The Dynamic Ocean FloorLab #7

Timeline

• 400 million years ago fish evolved• 200 million years ago Pangaea split apart,

and the start of the Triassic period (Dinosaurs)

• Broke up into two smaller super continents, called Laurasia and Gondwanaland, during the Jurassic period.

• 150 million years later, the Earth looks like it does now.

Plate Tectonics

• What exactly is it?

• The movement of plates and continents.

• Proposed in 1912 by Alfred Wegener.

• Not accepted until early 1960s.

• Has had a major impact on Earth Sciences.

• Occurs in the lithosphere by sea floor spreading

Lithosphere

• Is the outer most shell of the Earth

• This includes the crust and the mantle.

• Means stone sphere

• Continental crust is 35 km thick

• Oceanic crust on 5 km thick.

• Lithosphere total 100 km thick

• Oceanic crust and continental crust sits on top of the Lithosphere.

Asthenosphere

• Means weak sphere• Second layer of the Earth. • Approximately 600 km thick.• Outer section of this layer is liquid, this allows the

lithosphere to move. • Convection currents play an important role.

Plate Tectonics

• The plates along the Lithosphere are moving, and the continents are along for the ride.

• Its known that rocks are younger at the Mid-Ocean ridge then at the continents.

• Why?• This is verified by magnetic reversal information

preserved in the minerals of the mid-ocean ridge.

• The first positive proof that the plates were indeed moving.

Plate Boundaries

• Plate boundaries are found at the edge of the lithospheric plates.

• Earthquakes coincide with plate boundaries.

• There are four boundary types and they are characterized by their distinct motions.

Plate Boundaries

DivergentMoving apart

ConvergentMoving together

3 Types

Subduction (#1) One plate goes below anotherContinental Vs. Oceanic

Transform # 3 Plates slide past each other

Mid Atlantic Ridge

#2Continental Vs. ContinentalBoth plates push up Mts. form

Divergent Boundary

• Two plates move away from one another. • This is a zone of weakness.• As two plates move apart at the mid-ocean

ridges, magma from the mantle up wells through a crack in the oceanic crust and cooled by the sea creating new ocean floor.

• Energy is released in the form of earthquakes.

• Shallow focus earthquakes (less then 100 km in depth) occur in this area.

Mid Oceanic Ridges

Mid-Atlantic Ridge

• Mountain ranges along the ocean floor.

• It has a distance of 600 miles.

• Longer and also higher then any other mountain range on any continent.

• The ridges are home to Earth's highest mountains, and deepest canyons.

• The shape of the mid-ocean ridge is controlled by the rate the plates move apart.

Older crust is pushed laterally away from the ridge; thus the sea floor spreads apart.

•Where convection cells ascend sea floor spreading takes place.•Where the convection cells descend they drag crust down, causing subduction.•This pushing and pulling provides the forces that drives plate tectonics.

Convergent Plate Boundary - Subduction• Here one plate is overriding another forcing one into the

mantle.– Crust descends at angles from 35 to 90 degrees.– crust older than the Cretaceous period cannot be found in

any ocean basin.• Deep focus quakes (100-600 km) occur in this area.• As the magma melts pressure builds up and the result is

volcanic eruptions.

• When two continents meet head-on, neither is subducted.• The crust tends to buckle and be pushed upward.

Convergent Plate Boundary - Continental vs. Continental

Convergent Plate Boundary - Transform

• Third type of plate boundary. • Material is neither created nor destroyed

at these boundaries, but rather plates slide past each other.

• These are pressure valves • The location of shallow focus earthquakes Classic example: San Andreas fault in

California.

Convergent Plate Boundary Review

Evidence for Pangaea - Fossils

Fossils of glossopteris; a tree - like plant from the Permian Period and dominant plant of Gondwana are found

throughout India, South America, southern Africa, Australia, and Antarctica.

Evidence for Pangaea – Rock Formation• Additional evidence for Pangaea is found in the geology of adjacent continents, including matching geological trends between the eastern coast of South America and the western coast of Africa.• The continuity of mountain chains also provide evidence for Pangea. • One example of this is the Appalachian Mountains chain which extends from the United States to Ireland, Britain, Greenland, and Scandinavia. (See chart on page 151)

Paleomagnetism

• Ancient magnetism in rocks • Used to determine the rate and the time the

plates separated. • As magma rises to the ocean floor and cools,

the minerals align with the Earth’s magnetic field.

• When the Earth’s polarity changes the mineral pattern will also change.

• It is estimated that it takes 70,000 years for the polarity to rotate.

Paleomagnetismand the Ocean Floor

• See pages 149-150

• Chart on pg 149 shows the pattern of polarity over the past four million years.

• As new crust on the ocean floor is created it spreads out evenly on both sides of the mid ocean ridge.

Determining the rate of sea floor spreading

• This example is for the Pacific plate pg 150.• The green area is 2 million years old; thus the rock

needed 2,000,000 years to travel from the mid ocean ridge to that distance.

• From the ridge to the green area is a distance of 160 Km. • 1 Km = 100,000 cm • So 160 Km = 16,000,000cm (mult. 160 X 100,000)

Time

DistanceRate cm/yr8

yr 2,000,000

cm 16,000,000

Chap Summary page 155

• In Class:– Calculate for Atlantic Ocean on pg. 153 and

record values on pg. 153 - #5

• Home Work:– Questions #1 – 5 & 9 – 12 (Old Book)– Questions #1 – 5 & 8 – 12 (New Book)