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12.1 Evidence for Continental Drift

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Continental Drift Theory

• states that the continents have not always been in their present locations but have “drifted” there over millions of years.

• proposed by German scientist Alfred Wegener • He suggested that, millions of years ago, all the continents were joined as a

“supercontinent = PANGEA

EVIDENCE OF FOR CONTINENTAL DRIFT THEORY

THE CONTINENTS FIT

• The continents looked as though they might fit together like puzzle

pieces.

THE CONTINENTAL SHELVES FIT

• The continental shelves actually fit together even better than

continents

• CONTINENTAL SHELVES = submerged original shorelines of

continents

THE CONTINENTAL SHELVES FIT

• The continental shelves actually fit together even better than

continents

• CONTINENTAL SHELVES = submerged original shorelines of

continents

MATCHING FOSSILS, GEOLOGIC FEATURES AND ROCKS

• Wegener also realized that other evidence also supported his theory

MATCHING FOSSILS, GEOLOGIC FEATURES AND ROCKS

o There were matching geologic features and rocks on different continents.

EVIDENCE OF GLACIERS ON WARM CONTINENTS

o Wegner noted mountain ranges that begin on one continent, end at the

coastline and then appear to continue on a continent across an ocean

EVIDENCE OF GLACIERS ON WARM CONTINENTS

o Similar rocks (same age, same type) and rock structures found in

Newfoundland, Greenland, Ireland, Scotland, and Norway

MATCHING FOSSILS, GEOLOGIC FEATURES AND ROCKS

• Wegener also realized that other evidence also supported his theory

o There were matching fossils, like Mesosaurus, on different continents.

MATCHING FOSSILS, GEOLOGIC FEATURES AND ROCKS

• Wegener also realized that other evidence also supported his theory

o There were matching fossils, like Mesosaurus, on different continents.

EVIDENCE OF GLACIERS ON WARM CONTINENTS

o There was evidence of different climates, (eg. such as glaciers) on warm

continents.

How Can Continents Move?

PLATE TECTONIC THEORY

• Earth is broken into large, movable slabs of rock that move around the

surface of Earth, sliding over a layer of partly molten rock.

• These slabs are called tectonic plates

How Can Continents Move?

PLATE TECTONIC THEORY

• Earth is broken into large, movable slabs of rock that move around the

surface of Earth, sliding over a layer of partly molten rock.

• These slabs are called tectonic plates

EVIDENCE for PLATE TECTONIC THEORY

• It was noted that earthquakes and volcanoes appear in certain

patterns along the edges of tectonic plates

o Volcanoes are openings in Earth’s surface, that, when active, spew out gases,

chunks of rock, and melted rock

o An earthquake is a sudden, ground-shaking release of built-up energy at or

under Earth’s surface.

EVIDENCE for PLATE TECTONIC THEORY

• A long mountain range running north to south down the length of

the Atlantic Ocean was discovered = Mid-Atlantic Ridge

EVIDENCE for PLATE TECTONIC THEORY

• Determination of the age of rocks of the ocean floor revealed that

the youngest rocks were found closest to the ridge and the layer of

ocean sediment—the small particles of silt and organic debris

deposited on the ocean floor — became thicker the farther it was

from the ridge.

EVIDENCE for PLATE TECTONIC THEORY

• Determination of the age of rocks of the ocean floor revealed that

the youngest rocks were found closest to the ridge and the layer of

ocean sediment—the small particles of silt and organic debris

deposited on the ocean floor — became thicker the farther it was

from the ridge.

EVIDENCE for PLATE TECTONIC THEORY

• Determination of the age of rocks of the ocean floor revealed that

the youngest rocks were found closest to the ridge and the layer of

ocean sediment—the small particles of silt and organic debris

deposited on the ocean floor — became thicker the farther it was

from the ridge.

EVIDENCE for PLATE TECTONIC THEORY

• Paleomagnetism shows that iron-based rocks along the ridges are

magnetically striped with reversing magnetic fields.

Hess’ Explanation

• Hess tied together all the data on the age of rocks, sediment

thickness and magnetic striping . He suggested that:

o Magma (melted rock) rises to the Earth’s surface

o It cools down and hardens when it breaks through Earth’s surface forming new

sea floor = SPREADING RIDGE

o As more magma rises to the surface, it pushes the hardened material aside =

FLOOR SPREADING