Plate Tectonics

Theory of Continental Drift

In 1910, a German scientist named Alfred Wegener developed the Theory of Continental Drift, to prove that all the continents were once one super continent named Pangea. Wegener's theory consisted of:

     (1) Coastlines fit together like jigsaw puzzle

     (2) Similar rocks and structures on different continents

     (3) Similar fossils.   ex) rocks of Ordovician through Triassic age from both Africa and South America contained identical fossils

     (4) Similar glacial features (Paleoclimatology - study of ancient climates)

However, the problem with Wegener's theory is that at the time, it seemed impossible for the continents to be able to move so much, and Wegener was unable to provide an explanation for his theory.

Plate Tectonic Theory

A complete theory that explains how the plates move. It also explains Wegener's theory, as well as volcanoes, earthquakes, isostacy and the interior of the Earth. The Earth's lithosphere is made of about 12 'plates' that move over the asthenosphere. Seismic and tectonic activity mainly occurs at the plate boundaries. The lithosphere is about 100 km thick, and is made of the Earth's crust and upper mantle.

Tectonic Activity:

From the ancient Greek word for "construction", tectonic activity refers to the events of mountain building, volcanic eruptions, and earthquakes due to the plates grinding together at their boundaries.

Seismic Activity:

Fault: Any fracture in rock along which movement has occurred.

Plate Boundary: A fault separating one plate from another.

Continental Drift: The theory proposed by Alfred Wegener that all continents were once joined together and later split and drifted apart. The continental drift theory has been replaced by the more complete plate tectonics theory.

Pangaea: In Alfred Wegener's theory of continental drift, the supercontinent that included all present major continental masses(from the Greek root words for "all land"). Also, the northern part of Pangaea is called Laurasia and the southern part Gondwanaland.

Subduction: The process in which a lithospheric plate descends beneath another plate and dives into the asthenosphere.

Subduction Zone: The region or boundary where a lithospheric plate descends into the asthenosphere.

Transform fault: A strike-slip fault between two offset segments of mid-oceanic ridge.

Transform plate boundary: A boundary between two lithospheric plates where the plates are sliding horizontally past one another.

Convergent plate boundary: A boundary where two lithospheric plates collide head-on.

Divergent plate boundary: The boundary or zone where lithospheric plats separate from each other.

Mid-oceanic ridge: A continuous submarine mountain chain that forms at the boundary between divergent tectonic plates within oceanic crust.

Oceanic crust: **The 7- to 10-kilomenter-thick basaltic layer and the thin overlying sediment that underlie the ocean basins. It is more dense, thinner and newer than the continental crust.

Sea-floor spreading: The theory that segments of oceanic crust are separating at the mid-oceanic ridge.  Symmetric bands of magnetic striping on either side of the mid - ocean ridge. 

Isostasy: The condition in which the lithosphere floats on the asthenosphere as an iceberg floats on water.

Asthenosphere:The portion of the upper mantle beneath the lithosphere. It consists of weak plastic rock where magma may form.

 At the rate plates are moving (2-10cm a year), it is possible to end up with another Pangea in less than 200 years.

Plates

Oceanic plates: Oceanic plates relative to continental plates, are much denser, and thinner. They are also formed more recently than continental plates due to the diverging boundaries and convergent boundaries constantly cycling.

Continental plates: Continental plates are lighter, and much thicker than oceanic plates. They are also formed a much longer time ago.

Seafloor Spreading

-At mid-ocean ridges where magma rises and pushes apart each plate.

        PROOF: 1) Mid-Ocean Ridge, a massive mountain chain along seafloor

                      2) Youngest rock at the mid-ocean ridge--oldest rock far from midocean ridge

                      3) Sediment is thickest far from mid-ocean ridge near the continents

                                -more time to accumulate sediment

                      4) Magnetic Striping

                                -earth's magnetic field flips every ½ million years

                                -magma rises, the iron in the magma alligns w/ the earth's magnetic field.

                                -the earth's magnetic field flips as new magma rises and pushes aside old rock.

                                -over time, the seafloor has a symmetric pattern on either side of the mid-ocean ridge.