Plate Tectonics

-          Polar wandering: rocks record apparent movement of the magnetic pole

-          Oceanic crust: primarily mafic igneous rocks (basalts) with a thin layer of sediments derived from continents and from dead marine organisms

-          Continental crust: igneous, metamorphic, and sedimentary rocks, a majority of continental igneous rocks are felsic (granites)

·       Who originally proposed and defended the theory of Continental Drift?

o   Alfred Wegener proposed the continental drift hypothesis in 1915

·       What are the different lines of evidence for continental drift?

o   Geometric evidence: continents match up better using continental shelf rather than moder-day coast

o   Fossil evidence: land-dwelling animals and plant distributions match across continents

o   Paleoclimate evidence: evidence for ice sheets in places now in the tropics, coal swamps (tropical) in now temperate locations

o   Geological evidence: close match of rock types, structures, and mountain belts across the continents

·       Rock magnetism: normal vs. reversed polarity, concept of polar wandering, seafloor stripes

o   The minerals in the rocks are magnetized, the minerals algin with magnetic north

o   Polar wandering: rocks record apparent movement of the magnetic pole, indicates either that the continents have moved or magnetic north pole has moved over time

o   Earth’s magnetic field periodically changes polarity

o   Magnetic N sometimes at geographic S pole, known as magnetic reversal

o   Rocks that have the same magnetism as present have normal polarity

o   Rocks with opposite magnetism have reversed polarity

o   The high intensity stripes show normal polarity, which is closest to the magma

o   Low intensity stripes show reversed polarity

o   The seafloor strips, magma upswells at mid ocean ridges and records magnetic field, pushed away from ridge as new magma rises, stripes of same magnetism on either side

·       Stress and strain definitions

o   Stress: force applied per unit area

o   Strain : change in shape and/or volume as result of imposed stress

o   3 different types of stress

§  Compressional: shortens rock bodies

§  Tensional: lengthens rock bodies

§  Shear: horizonal displacements along the fault

·       Brittle deformation vs. ductile deformation

o   Brittle deformation: rocks break in response to stress, resulting in faults

o   Ductile deformation: rocks bend and distort without breaking, resulting in folds

o   Factors that influence the strength of a rock and how it will deform is due to temperature, confining pressure, strain rate, and rock minerals

·       Definitions of the lithosphere  and asthenosphere

o   Lithosphere: crust and upper part of the mantle, the plates and rigid

o   Asthenosphere: part of a mantle below lithosphere, weak solid (close to melting), allows plates to move

·       The three types of stress (compressional, tensional, shear) AND be able to associate these with plate boundary types.

o   Transform = shear stresses, plates slide horizontally past each other

o   Divergent = tensional stresses, plates move apart and create new lithosphere

o   Convergent = compressive stresses, plates collide and one is pulled into the mantle and recycled

·       Differences between oceanic and continental crust: thickness, density

o   Oceanic crust is denser and thinner than continental crust

·       The three types of plate boundaries:

o   Motions (direction) and associated stress type

o   Where on earth these boundaries occur (in general, and know the examples of specific locations that were mentioned in lecture)

§  Divergent Plate Boundary

·       At mid-ocean ridges, areas of seafloor spreading, thus creation of new oceanic crust. Also, continental rifting, continental crust splits apart at rift, leading to creation of a new ocean

§  Convergent Plate Boundary

·       At oceanic-continental boundaries, oceanic crust cools with increasing age, plate subducts beneath to subduction zones, oceanic trenches and volcanic activity on overriding plate, the destructive plate boundary crust is destroyed

·       Oceanic-continental convergence, ex. Andes, cascades

·       Oceanic-oceanic convergence, age of plate determines which will subduct (older = more dense) and creates volcanic island arcs

·       Ex. Aleutian Islands, Alaska

·       Continental-continental convergence, continental crust too buoyant to subduct, collision zone = mountain building, ex. Himalayas 

§  Transform Fault Boundaries

·       Plates slide past one another, no new lithosphere is created or destroyed (conservative margin), ex. San Andreas fault

·       Fracture Zones: most join two segments of a mid-ocean ridge along breaks in the oceanic crust

·       The sub-types of the boundaries (as they relate to movement among different types of crust, e.g. continent-continent convergence vs. continent-ocean convergence)

·       What is seafloor spreading, and at what type of boundary does this occur?

o   Happens at divergent plate boundary, seafloor spreading: creation of new oceanic crust

·       At what type of boundary do subduction zones occur? Types of geologic features are common at subduction zones?

o   Convergent plate boundary, geologic features are common are oceanic trenches and volcanic activity

·       Geologic features common at areas continent-continent convergence

o   Mountains are common

·       At what type of boundary are mid-ocean ridges found?

o   Divergent plate boundaries

·       Age of ocean crust relative to mid ocean ridges

o   Crust that is youngest is at the mid-ocean ridge and the further from the ridge is, the older the crust is

·       What is a hot spot? Be familiar with the hot spot(s) that we discussed in lecture

o   A hot spot is an area under the rocky outer layer of Earth where magma is hooter than the surrounding magma

o   The Hawaiian Islands and Emperor Seamounts have hot spots