Chapter 3 –
Plate Tectonics – Study Guide
· Know all
your vocabulary. Be able to match each
definition to the word it defines.
· Describe
Alfred Wegener’s hypothesis about the continents.
Answer: Wegener’s hypothesis was that all the
continents were once joined together in a single landmass and have since
drifted apart. Wegener was a German
scientist, and his hypothesis was formulated in 1910.
· What name
did Wegener give the supercontinent, or single landmass, that existed about 300
million years ago?
Answer: Pangaea
· What
evidence did Wegener suggest supported his hypothesis?
Answer: He pieced together maps of Africa and South
America, and noticed that mountain ranges on the continents line up. He also noticed that coal fields in Europe
and North America also match up. Also,
Wegener used fossils to support the hypothesis because they contained fernlike
plants and freshwater reptiles found in places now separated by oceans. The plants couldn’t have grown today in some
of the places the fossils were discovered, and the reptiles couldn’t have swum
the distance across the oceans.
· More
evidence confirming the hypothesis of the continental drift was published in
1957 by geologist Marie Tharp. What
evidence did she present?
Answer: Data taken from ships showed how the height
of the ocean floor varied, and that in certain places the floor of the ocean
appeared to be stitched together like the seams of a baseball. The seams formed mountain ranges that ran
along the middle of some ocean floors.
These were called mid-ocean ridges.
· What device
using sound waves to measure the distance to an object was used in the
mid-1900s by scientists to map mid-ocean ridges?
Answer: sonar
· Describe the
process of sea-floor spreading.
Answer: Sea-floor spreading begins at a mid-ocean
ridge, which forms along a crack in the oceanic crust. Along the ridge, new molten material from
inside Earth rises, erupts, cools, and hardens to form a solid strip of
rock. More crust is added to the ocean
floor. At the same time, older strips of
rock move outward from either side of the ridge.
· Name 3 types
of evidence that geologists have found for sea-floor spreading.
Answer:
1-Ocean-floor Material shows
rocks shaped like pillows that only form when molten material hardens quickly
after erupting under water.
2-Magnetic Stripes-as magma
erupts, cools, and hardens, magnetic minerals inside the rock line up in the
direction of Earth’s magnetic poles. The
pattern of magnetic stripes on one side of a mid-ocean ridge is usually a
mirror image of the pattern on the other side of the ridge.
3-Drilling Samples-Rock
samples from the ocean floor show that the farther away from a ridge a rock
sample was taken, the older the rock was.
The youngest rocks were always found at the center of the ridges.
· What happens
at deep-ocean trenches?
Answer: In a process taking tens of millions of
years, part of the ocean floor sinks back into the mantle at deep-ocean
trenches.
· Explain
subduction.
Answer: Crust closer to a mid-ocean ridge moves away
from the ridge and toward a deep-ocean trench.
The new oceanic crust is hot, but as it moves away from the mid-ocean
ridge, it cools. As it cools, it becomes
more dense. Eventually, as it moves, the
cool, dense crust might collide with the edge of a continent. Gravity then pulls the older, denser oceanic
crust down beneath the trench and back into the mantle. This is called subduction.
· Earth’s
plates meet at boundaries. What are the
three types of boundaries? Explain each.
Answer:
1. Divergent Boundary-plates that move apart, or
diverge from each other;
2. Convergent Boundary-plates that come together,
or converge;
3. Transform Boundary-plates that slip past each
other, moving in opposite directions.
The sides of the plates are rocky and jagged, so the two plates can grab
each other and “lock” in place. Forces
inside the crust can cause the two plates to unlock. Earthquakes often occur when this
happens. Crust is neither created or
destroyed at transform boundaries!!!!!
· What is the
‘theory of plate tectonics’?
Answer: It states that Earth’s plates are in slow,
constant motion, driven by convection currents in the mantle.
