17.1 ~ Drifting Continents Did Pangea Exist?

The Theory of Continental Drift  Wegener’s idea that the continents slowly moved over the earth became known as continental drift.  Unfortunately Wegener could not provide a satisfactory explanation for the force that pushed or pull the continents. Therefore most geologists rejected his idea.

Evidence of Continental Drift  From Rock Formation  From Fossils  From Ancient Climate

Evidence of Continental Drift  Evidence from landforms/rock formations:  Mountain ranges and other features Example - Mountain ranges in South Africa line up with mountain ranges in Argentina.

Evidence for Continental Drift  Evidence from fossils  A fossil is any trace of an ancient organism that has been preserved in rock.  Glossopteris (a seed fern) fossils have been found in rocks in Africa, South America, Australia, India, and Antarctica.

The Theory of Continental Drift  A German scientist named Alfred Wegener formed the hypothesis that the continents had moved!  He proposed that all the continents had once been joined together in a single landmass and have since drifted apart.

Evidence for Continental Drift  Evidence from climate  Fossils of tropical plants have been found too far south to have survived.  Glacial deposits are found too far north to have existed.  These clues provide evidence that continental drift really happened.

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 Spreading Is the ocean floor flat?

Technology allowed scientists to study the ocean floor  American geophysicist Harry Hess helped develop the theory of plate tectonics.  Echo-sounding methods:  Sonar – uses sound waves to measure water depth  Magnetometer – detects small changes in the magnetic fields

Topography of Ocean Floor Mid Ocean Ridges – underwater mountain chains that run through the Earth’s Basins Mid Ocean Ridges – underwater mountain chains that run through the Earth’s Basins Deep-sea trenches – a narrow, elongated depression in the seafloor with very steep sides Deep-sea trenches – a narrow, elongated depression in the seafloor with very steep sides Magma rises to the surface and solidifies and new crust forms Magma rises to the surface and solidifies and new crust forms Older Crust is pushed Older Crust is pushed farther away from the ridge

Ocean Rocks and Sediments  Analysis:  Age of rocks vary in place, and the age of oceanic crust consistently increases with distance from a ridge  Sediments are deposited on the ocean floor – thickness of the sediments increases with distance from the ocean ridge

Magnetism Rocks contain iron-bearing minerals that provide a record of Earth’s magnetic field.  Paleomagnetism – studies this magnetic recorded “paleo”=  Paleomagnetism – studies this magnetic recorded “paleo”=Ancient; prehistoric; old  For example: Basalt, it is rich in iron-bearing minerals, provides and accurate record of ancient magnetism.

Magnetism  Basaltic lava cools, the iron becomes oriented parallel to Earth’s magnetic field.  Therefore, these minerals are compass needles

Geomagnetic Time Scale  Magnetic reversal – is a change in Earth’s magnetic field. Long terms changes are called epochs short terms are called events  If is has the same orientation as the present field = normal polarity (+)  If it is opposite = reversed polarity (-)

Seafloor Spreading  Theory – states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches What actually happens during seafloor spreading?  magma is forced toward the crust along an ocean ridge and fills the gap that is created. When it hardens a new ocean floor is added

Are the continents moving or are they passengers that ride with the ocean crust?  Sea floor spreading is the final answer Wegner needed to complete his model of continental drift.

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