Plate Tectonics

The Plates

How do we know this?Part 1 – older evidence

• Biology – same fossils in different parts of the world

• Geology – same rocks in different parts of the world

• Climate – fossils of plants that live in tropical conditions in Antarctica

How do we know this?Part 2 – New evidence

• The Mid Atlantic ridge (1948)

• Paleomagnetism and the reversal of the earths magnetic field (1950’s)

• Sea floor spreading – dating of the sea floor rocks (1960’s)

Structure of the earth• 1) Layers:

– i) Continental and oceanic crust – ii) Mantle– iii) Inner and outer Core

• 2) Lithosphere and asthenosphere; Describe the nature of the Lithosphere (rigid), Asthenosphere, Low Velocity Zone,

• 3) Discontinuities: Depth and nature of discontinuities: Inner core, outer core, mantle, continental crust and oceanic crust.

• 4) Direct evidence: Such as deep mines, surface rocks, volcanic vents and ophiolites.

• 5) Indirect evidence: Such as seismic wave velocities, shadow zones, whole earth and rock density and meteorites.

Structure of the earth

• Crust: the upper layer which is solid (divided into 2 types).

• Lithosphere: the crust plus the upper part of the mantle (solid).

• Mantle:– Upper part is solid and is

part of the lithosphere.– Below this is the

asthenosphere which is partly molten and can flow.

– Rest of mantle is liquid

• Outer Core: Liquid.• Inner Core: Solid.

The plates

• Lithospheric plates comprise of 2 parts:– An upper part which is

crust.– A lower part which is a

solid part of the upper mantle.

• This makes a lithospheric plate

• There are two kinds – continental and oceanic

How do we know this?

• As early as the 16th century, the fit between the continents was discussed by Abraham Ortelius, Francis Bacon and other well educated men of the age

• Alfred Wegener (1880-1930), a German meteorologist and geologist, was the first person to propose the theory of continental drift as a unified theory

• In his book, "Origin of Continents and Oceans," he calculated that 200 million years ago the continents were originally joined together, forming a large supercontinent. (note the symmetry between the theory of evolution and tectonics)

• He named this supercontinent Pangaea, meaning "All-earth".

Continental Drift???• However, it was not until 1912

that the idea of moving continents was seriously considered as a full-blown scientific theory -- called Continental Drift -- introduced in two articles published by Wegener.

YOU NEED TO NOTE DOWN THE OUTLINE OF THE STORY…

• He contended that, around 200 million years ago, the supercontinent Pangaea began to split apart.

Evolution of the theory

• Alexander Du Toit, Professor of Geology at Johannesburg University and one of Wegener's staunchest supporters, proposed that Pangaea first broke into two large continental landmasses, Laurasia in the northern hemisphere and Gondwanaland in the southern hemisphere.

• Laurasia and Gondwanaland then continued to break apart into the various smaller continents that exist today.

• Wegener's theory was based in part on what appeared to him to be the remarkable fit of the South American and African continents, first noted by Abraham Ortelius three centuries earlier.

• Wegener was also intrigued by the similar layout (structure) and lithology on opposite sides of the ocean. He was not the first to remark that finding identical rocks and fossils on either side of the Atlantic reasonably meant at one time they were joined

You have a cut out and stick together map of this we will do later in the week..... Good old fashioned geography with colouring pencils

• The most convincing sequences were the laurasian Cyclotherms – repeated swamp and river deposits. These are coal bearing formations, and the same fossilised plants were found in S America and Africa

• The fact that there are fossils of tropical plants (in the form of coal deposits) in Antarctica led to the conclusion that this frozen land previously must have been situated closer to the equator, in a more temperate climate where lush, swampy vegetation could grow. The earth has never been that hot while life has existed!

• Continental drift was not an accepted theory, for one significant reason, at the time, scientists believed continents were fixed – there was no way of measuring movement, and that if they could move, what kind of force would make masses of rock float and move

• Wegener's lack of a movement mechanism meant that his theory was not accepted in his lifetime.

The fate of Wegener...• On September 22, although the route markers were by now largely buried

by snow, Wegener set out with 13 Greenlanders and his meteorologist Fritz Loewe to supply the camp by dog sled. During the journey the temperature reached −60 °C (−76 °F) and Loewe's toes became so frostbitten they had to be amputated with a penknife without anaesthetic. Twelve of the Greenlanders returned to West camp while the remaining three reached Eismitte on October 29.

• With only enough supplies for three at Eismitte, Wegener and Rasmus Villumsen took two dog sleds and made for West camp. They took no food for the dogs and culled them to feed the rest until they could only run one sled. While Villumsen rode the sled, Wegener had to use skis. Wegener's body was found buried in a sleeping bag cover six months later on May 12, 1931 halfway between Eismitte and West camp. At 50 years of age and a heavy smoker his suspected cause of death was heart failure through overexertion. Villumsen was never found. It is unlikely Wegener or Villumsens bodies will ever be recovered as it is estimated that due to accumulation of ice both now lie at a depth of more than 100 metres (330 ft).

• However, after his death, new evidence from ocean floor exploration and other studies rekindled interest in Wegener's theory, ultimately leading to the development of the theory of plate tectonics.

• Plate tectonics has proven to be as important to the earth sciences as the discovery of the structure of the atom was to physics and chemistry and the theory of evolution was to the life sciences.

• Even though the theory of plate tectonics is now widely accepted by the scientific community, aspects of the theory are still being debated today.

• http://www.mrsciguy.com/convection.html

Convection

• Convection is a widely used term, and refers to a warmer section of liquid moving away from the heat source, until a level is reached at which the current must diverge, liquid spreads, cools, sinks and is heated again

• All liquids if not heated evenly from the base will display convection

Convection – driving forces

• Very slow convection currents flow in the asthenosphere.

• These currents provide horizontal forces on the plates of the lithosphere much as convection in a pan of boiling soup (must be Heinz tomato) causes a crouton on the surface of the water to be pushed sideways.

Heat source?• The structure of the earth is

understood through earthquake wave refraction, which we will discuss next term.

• The difference in material at the core mantle boundary is at a known depth, and scientists believe that the outer core generates thermonuclear reactions which send plumes of superheated magma to the surface, and lateral movement starts at the base of the lithosphere

MOR

• In the 1950’s the ridges were discovered when we started to map the ocean floors.

• Columbia University mapped the Mid Atlantic ridge and named it, it is however the only ridge in the middle of the ocean!

• It was this discovery that led to discussion of sea floor spreading

• The ridges overall total over 80,000km of mountain chain

Sea Floor Spreading

• Proposed by Harry Hess of Princeton University

• The magnetic stripes are symmetrical around the ridges, giving a driving force for separating continents

• He discovered the flat topped old volcanoes either side of the ridge and named them Guyots, and was the first to propose movement away from the ridges

• His book, “The History of Oceans” in 1962 brought Wegener's theories back into science and explained how the continents could move

Magnetic sea floor stripes

• The mid-ocean ridge systems form new oceanic crust.

• As crystallized basalt extruded at a ridge axis cools below Curie points of appropriate iron-titanium oxides, magnetic field directions parallel to the Earth's magnetic field are recorded in those oxides.

• Because the field has reversed directions at irregular intervals throughout its history, the pattern of reversals in the ocean crust can be used as an indicator of age.

• Likewise, the pattern of reversals together with age measurements of the crust is used to help establish the history of the Earth's magnetic field.