Transcript continental drift

CHAPTER 10 SECTION 1
CONTINENTAL DRIFT
Objectives
• Summarize Wegner’s hypothesis of continental drift
• Describe the process of sea floor spreading
• Identify how paleomagnetism provides support for
the idea of sea floor spreading
• Explain how sea-floor spreading provides a
mechanism for continental drift
CONTINENTAL DRIFT
• One of the most exciting
recent theories in Earth
Science began with
observations made over
400 years ago
• As early explorers sailed
the globe they collected
information on the
continents coastlines
• Map makers used this
information to make the
first reliable world maps
CONTINENTAL DRIFT
In 1912 a German
scientist named Alfred
Wegner proposed the
hypothesis of continental
drift
He hypothesized that the
continents all formed
one land mass called a
super continent, at one
time and then began to
break apart about 250
million years ago, during
the Mesozoic Era
CONTINENTAL DRIFT
• In addition to the
similarities of the
coastlines Wegner
proposed that fossil
evidence should exist
between areas that
once were
connected
• A identical fossil of
Mesoasaurus was
discovered in both
South America and
Western Africa
CONTINENTAL DRIFT
• The theory of
continental drift is
also supported by
the age, type, and
shapes of the rocks
found on the edges
of divergent
continents
CONTINENTAL DRIFT
• Changes in climate
patterns also suggest
that continents are
not where they once
were
• South Africa displays
evidence of glacial
activity
• Fossils of tropical
plants found on
Antarctica
CONTINENTAL DRIFT
• Despite the evidence
Wegner had many
scientists at the time
rejected his
hypothesis of
continental drift
• He spent the rest of
his life trying to find
the mechanism for
continental drift, but
died before the
discovery occurred
CONTINENTAL DRIFT
• Evidence for Wegner’s
theory came two decades
after his death
• Scientists who set out to
map the Mid-Atlantic Ridge
made some discoveries
• Amount of sediment near
the ridge is less than further
away
• Radiometric dating of rocks
showed rocks get older as
the move away from the
ridge
• Ocean rocks are much
younger than continental
rocks
CONTINENTAL DRIFT
• In the 1950’s
geologist Henry Hess
proposed that the
mechanism for
Wegner’s hypothesis
was sea floor
spreading
• At the center of the
valley is a rift were
new magma is rising
and cooling creating
new sea floor
CONTINENTAL DRIFT
• If you have ever used
a compass you know
Earth has a magnetic
pull
• As magma cools the
iron rich minerals in
the magma align to
Earth’s magnetic field
and it is permanent
(paleomagnetism)
CONTINENTAL DRIFT
• Earth’s magnetic field
has not always
pointed north
• Rocks with normal
polarity have the
same dating periods
as other normal
polarity rocks
• The same holds true
for rocks with reverse
polarity (south
aliened)
CONTINENTAL DRIFT
• The stripped pattern
on one side of the
mid ocean ridge
mirrors the pattern
on the other side of
the ridge
• Age of the ocean
rocks are
symmetrical
CONTINENTAL DRIFT
• Rocks on land have
magnetic symmetry
also
• Similar layers and
age in rocks
• Further proof
verifying Wegener's
hypothesis