From Continental Drift Hypothesis to Plate Tectonic Theory

Observation: Some of the boundaries of the continents look as though they had once fit together Initial hypothesis, put forth by A. Wegener: In the geologic past, all the continents formed one supercontinent, which subsequently broke up and drifted apart.

Supporting evidence: fossils of the same type found at these now-separated continental boundaries

Supporting evidence: evidence of similar climates at the now-separated areas

Supporting evidence: matching rock types and orientation of mountain ranges at the now-separated areas

Continental Drift Hypothesis Rejected! How did the continents move through the oceans? Tidal influence of the moon? Theoretical calculations proved this to be false. Continental crust plows through or slides over ocean crust? Observations proved this false. The conclusion of the scientific community at the time was that the continental drift hypothesis, as presented by Wegener could not be true.

Sea-floor spreading –modifying the hypothesis The sea-floor spreading hypothesis provided a new mechanism for the change in the position of the continents, based largely on post-WW2 observations: mapping of underwater mountain ranges (oceanic ridges) that circle the globe, often parallel to continental boundaries dredging of sea floor sediment and rocks indicated the age of the oldest ocean crust was much younger than that of continental crust. Recurring patterns of earthquakes and volcanoes in places such as the Circum-Pacific Belt and along the oceanic ridges.

Sea Floor Spreading Hypothesis led to the present Plate Tectonic Theory

The Major Tectonic Plates

Plates move apart. The mechanism called Sea-Floor Spreading Divergent Boundaries Plates move apart. The mechanism called Sea-Floor Spreading

Sea-Floor Spreading Two plates move apart, creating a rift valley between them Magma (molten rock) from the asthenosphere pushes into the rift and creates new seafloor Oceanic ridges (underwater mountain range) develop along well-developed divergent boundaries Mid-Atlantic Ridge East Pacific Rise

Figure 15.10

Sea Floor Spreading on Land? Sea floor spreading adds thin, low-elevation ocean crust to landmass. In time, water fills in and an ocean basin will develop Arabian peninsula split from African continent Process continues in East Africa rift valleys (note lakes filling in low lying ocean crust).

Some geologists are calling this the divergent boundary of the Somali Plate

Forming a Divergent Boundary To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Convergent Boundaries These are boundaries where plates come together and lithosphere is being deformed and/or destroyed There are 3 types of convergent boundaries In 2 of the 3 types the process that occurs is called subduction, where oceanic lithosphere on one tectonic plate is thrust into the mantle beneath another plate.

Oceanic-continental convergence Denser oceanic slab sinks into the asthenosphere (subduction) Pockets of magma develop and rise Continental volcanic arcs form (e.g. Andes, Cascades).

Figure 15.14a

Motion at Plate Boundaries To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Oceanic-oceanic convergence Two oceanic slabs converge and the older, denser one subducts beneath the younger, more buoyant one. Trench and volcanic island arc forms as volcanoes emerge from the sea. Examples include the Aleutian, Mariana, and Tonga islands, and those shown in the map.

Motion at Plate Boundaries To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Continental-continental convergence When converging plates contain continental material at their respective boundaries, subduction cannot occur, as the plates are too buoyant to sink. The continents collide, causing the crust to shorten and thicken This process produces high-elevation, non-volcanic mountains over a large area, such as we see in the Himalayas today.

Figure 15.14c

Convergent Margins: India-Asia Collision To view this animation, click “View” and then “Slide Show” on the top navigation bar.

The Major Tectonic Plates

Transform Fault Boundaries Conservative boundary (no loss or gain of lithosphere) Plates slide past one another Most transform faults join two segments of sea-floor spreading Significant non-oceanic transform fault boundaries include San Andreas Fault, Alpine Fault Anatolian Fault (Turkey)

Figure 15.16

Figure 15.17

Breakup of Pangaea To view this animation, click “View” and then “Slide Show” on the top navigation bar.