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Plate Tectonics DBQ
Date____________________
The Earth is composed of four layers: the inner core, outer core, mantle, and crust. The crust (or
lithosphere) is divided into sections called plates. The mantle, also referred to as the asthenosphere, is a
hot liquid layer, made up of magma, or molten rock. The magma closest to the core is the hottest, and
thereby less dense than the magma near the crust. What happens next? The less dense magma cannot
support the denser magma so the hot, less dense magma shifts upward while the denser molten rock near
the core sinks. This is all repeated again, continuously as the cooler magma is heated. This process is
called convection currents. The plates in the lithosphere glide over the top of this magma, causing them
to reposition themselves over the globe.
With this plate tectonic theory, proposed by Alfred Wegener, we cannot go outside and see the ground in
motion, nor can we obtain a satellite picture and see a change in the continents from what they looked like
yesterday, or even last decade. So we would naturally be forced to think this is false, which is what
scientists did when Wegener conceived this hypothesis. After initially being dismissed, evidence came
forth that facilitated the proving of plate tectonics. It was found that the continents have a "puzzle-like"
fit, telling us that they could have one day been together and broken apart. Also the same fossils of
organisms only thought to have been in Africa were found as well in the Americas. These creatures could
not have swam across the ocean, so there must have been land connecting them. The same rock layer
have even been found in different continents, indicating that they were once joined. Given this evidence,
we can now support this theory, as scientists today do.
Additionally, the discovery of magnetic striping and the stripes being symmetrical around the crests of the
mid-ocean ridges suggested a relationship. In 1961, scientists began to theorise that mid-ocean ridges
mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge
crest. New magma from deep within the Earth rises easily through these weak zones and eventually
erupts along the crest of the ridges to create new oceanic crust. This process, later called seafloor
spreading, operating over many millions of years continues to form new ocean floor all across the 50,000
km-long system of mid-ocean ridges. This hypothesis was supported by several lines of evidence:
1. at or near the crest of the ridge, the rocks are very
young, and they become progressively older away
from the ridge crest;
2. the youngest rocks at the ridge crest always have
present-day (normal) polarity;
3. stripes of rock parallel to the ridge crest alternated in
magnetic polarity (normal-reversed-normal, etc.),
suggesting that the Earth's magnetic field has flipflopped many times.
Three types of plate boundaries exist, characterized by the way the plates move relative to each other.
They are associated with different types of surface phenomena. The different types of plate boundaries
are:
Transform boundaries occur where plates slide or, perhaps more accurately, grind past each other along
transform faults. The relative motion of the two plates is either sinistral (left side toward the observer) or
dextral (right side toward the observer).
Divergent
boundaries
occur where
two plates
slide apart
from each
other
(examples of
which can be
seen at midocean ridges
and active
zones of
rifting (such
as with the
East Africa
rift)).
Convergent boundaries (or active margins) occur where two plates slide towards each other commonly
forming either a subduction zone (if one plate moves underneath the other) or a continental collision (if
the two plates contain continental crust). Deep marine trenches are typically associated with subduction
zones. Because of friction and heating of the subducting slab, volcanism is almost always closely linked.
Examples of this are the Andes mountain range in South America and the Japanese island arc.
1. Who proposed the idea of continental drift and what is believed to be the driving force behind the plate
movement? _______________________________________________________________________
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2. List 4 evidences that support the theory of Plate Tectonics
1._________________________________________________________________________
2._________________________________________________________________________
3._________________________________________________________________________
4._________________________________________________________________________
3. List the 4 main layers the Earth is composed of, starting with the inner most layer. Using page 10 of your Earth
Science Reference Tables’s, add their thicknesses in kilometers.(Combine the plastic mantle and the stiffer mantle
when doing thickness)
____________________
______________km
____________________
______________km
____________________
______________km
____________________
______________km
4. What are the main evidences that help support he theory of continental drift?
1._________________________________________________________________________
_________________________________________________________________________________
2. _________________________________________________________________________
_________________________________________________________________________________
3._________________________________________________________________________
_________________________________________________________________________________
4._________________________________________________________________________
_________________________________________________________________________________
5. List and briefly define the 3 types of plate boundaries.
1._________________________________________________________________________
_________________________________________________________________________________
2._________________________________________________________________________
_________________________________________________________________________________
3._________________________________________________________________________
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Regents Questions
The diagram below shows a portion of Earth's interior. Point A is a location on the interface between
layers.
______1. The arrows shown in the asthenosphere represent the inferred slow circulation of the plastic
mantle by a process called
A) radiation
B) insolation
C) convection
D) conduction
______2. The map below shows the crustal plate boundaries located along the Pacific coastline of the
United States. The arrows show the general directions in which some of the plates appear to be moving
slowly.
The best way to find the direction of crustal movement along the San Andreas fault is to
A) match displaced rock types from opposite sides of the fault
B) measure gravitational strength on opposite sides of the fault
C) observe erosion along the continental coastline
D) study the Earth's present magnetic field