Plate Tectonics Chapter 4 & 5

Early Ideas Early mapmakers (Columbus and Magellan) recorded observations about coastlines –said to look like puzzle pieces. Alfred Wegener –German scientist, 1912, Theory of Continental Drift. Using rock, fossil and climatic evidence, built upon early mapmaker theory. Used fossil of Mesosaur found in W. Africa and E. South America. Named Pangea

Continental Drift Proposed movement of continents 200 mya through movement of continental crust (lithosphere) over mantle (asthenosphere)

Seafloor spreading From ’s, new technology to study seafloor revealed at mid-ocean ridges that rock was not old debris of continents, but new material. Hess concluded that magma at the ridges was adding new material. By 1960’s, discovery of paleomagnetism supported both seafloor spreading and continental drift.

Paleomagnetism

The Earth's magnetic field changes strength and reverses polarity from time to time. When liquid magma cools and becomes rock, some of the minerals in the rock preserve a record of the direction and strength of the magnetic field at that time. This is the process called paleomagnetism. Since radioactive dating can be used, geologists have used paleomagnetism to make a geomagnetic reversal time scale, showing how the Earth's magnetic field has changed over the past couple hundred million years or more.

Theory of Plate Tectonics Proposes that continents have moved, also attempts how and why (something Wegener could not) Lithosphere (rigid crust) is broken into plates that move over the malleable asthenosphere (rock that can flow). Crust is categorized as oceanic (thin, dense, mafic rock) or continental (thick, less dense, felsic or granitic rock)

Theory of Plate Tectonics How plates are thought to move: convection currents As rock/magma heats it becomes less dense and rises, when it reaches the top (lithosphere) it cools, becomes more dense and sinks. Any new, hot material pushes the cold material away. This creates a cycle or current.

Convection Currents The force thought to be responsible for plate movement.

Plate Boundaries The following two slides show maps of the current major plate boundaries and their relative motions. You should be able to describe various landforms created by different plate movements.

Plate Boundaries Divergent Boundary – moving _____ Convergent Boundary – moving ________ Transform Fault Boundary – moving ________________________ apart together sideways past each other

Divergent boundary of two continental plates. Creates a __________. Example: _____________ rift valley East African Rift

Convergent boundary of two oceanic plates. Creates an ________ and a _____. Example: _____ island arctrench Hawaiian Islands

Convergent boundary of an oceanic plate and a continental plate. Forms a _______ mountain range and a ______. Examples: _______ or _______ Mts volcanic trench Cascades Andes

Convergent boundary of two continental plates. Forms a ______ mountain range. Examples: ___________, Alps, ______________ folded Himalayas Appalachians

Transform-fault boundary where the North American and Pacific plates are moving ____ each other. Example: ________________ in California past San Andreas Fault

Plate Boundaries Review Places where plates move apart are called _____________ boundaries. When continental plates diverge a ___________ is formed. When two oceanic plates converge what is created? _________________ The Appalachians formed mainly from continental plate collisions and therefore are a __________ mountain range. The force moving the plates is ____________. Convection currents divergent rift valley an island arc and a trench folded

Theory of Suspect Terranes Plate Tectonics cannot say HOW the continents were formed, this one attempts to. Based on geological evidence, suggests that each continent is a mash-up of many terranes (or pieces of land) Suggest that movement from seafloor spreading moves terranes to continents.