Continental Drift

• The crust, beneath the continents and oceans, together with the upper part of the mantle is divided into huge slabs called plates.

• There are 8 identified major plates plus an assortment of smaller ones.

The plates are capped by both the oceanic and continental crust. Most volcanoes are found along and around the plate edges.

•Eurasian•African •North American•South American Antarctic Indoaustralian•Pacific•Nazca

German scientist Alfred Wegener formed this idea of Continental Drift.  He argued that today’s continents once formed a single landmass, which he named Pangaea (Greek for "all land"). 

Pangaea

• broke into pieces due to the weaknesses in the earth's crust

• which drifted centimeter by centimeter over millions of years until they

• arrived at where they are now. 

Evidence From Landforms

• A mountain range in South Africa lines up identical with mountain ranges in Argentina (South America)

• Brazilian coal fields match up identical with coal fields in South Africa

The presence of similar mountain ranges indicatesthat Africa and South America were once joined.

The continents of Africa and South America best match up like jigsaw-puzzle pieces

Fossil: A trace of an organism that has been preserved in rock

• Glossopteris fossils have been found in rocks in Africa, South America, Australia, India, and Antarctica unexpectedly

• Seeds could not have traveled that far over the oceans to reach other continents

Climate Change as evidence for Continental

Drift• The Island of Spitsbergen

(Artic Ocean) has evidence of tropical plants

• Deep scratches in rocks were found in South Africa

• These scratches support evidence of glaciers

• Wegener could not provide a satisfactory explanation for the force that pushes or pulls the continents

• He could not identify the cause of continental drift

• Geologists needed more evidence of how the continents and mountains were formed

Sea Floor Spreading

Mid-Ocean Ridge

• Mid-Ocean Ridge: The undersea mountain chain where new ocean floor is produced; a divergent plate boundary

Sonar

• Sonar: A device that determines the distance of an object under water by recording echoes of sound waves

• The sonar is used to map the ocean floor• Sonar bounces sound waves off underwater objects

and then records the echoes of these sound waves • The time it takes for the echo to arrive indicates the

distance to the object

Sea-Floor Spreading

• Sea-floor spreading: The process by which molten material adds new oceanic crust to the ocean floor

•At the mid-ocean ridge, molten material rises from the mantle and erupts. •The molten material then spreads out, pushing older rock to both sides of the ridge.

•Over tens of millions of years, the process continues until the oldest ocean floor collides with the continental crust

Evidence for sea-floor spreading

• Evidence from molten material

• Evidence from magnetic stripes

• Evidence from drilling samples

Evidence From Molten Material

• Alvin’s crew found strange rocks shaped like pillows or like toothpaste squeezed from a tube

• Such rocks can form only when molten material hardens quickly after erupting under water

• The presence of these rocks showed that molten material has erupted again and again from cracks along the central valley of the mid-ocean ridge.

http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/pillow_lava.html

Evidence From Magnetic Stripes

• Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes”

• 780,000 years ago, magnetic poles reversed themselves

•If they reversed today, the needle in a compass would point south instead of north

• The rock in the ocean is made of iron, which began as molten material

Evidence From Drilling Samples

• When scientists sampled the rocks, they found that the farther away from the ridge the rocks were the older they were

• The younger rocks were always in the center of the ridges

Seafloor Spread in the Atlantic• The Atlantic Ocean

is expanding at a greater rate than subducting

• This is because of the low number of trenches in the Atlantic

• Over time the entire ocean gets larger and pushes against the continents

Deep-Ocean Trenches• Deep-Ocean

Trenches: A deep valley along the ocean floor through which oceanic crust slowly sinks towards the mantle

• This process is called subduction

SubductionSubduction: The process by which oceanic crust sinks through a deep-ocean trench and back into the mantle; a convergent plate boundary

• At deep-ocean trenches, two plates collide causing the denser of the two plates to dive back to the mantle.

• Over tens of million of years, this material melts back into molten material and may rise again as new oceanic crust.

Subduction in the Pacific Ocean• The more dense oceanic crust subducts (sinks) back

into the mantle at a deep-ocean trench• Subduction in the Pacific Ocean is occurring at a

greater rate than sea-floor is expanding• This is caused by the large amount of trenches

present.