Ch.2: Plate Tectonics Flashcards
Wegner’s hypothesis
continents are mobile and once formed a supercontinent, Pangaea
didn’t know the mechanism that moves
the continents—that distinguishes Continental Drift
from Plate Tectonics
Wedger’s Evidence of Pangea (4)
“Jigsaw Fit” of continents • Distribution of climate belts preserved in rocks (ex. glacial deposits, coal deposits) •Distribution of fossils •Matching rock type and structures
Newer evidence to support plate tectonics (7)
Bathymetric profiles and ocean floor maps Patterns of earthquakes Seafloor spreading High heat flow at oceanic ridges Paleomagnetism Magnetic reversals Marine magnetic anomalies
Bathymetric Profiles
illustrates that mid-ocean ridges are elevated above deeper abyssal plains
Seafloor spreading
Harry Hess, 1962
• Measured high heat flow at ocean ridges
New ocean crust is created at mid-ocean ridges
(regions of high heat flow), moves away from the
ridges, and then sinks back into the mantle.
Robert Dietz termed this process subduction.
Magnetic Inclination
Magnetic inclination indicates the paleo-latitude
of where the rock originally formed.
For example, a rock with magnetic inclination of 0°
formed at the magnetic equator.
Paleomagnetism: Apparent Polar Wandering
Magnetic changes in a layered succession of lavas can
preserve the history of tectonic plates moving a continent.
Magnetic reversals
During reverse magnetic polarity, the magnetic dipole
points in the opposite direction.
During these conditions, a compass needle would
point to the south magnetic pole. Points up
Paleomagnetism: “Frozen” into Basalt
Iron-rich minerals in rocks record the magnetic
field at time the rock formed.
In hot magma, magnetic dipoles are randomly
oriented and no magnetization is possible.
As magma cools, magnetic dipoles in iron-bearing
minerals become aligned with Earth’s magnetic field.
The magnetic signature is then frozen in the rock.
Marine magnetic anomalies
Magnetometers towed by ships moving across the midocean ridges recorded alternating strong and weak
magnetic fields.
Close to MOR
Divergent
plates move away from each other
Two plates move away from one another; seafloor
spreading produces new oceanic crust at a mid-ocean
ridge. Also called a constructive plate boundary because new crust is formed.
Convergent
plates move towards each other
The denser plate subducts beneath the overriding plate in this destructive plate margin.
Transform
plates move sideways past each other
Rifting
When continental lithosphere stretches and thins, the upper crust breaks. The upwelling asthenosphere
initiates volcanism. May eventually split a continent apart
Ocean-continent convergence
forms a magmatic arc on the edge of the overriding
continental plate.
Ocean-ocean convergence
forms a volcanic island arc on the edge of the
overriding oceanic plate
Subduction
The Wadati-Benioff zone is an inclined plane of earthquakes that defines the position of the
downgoing tectonic plate.
When denser plate subducts under overriding plate, water (oceanic crust under continental) melts rock from mantle
Continent-continent convergence
starts after subduction has consumed the oceanic plate that was once between two continents. Continental crust is too buoyant to subduct; when two continents converge, rock undergoes compression and shearing,
and a mountain range like the Himalayas develops.
Volcanoes
As a plate drifts over a mantle plume, a volcano
forms over the hot spot. A chain of nowextinct volcanoes forms behind the active volcano (like in Hawaii). First proposed by J. Tuzo Wilson,
1960s.
Youngest vs. oldest seas floor
youngest close to MOR
oldest away from MOR
Ridge push
“Ridge push” develops by the gravitational
energy associated with the topographic
elevation of the midocean ridge.
Divergence at MORproduces high ridge that pushes down
Convection
Warm areas in mantle rises, cool areas sink
Slab pull
“Slab pull” develops because the old oceanic
lithosphere is more dense than the underlying
asthenosphere, so it sinks
Weight of downgoing slab pulls and out and away from ridge subducting plate down
Active
Active plate boundaries
Passive
Not active plate boundaries, result
