Name: ____________________________
Date: ___________________________
Chapter 10: Plate Tectonics
Section 1: Continental Drift
Wegener’s Hypothesis
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Continental drift - the hypothesis that a single large landmass broke up into smaller landmasses
to form continents, which then drifted to their present location.
 As people studied continental coastlines on maps, they noticed that the continents
looked as though they would fit together like parts of a giant
_____________________________________________________.
The hypothesis of continental drift was first proposed by German scientist
_______________________________________ in 1912.
 Wegener used several different types of evidence to support his hypothesis.
Fossil Evidence
 Fossils of the same _____________________and______________________ could be
found in areas of continents that had once been connected.
Evidence from Rock Formations
 Ages and types of rocks in the ______________________________________ of widely
separated areas matched closely.
Climactic Evidence
 Changes in _________________________________________________ suggested the
continents had not always been located where they are now.
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Similar rock formations and fossil evidence supported Wegener’s hypothesis.
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Missing Mechanisms
 Despite the evidence that supported his hypothesis, Wegener’s ideas were strongly
___________________________________________.
 Other scientists of the time rejected the mechanism proposed by Wegener to explain
how _______________________________________________________________.
 Wegener suggested that the continents moved by
_______________________________________________________________________
of the ocean floor. This idea was shown to be physically impossible.
 Wegener spent the rest of his life searching for a _______________________________
for the movement of continents.
Name: ____________________________
Date: ___________________________
READING CHECK
 Why did many scientists reject Wegener’s hypothesis of continental drift?
Mid-Ocean Ridges
 Mid-ocean ridge a _____________________________________________________
that has a steep, narrow valley at its center
 Forms as _______________________________________ from the
asthenosphere
 Creates new oceanic ________________________ (sea floor) as tectonic plates
move apart
 The evidence that Wegener needed to support his hypothesis was discovered nearly
____________________________________________ after his death.
 In 1947, a group of scientists set out to map the ____________________________.
While studying the Mid-Atlantic Ridge, scientists noticed
__________________________ surprising trends.
1. The sediment that covers the sea floor is ________________ closer to a ridge
than it is farther from the ridge.
2. The ocean floor is ______________________________. While rocks on land are
as much as __________________________________, none of the oceanic rocks
are more than __________________________________________.
 Rocks _____________________to a mid-ocean ridge are __________________ than
rocks farther from the ridge. Rocks _______________________to the ridge are covered
with _______________________________________ than rocks farther from the ridge.
Sea-Floor Spreading
 Sea-floor spreading - the process by which new _____________________________________
(sea floor) forms as magma rises to Earth’s surface and solidifies at a mid-ocean ridge
Name: ____________________________
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Date: ___________________________
In the late 1950s, geologist _________________________________________ proposed
that the valley at the center of a mid-ocean ridge was a crack, or
___________________, in Earth’s crust.
As the ocean floor moves away from the ridge, molten rock, or magma,
_________________________ to fill the crack.
Hess suggested that if the _______________________________ is moving, the
continents might be moving also. He suggested this might be the mechanism that
Wegener was searching for.
As the ocean floor spreads apart, magma rises to fill the rift and then cools to form new
rock.
READING CHECK
 How does new sea floor form?
Paleomagnetism
 Paleomagnetism - the study of the alignment of ______________________________________
in rock, specifically as it relates to the reversal of Earth’s magnetic poles; also the magnetic
properties that rock acquires during formation
 As magma solidifies to form rock, _________________________________ minerals in the
magma align with Earth’s magnetic field. When the rock hardens, the magnetic orientation of
the minerals becomes ______________________________.
 Magnetic Reversals
o Scientists have discovered rocks whose magnetic orientations point
_________________________of Earth’s current magnetic field.
o Rocks with magnetic fields that point ______________________________, or
_______________________________________, are classified in the same time interval.
Rocks with magnetic fields that point _______________________________, or
________________________________, also fall into specific time intervals.
o When scientists placed these periods of normal and reversed polarity in
____________________________________________, they discovered a pattern of
alternating normal and reversed polarity in the rocks.
Name: ____________________________
Date: ___________________________
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Scientists used this pattern to create the
_________________________________________________________________.
Magnetic Symmetry
o Scientists discovered a ___________________________________________________
on the ocean floor on each side of a mid-ocean ridge.
o The pattern on one side of the ridge is a ________________________________ of the
pattern on the other side.
o When drawn on a map, these patterns ____________________ the geomagnetic
reversal time scale.
o The symmetry of magnetic patterns and the symmetry of ages of
_____________________________ rocks supports Hess’s idea of sea-floor spreading.
Magnetic Symmetry
o The pattern of magnetic symmetry and age of rock formation indicate that new rock
forms at the center of a ridge and then move away from the center in opposite
directions.
READING CHECK
o How are magnetic patterns in sea-floor rock evidence of sea-floor spreading?