Presentation on theme: "From Continental Drift Hypothesis to Plate Tectonic Theory"— Presentation transcript:
1From Continental Drift Hypothesis to Plate Tectonic Theory
2Observation: 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.
3Supporting evidence: fossils of the same type found at these now-separated continental boundaries
4Supporting evidence: evidence of similar climates at the now-separated areas
5Supporting evidence: matching rock types and orientation of mountain ranges at the now-separated areas
6Continental 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.
7Sea-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 boundariesdredging 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.
8Sea Floor Spreading Hypothesis led to the present Plate Tectonic Theory
10Plates move apart. The mechanism called Sea-Floor Spreading Divergent BoundariesPlates move apart. The mechanism called Sea-Floor Spreading
11Sea-Floor SpreadingTwo plates move apart, creating a rift valley between themMagma (molten rock) from the asthenosphere pushes into the rift and creates new seafloorOceanic ridges (underwater mountain range) develop along well-developed divergent boundariesMid-Atlantic RidgeEast Pacific Rise
13Sea 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 developArabian peninsula split from African continentProcess continues in East Africa rift valleys (note lakes filling in low lying ocean crust).
14Some geologists are calling this the divergent boundary of the Somali Plate
15Forming a Divergent Boundary To view this animation, click “View” and then “Slide Show” on the top navigation bar.
16Convergent Boundaries These are boundaries where plates come together and lithosphere is being deformed and/or destroyedThere are 3 types of convergent boundariesIn 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.
17Oceanic-continental convergence Denser oceanic slab sinks into the asthenosphere (subduction)Pockets of magma develop and riseContinental volcanic arcs form (e.g. Andes, Cascades).
19Motion at Plate Boundaries To view this animation, click “View” and then “Slide Show” on the top navigation bar.
20Oceanic-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.
22Motion at Plate Boundaries To view this animation, click “View” and then “Slide Show” on the top navigation bar.
23Continental-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 thickenThis process produces high-elevation, non-volcanic mountains over a large area, such as we see in the Himalayas today.
27Transform Fault Boundaries Conservative boundary (no loss or gain of lithosphere)Plates slide past one anotherMost transform faults join two segments of sea-floor spreadingSignificant non-oceanic transform fault boundaries includeSan Andreas Fault,Alpine FaultAnatolian Fault (Turkey)