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12.1 EVIDENCE FOR CONTINENTAL DRIFT
(PART 2)
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(Reference: pp. 506 - 517 of BC Science 10)
HOW CAN CONTINENTS MOVE?

Wegener’s evidence for continental drift did not explain how
entire continents could change locations.
 In Wegener’s time, scientists did not know that the
surface of Earth is broken into large, movable slabs of
rock called __________ __________.

New scientific equipment allowed scientists to measure the slow
but steady drift of Earth’s tectonic plates.
Observations related to the movement of the tectonic plates
1. It was noted that earthquakes and volcanoes appear in
certain patterns along the edges of tectonic plates.
 ______________ are openings in Earth’s surface,
that, when active, spew out gases, chunks of rock,
and melted rock.
 An ________________ is a sudden, groundshaking release of built-up energy at or under
Earth’s surface.
2. Mapping of the ocean floor revealed the _______________ ________, a long mountain range running
down the middle of the Atlantic Ocean.
Evidence from ocean rock and sediments

The rocks closest to the Mid-Atlantic Ridge were
_______________ than other ocean rocks.

Sediments along the ridge became _____________
farther away from the ridge.
Evidence from paleomagnetism

Like a bar magnet, Earth has north and south magnetic
______ and a ____________ ________.
 Iron and other magnetic metals in rocks also
usually align with these field lines.

Earth’s magnetic field is not unchanging, however.
 It not only changes in strength from time to time
but, over thousands of years, its direction can
completely reverse, a process known as
_____________ __________________.

scientists think that Earth’s magnetic field
is produced by the motion of _______
_____ in the planet’s interior.

normal polarity: when Earth’s magnetic poles
are ___________ to its geographic poles,
as they are now.
 A compass needle would point in the
general direction of the geographic
_______ Pole.

reverse polarity: when Earth’s magnetic
poles reverse so that they are
____________ to Earth’s geographic poles.
 A compass needle would point in the
general direction of the geographic
________ Pole.
 Paleomagnetism: _________________________
_________________________________________

rocks along the ridges show a pattern of
stripes in the direction that iron-based
minerals are pointing on the sea floor.

The pattern, called ___________
___________, occurs on both sides of the
ridge and corresponds with reversing
magnetic fields
SEA FLOOR SPREADING: AN EXPLANATION
•
Hess suggested that magma rose to form new rock at certain
places.
 Magma: ____________________________________

Like other fluids, it rises and falls due to
____________ differences (convection).
 The convection currents of magma formed a
______________ ___________where they broke
through Earth’s crust.
 sea floor spreading: ___________________________
___________________________________________

Like a “new crust” conveyer belt

Magnetic striping of basalt rock shows long stripes
of new rock moving away from ocean ridges and
also reveals the direction of Earth’s magnetic field
at that time.
•
Wilson combined the concepts of sea floor spreading and
paleomagnetism to explain continental drift.
 He suggested that chains of volcanic islands, such as the
______________ ____________, were formed when a
tectonic plate passed over a stationary _____ ______.

hot spot: ______________________________
______________________________________
 He thought that continents must break up at certain
areas, move across Earth’s surface, and then rejoin.

This explained the formation of ____________
and _______ _______ and the cause of
_______________ and ___________ eruptions.

This also gave a reason for the movement of
___________ ________ and helped explain the
transformation of rocks from one type to another
in the rock cycle.
 Later named the ________ _________
_________, the unifying theory of geology.
SUMMARY: