this dynamo mechanism, fluid motion in the Earth's outer core moves
conducting material (liquid iron) across an already existing, weak magnetic field and generates an electric
current. (Heat from radioactive decay in the core is thought to induce the convective motion.)
3.1.1 Stability of Earth’s Magnetic Field
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Document Summary
Earth is different from other planets because we have plate tectonics. " to geological engineers, plate tectonics is important because it explains why and where significant deformation of earth"s surface occurs (thus where not to build bridges, for instance). To economic earth scientists it explains the type and location of many metallic mineral deposits. It further explains the global distribution of earthquake and volcanic hazards. Earth is the only planet in the solar system whose surface is totally determined by the processes called plate tectonics. Everything was formed by their movements (continents, ocean basins, and mountain ranges). The first step toward formulation of a theory was made by alfred wegener; he grouped his ideas under the heading continental drift" and published them in 1912. Nothing exceptional happened toward the development of the plate tectonics theory until after the war when there was a need to detect both sunken ships and lurking submarines.
