CAS ES 105 Chapter Notes - Chapter 2: Continental Margin, Mariana Trench, Jigsaw Puzzle
30 Jul 2018
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Ch. 2 The Way Earth Works
Plate Tectonics
• Introduction
o Alfred Wegner, aimed to resupply weather observers stranded at a remote
camp
▪ Died
▪ Proposed that historically, continents fit together like pieces of giant
jigsaw puzzle in one supercontinent (Pangea), later fragmented into
separate continents
▪ Continental drift: the process by which over time, these continents
moved apart into their present positions
o Seafloor spreading: a new seafloor forms between 2 continents as they move
apart (Hess)
o Subduction: continents can move toward each other when the old ocean floor
between them sinks back down into the Earth’s interior
o Lithosphere: the Earth’s outer, relatively rigid shell
▪ Plates: slowly move relatively to one another
▪ Theory of plate tectonics
• Continental Drift
o Wegner’s evidence
▪ Fit of the Continents – showed that all continents would fit together to
form Pangea with barely any overlaps – too good to be coincidence
o Distribution of climate belts in the past
▪ Diff climate belts occur at diff latitudes
▪ Sedimentary-rock beds (layers) at a location that changed temp due
to these shifts in latitude
▪ Glacial striation: scratches carved by glaciers in areas that now are in
non-polar latitudes
▪ Fossils of tropical trees in now cold areas
o Distribution of Fossils
o Matching Geologic Units and Mountain Belts
• The opposing view: Drift Denial
o No forces are strong enough to move continents???
o Mobilist view: movement of continents is happening because of plate
tectonics
▪ First step was recognizing seafloor spreading
• Take-home message
o Alfred Wegener proposed his hypothesis of continental drift based on his
observations of the shape of coastlines, on the record of past climates
preserved in sedimentary rocks, on the distribution of fossils, and on the
matching of rocks and mountains across oceans.
o He argued that the continents formerly made up one supercontinent,
Pangaea, that later broke apart.
o The hypothesis was not widely accepted until decades after Wegener’s death
• The Discovery of Seafloor Spreading
o 1930-1960
▪ Bathymetric features of the seafloor
▪ Bathymetry: depth variations of the seafloor
• Used to be tedious because did it through “sounding” → sonar
• Bathymetric map: shows overall shape of seafloor
• Ocean basins: distinctly lower than continents
o Why do they exist?
▪ Continental lithosphere = thicker than oceanic
lithosphere
▪ Upper layer = less dense than that of oceanic
lithosphere
▪ Surface of a raft of continental lithosphere sits
higher than the surface of a one made up
continental lithosphere
o Continental margin: boundary between an ocean basin
and a continent
• Abyssal plains: flat regions, in large areas of ocean basins
• Mid-ocean ridges: long submarine mountain ranges with crests
o Usually symmetrical and segmented
• Fracture zones: where segments of a mid-ocean ridge
terminate, end roughly at right angles
• Deep-sea trenches: in/along edges of ocean basins
o Some border continents, some don’t
o Not all continental edges border trenches
• Oceanic island: peak of igneous rock that rises above sea level
• Seamount: peak of igneous rock that lies below sea level
o These and oceanic islands usually occur in chains
▪ Character of oceanic crust
• Experiments here taught us that the upper layer of oceanic
crust beneath abyssal plains = relatively thin sediment that
settled down from overlying seawater
• Beneath sediment – oceanic crust = basalt (dense gray igneous
rock)
▪ Heat flow: the rate at which heat rises from the Earth’s interior
• Heat flow beneath mid-ocean ridges = greater than beneath
abysses
▪ Distribution of Earthquakes
• Fault: a fracture in the Earth’s crust on which slip (sliding)
takes place
• Earthquakes: episodes of ground shaking, mostly due to fault
slip (sudden movement generates vibrations that travel
through the Earth in wave form, when they reach the ground
surface, they cause shaking)
• Seismic belts: distinct belts of earthquakes, lie along trenches,
mid-ocean ridge axes, along parts of fracture zones, and on
other faults
• Harry Hess
o Thinness of ocean floor means it can’t be as old as the Earth
o New ocean floor must be forming at ridges so ocean basin can grow wider
with time
o Hypothesis: ocean basins form by seafloor spreading
▪ Seafloor stretched apart along axis of a mid-ocean ridge, new oceanic
crust formed from magma rose and filled the space, then new seafloor
moved away from ridge so an ocean basin could grow wider over time
▪ Seafloor spreading – explained how continental drift occurs
▪ Continents move apart due to seafloor spreading at mid-ocean ridges;
continents move toward each other as seafloor between them is
consumed at trenches
• An essay in geo-poetry
o Take-home message
▪ Observaitons of seafloor bathymetry, sediment cover, heat flow, and
seismicity led to the proposal that ocean basins open by seafloor
spreading, during which new seafloor forms at mid-ocean ridges, and
they close by subduction, when old seafloor sinks back into the mantle
at trenches
• Modern plate tectonics theory
o If seafloor spreading and subduction take place, and then continents move,
then the outer layer of the earth (the shell we live on) must be mobile!
o Rigid outer shell: bends and breaks, but doesn’t flow; not completely intact,
has separate “plates” that move relative to one another
o Plate tectonics = grand unifying theory of geology
• Lithosphere and asthenosphere
o Lithosphere: the crust + the uppermost part of the upper mantle
o Lithospheric mantle: the part of the mantle within the lithosphere
o Asthenosphere: the portion of the mantle that lies beneath the lithosphere
▪ Mantle = plastic: flows slowly without breaking
o Crust has different composition than mantle
▪ Moho = base of crust, not base of lithosphere
o Lithosphere has different response to force than asthenosphere
▪ Lithosphere = rigidly
▪ Athenosphere = plastically
• Heat makes rock, like wax, softer
o Lithosphere
▪ Plates: lithosphere plates
▪ Plate boundaries: the dividing lines between plates
▪ Plate interior: a portion of a plate that is not near a plate boundary
• Remain pretty earthquake-free bc the plates are rigid
▪ 7 large plates, several other smaller ones
