Review sheet – Chapter 3
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Understand that the Earth is density-stratified; that each deeper layer contains elements that
are more dense (heavier) than the layers above (lightest, least dense elements on surface)
Know that density is measured as mass over (divided by) volume
Know that the core contains materials with the greatest density, while the crust contains the
least dense materials (elements)
Know that the Earth is layered (contains a series of concentric layers)
Understand that the compositional (based on chemical properties) layers of the Earth are the
crust (thin, outermost), mantle (thick, middle layer), and core (densest, innermost layer)
Understand that the Earth is further classified into layers based on physical differences; these
layers are the lithosphere (cool and rigid, uppermost; include all of the crust and the upper,
rigid portion of the mantle), asthenosphere (hot, partially melted portion of the upper
mantle), lower mantle (portion of the mantle that extends to the core, rigid) and core
(includes the inner and outer core; the inner core is solid, while the outer core is viscous)
Understand that both oceanic and land crust is found in the lithosphere
Understand that the lithosphere floats on the asthenosphere
Understand that the source of the Earth’s heat is radioactive decay (then and now)
Know the difference between oceanic and land crust: oceanic crust is thin and made up of
heavy/dense basalt, while continental crust is thick and made up of (relatively) light granite
Understand that the less dense continental crust does not sink as deep into the asthenosphere
and so rises high above the oceanic crust, which is heavier and sinks much deeper into the
asthenosphere
Know that buoyancy is the ability of objects to float in a fluid by displacing a volume of fluid
equal in weight to the floating object’s own weight
Understand that lithospheric plates float in the asthenosphere because of buoyancy
Know the evidence for continental drift: parallelism of the shorelines (South America and
Africa), discovery of coal (tropical plants) in Antarctica, similarities in fossils across (now)
separate continents, “ring of fire” in Pacific Ocean
Know that Alfred Wegener proposed the theory of continental drift
Know that Pangaea was the supercontinent which consisted of all the world’s continents before
they separated (into the continents we see today)
Know that the age of oceanic crust is no less than 200 million years old
Know that the age of continental crust is ~4 billion years old (much, much older than oceanic
crust)
Understand that scientists use radioactive (radiometric) dating to age the continental and
oceanic crusts
Understand the process of seafloor spreading; that new ocean floor is generated at mid-ocean
ridges and spreads outward; understand that this pushes (spreads apart) continents on either
end of this ridge as a result
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Know that seafloor spreading is powered by convection currents; know that convection currents
refer to slow-flowing circuits of material in the asthenosphere
Understand the theory of plate tectonics; the heat-driven large-scale movements of the Earth’s
lithosphere over the asthenosphere; a key principle of plate tectonics is that the lithosphere
exists as separate and distinct tectonic plates (~13) which ride on the fluid/plastic
asthenosphore
Know that newly-formed oceanic crust is hot, and so less dense; therefore it sits higher on the
asthenosphere, whereas older oceanic crust is cool, and so denser; therefore it sits lower on the
asthenosphere. As you move away from the spreading center (mid-ocean ridge), the ocean
contour shrinks in volume, with the warmest (newest) regions existing as tall ridges rising from
the ocean floor
Know that oceanic crust ages as it spreads outward from the spreading center (towards the
continents); the oldest oceanic crust would therefore be closer to the continents, and the
newest would be at or near the spreading center (mid-ocean ridge)
Know that the rate of seafloor spreading is ~2-12cm per year
Understand that the constant creation of new, oceanic crust must be balanced by the
destruction of crust elsewhere
Understand the process of subduction; whereby crust plunges back down into the mantle
(regions of crust destruction are called subduction zones)
Understand that because oceanic crust is denser, when it meets continental crust, it subducts
beneath the continental plate
Understand that lithospheric plates interact with one another; diverging (coming apart);
converging (coming together) or sliding past one another
Know that divergent plate boundaries are regions of crustal formation; mid-ocean ridges
Know that the Mid-Atlantic Ridge is the Atlantic Ocean’s mid-ocean ridge system (the East
Pacific Rise occurs in the Pacific)
Understand that divergent plate boundaries originate beneath continental crust and begin as a
rift valley, which slowly develops into an ocean basin
Know that the Red Sea is a ‘juvenile’ ocean
Understand that plates may converge in one of three ways: continental to continental;
continental to oceanic; oceanic to oceanic
Understand that the denser plate will always subduct beneath the less dense plate during
convergence; therefore an oceanic plate will always subduct beneath a continental plate, and
the older (denser) oceanic plate will subduct beneath the newer (less dense) oceanic plate
Know that volcanoes frequently form when an oceanic plate converges with a continental plate,
or when 2 oceanic plates converge
Know that the Marianas Trench is the world’s deepest oceanic trench (the deepest part of the
world’s ocean), and was formed by the convergence of 2 oceanic plates (1 subducting beneath
the other)
Know that the Marianas Trench is ~11,000 meters (36,000 feet)
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Understand that when 2 continental plates converge, neither is subducted (same age; same
density) so they are instead pressed and folded into each other, creating mountain ranges
Know that the Himalayas (the highest of the world’s mountain ranges) are an example of 2
continental plates converging (the Indian plate and the Asian plate)
Understand that hot spots are further evidence of plate tectonics; hot spots are not located at a
plate boundary, but instead are a single source of rising magma. Volcanic island chains form as a
result of the plate moving over the stationary hot spot. Know that the Hawaiian Islands are an
example of a plate moving slowly over a stationary hot spot (the Hawaiian Islands are NOT the
result of convergence or divergence)
Understand that transform boundaries are regions where 2 plates slide past one another
Understand that transform boundaries are offset horizontally from a vertical spreading center
(the result of spreading occur on a spherical Earth)
Know that mid-oceanic ridges are marked by numerous transform faults, and that California’s
San Andreas fault is an example of a transform fault
Understand that earthquakes may result when 2 plates slide past each other in opposite
directions
Know that oceans cannot grow indefinitely – eventually they will collide with land on the other
side of the planet
Know that the Wilson Cycle is the collective stages of ocean basin formation and destruction,
consisting of an embryonic (rift valley), juvenile (Red Sea), mature (Atlantic Ocean), declining
(Pacific Ocean), terminal (Mediterranean Sea), and suturing (Himalayas) stage
Be comfortable with examples of each of the stages in the Wilson Cycle (as summarized above)