17.1 ~ Drifting Continents Did Pangea Exist?. The Theory of Continental Drift Wegener’s idea that the continents slowly moved over the earth became.

17.1 ~ Drifting Continents17.1 ~ Drifting ContinentsDid Pangea Exist?Did Pangea Exist?

The Theory of Continental DriftThe Theory of Continental Drift Wegener’s idea that Wegener’s idea that

the continents slowly the continents slowly moved over the earth moved over the earth became known as became known as continental drift.continental drift.

Unfortunately Wegener Unfortunately Wegener could not provide a could not provide a satisfactory explanation satisfactory explanation for the force that for the force that pushed or pull the pushed or pull the continents. Therefore continents. Therefore most geologists most geologists rejected his idea.rejected his idea.

Evidence of Continental DriftEvidence of Continental Drift

From Rock FormationFrom Rock Formation From FossilsFrom Fossils From Ancient ClimateFrom Ancient Climate

Evidence of Continental DriftEvidence of Continental Drift Evidence from Evidence from

landforms/rock landforms/rock formations:formations: Mountain ranges and Mountain ranges and

other features other features

ExampleExample - Mountain - Mountain ranges in South Africa ranges in South Africa line up with mountain line up with mountain ranges in Argentina.ranges in Argentina.

Evidence for Continental DriftEvidence for Continental Drift

Evidence from fossilsEvidence from fossils A fossil is any trace of A fossil is any trace of

an ancient organism an ancient organism that has been that has been preserved in rock.preserved in rock.

Glossopteris (a seed Glossopteris (a seed fern) fossils have been fern) fossils have been found in rocks in Africa, found in rocks in Africa, South America, South America, Australia, India, and Australia, India, and Antarctica.Antarctica.

The Theory of Continental DriftThe Theory of Continental Drift

A German scientist named A German scientist named Alfred Wegener formed Alfred Wegener formed the hypothesis that the the hypothesis that the continents had moved!continents had moved!

He proposed that all the He proposed that all the continents had once been continents had once been joined together in a single joined together in a single landmass and have since landmass and have since drifted apart.drifted apart.

Evidence for Continental DriftEvidence for Continental Drift Evidence from climateEvidence from climate

Fossils of tropical plants Fossils of tropical plants have been found too far have been found too far south to have survived.south to have survived.

Glacial deposits are Glacial deposits are found too far north to found too far north to have existed.have existed.

These clues provide These clues provide evidence that evidence that continental drift really continental drift really happened.happened.

Rejection!!Rejection!!

Even with the evidence, most scientists rejected Alfred Wegener’s theory for about a half a century, from the 1920s to the 1960s.

17.2 ~ Sea-floor Spreading17.2 ~ Sea-floor Spreading

Is the ocean floor flat?Is the ocean floor flat?

Technology allowed scientists to Technology allowed scientists to study the ocean floorstudy the ocean floor

American geophysicist Harry Hess helped American geophysicist Harry Hess helped develop the theory of develop the theory of plate tectonicsplate tectonics..

Echo-sounding methods:Echo-sounding methods: Sonar – uses sound waves to measure water Sonar – uses sound waves to measure water

depthdepth Magnetometer – detects small changes in the Magnetometer – detects small changes in the

magnetic fieldsmagnetic fields

Topography of Ocean FloorTopography of Ocean Floor• Mid Ocean Ridges Mid Ocean Ridges – –

underwater mountain chains underwater mountain chains that run through the Earth’s that run through the Earth’s BasinsBasins

• Deep-sea trenches Deep-sea trenches – a narrow, – a narrow, elongated depression in the elongated depression in the seafloor with very steep sidesseafloor with very steep sides

• Magma rises to the surface and Magma rises to the surface and solidifies and new crust formssolidifies and new crust forms

• Older Crust is pushedOlder Crust is pushed

farther away from the ridgefarther away from the ridge

Ocean Rocks and SedimentsOcean Rocks and Sediments Analysis:Analysis:

Age Age of rocks vary in of rocks vary in place, and the age of place, and the age of oceanic crust oceanic crust consistently increases consistently increases with distance from a with distance from a ridge ridge

Sediments are Sediments are deposited on the deposited on the ocean floor ocean floor – – thickness thickness of the of the sediments increases sediments increases with distance from the with distance from the ocean ridgeocean ridge

MagnetismMagnetismRocks contain iron-bearing minerals that Rocks contain iron-bearing minerals that provide a record of Earth’s magnetic field.provide a record of Earth’s magnetic field.PaleomagnetismPaleomagnetism – studies this magnetic – studies this magnetic recorded recorded “paleo”=“paleo”=Ancient; prehistoric; old

For example: Basalt, it is rich in iron-bearing For example: Basalt, it is rich in iron-bearing minerals, provides and accurate record of minerals, provides and accurate record of ancient magnetism.ancient magnetism.

MagnetismMagnetism

Basaltic lava cools, the iron becomes Basaltic lava cools, the iron becomes oriented parallel to Earth’s magnetic field.oriented parallel to Earth’s magnetic field.

Therefore, these minerals are compass Therefore, these minerals are compass needlesneedles

Geomagnetic Time ScaleGeomagnetic Time Scale Magnetic reversal – is Magnetic reversal – is a change a change in Earth’s in Earth’s

magnetic field. Long terms changes are magnetic field. Long terms changes are called called epochs epochs short terms are called short terms are called eventsevents

If is has the same orientation as the present If is has the same orientation as the present field = field = normal polarity (+)normal polarity (+)

If it is opposite = If it is opposite = reversed polarity (-)reversed polarity (-)

Seafloor SpreadingSeafloor Spreading

Theory – states that new ocean crust is Theory – states that new ocean crust is formed at ocean ridges and destroyed at formed at ocean ridges and destroyed at deep-sea trenchesdeep-sea trenches

What actually happens during seafloor What actually happens during seafloor spreading?spreading?

magma is forced toward the crust magma is forced toward the crust along an ocean ridge and fills the gap that is along an ocean ridge and fills the gap that is created. When it hardens a new ocean floor created. When it hardens a new ocean floor is addedis added

Are the continents moving or Are the continents moving or are they passengers that ride are they passengers that ride

with the ocean crust?with the ocean crust?

Sea floor spreading is the final answer Sea floor spreading is the final answer Wegner needed to complete his model of Wegner needed to complete his model of continental drift.continental drift.

Lab ActivityLab Activity

In this lab you will build a model to help understand the process of sea-floor spreading and how magnetic reversals are recorded in the ocean floor.

Sea-floor Spreading ModelSea-floor Spreading Model

Rubric – Rubric – Project gradeProject grade Model is labeled: Title/ Name/ Period- 10 ptsModel is labeled: Title/ Name/ Period- 10 pts Lines are measured accurately – 10 ptsLines are measured accurately – 10 pts Label magnetic polarity – 10 ptsLabel magnetic polarity – 10 pts Label age of rocks – 10 ptsLabel age of rocks – 10 pts Lines vary in thickness – 10ptsLines vary in thickness – 10pts Student Data Write up packet– 50ptsStudent Data Write up packet– 50pts