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