PLATE TECTONICS Chapter 7 · !Plate tectonics is the study of how the Earth's crust is shaped by geological forces. It relies on the ... Ocean floors move like conveyor belts- carrying

Chapter 7 PLATE TECTONICS

Section 1 CONTINENTAL DRIFT

PLATE TECTONICS

Ø The Earth's crust is made of pieces of rock called tectonic plates.

Ø Plate tectonics is the study of how the Earth's crust is shaped by geological forces. It relies on the understanding that the crust is divided into large pieces, or plates, that sit on the molten interior of the planet.

MOVEMENT OF CONTINENTS

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

Continental drift is the idea that the continents move around on Earth’s surface.

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PANGAEA = ALL EARTH

¡ 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

EVIDENCE OF CONTINENTAL DRIFT

Wegener's theory was based in part on what appeared to him to be the remarkable fit of the South American and African continents.

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GEOLOGICAL EVIDENCES

Same rocks and structures on each side of the Atlantic ocean

Continents fit together along the continental shelves

PALEOCLIMATIC EVIDENCE

Glacial moraines in tropical regions

¡  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 l ifetime.

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.

§ Each continuing to break apart into the smaller continents that exist today.

EVOLUTION OF THE THEORY

¡  On September 22, although the route markers were by now largely buried by snow, Wegener set out with 13 Greenlanders and his meteorologist Fr itz 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 Vi l lumsen took two dog sleds and made for West camp. They took no food for the dogs and culled them to feed the rest unti l they could only run one sled. While Vi l lumsen rode the sled, Wegener had to use skis. Wegener's body was found buried in a sleeping bag cover s ix 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 fai lure through overexertion. Vi l lumsen was never found. It is unl ikely Wegener or Vi l lumsens bodies wil l ever be recovered as it is est imated that due to accumulation of ice both now l ie at a depth of more than 100  metres (330  ft).

THE FATE OF WEGENER...

¡ After Wegener’s 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.

¡  Plate tectonics has become the unifying theory of geology

¡  It explains the connection between continental drift and the formation and destruction of crust along plate boundaries.

¡  It also helps to explain the occurrence of earthquakes, volcanoes, and mountains

A THEORY IN PROGRESS

Section 2 SEAFLOOR

SPREADING

evidence for

sea-floor spreading

1. molten

material

2. magnetic

stripes

3. drilling

samples

World War II Submarine warfare

MAPPING THE OCEAN FLOOR

¡ Sound waves echo off the ocean bottom – the longer the sound waves take to return to the ship, the deeper the water is.

TECHNOLOGY DEVELOPED DURING WWII

¡ Sonar - image the deep ocean using reflected sound.

¡ Magnetometer - detect faint magnetic fields.

¡ Developed to search for U-boats

¡ 1968, establishment of the Deep Sea Drilling Project (DSDP).

¡ Specially outfitted ship for drilling into the ocean floor and retrieving rock samples.

¡ Sediment layers and igneous rock dated to reveal the age of the ocean floor in different places.

Deep Sea Drilling Project ship - Glomar Challenger

Harry Hess

• Geologist at Princeton University and WWII Submarine Captain

•  Sea floor spreading hypothesis (early 1960’s).

•  Suggested that the ocean

floor itself might be moving.

•  Suggested that the sea floor was significantly younger than the continents.

¡  Proposed by Harry Hess of Pr inceton University

¡  The magnetic str ipes are symmetrical around the r idges, giving a driving force for separating continents

¡  He discovered the f lat topped old volcanoes either s ide of the r idge and named them Guyots, and was the f i rst to propose movement away from the r idges

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

SEA FLOOR SPREADING

HARRY HESS’ IDEA-

Ocean floors move like conveyor belts- carrying continents with themà

Movement begins at mid-ocean ridgeà

Molten material rises from mantle & eruptsà

Molten material spreads out- pushing older rock to both sides of the ridge

THERE IS A CHAIN OF SUBMERGED MOUNTAINS RUNNING THROUGH THE

CENTER OF THE ATLANTIC OCEAN.

¡  This chain is called the Mid-Atlantic Ridge. ¡  This ridge is part of a system of ocean ridges. ¡ Mid-ocean ridges are underwater mountain

chains that run through Earth’s ocean basins.

THE EARTH’S LONGEST MOUNTAIN RANGE IS

UNDERWATER

Mid-Ocean Ridge

https://www.ocean.udel.edu/extreme2004/geology/mission/divelocation/ridge.html

¡ The mid-ocean ridge (shown in red) winds its way between the continents much like the seam on a baseball.

¡ Hess proposed that hot, less dense material below Earth’s crust rises toward the sur face at the mid-ocean ridges. Then, it flows sideways, carrying seafloor away from the ridge in both directions. This theory is known “seafloor spreading”.

Ø The key was the discovery that there are “magnetic patterns” in the rocks on either side of the mid-ocean ridges.

Ø Matching magnetic patterns and the age of rocks on either side of mid-ocean ridges provided strong evidence for sea-floor spreading.

SEA FLOOR SPREADING HYPOTHESIS

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BECAUSE OF SEA-FLOOR

SPREADING, THE DISTANCE BETWEEN

EUROPE AND NORTH AMERICA IS INCREASING

BY A FEW CENTIMETERS

PER YEAR.

Growing an Ocean

MID

-AT

LA

NT

IC R

IDG

E

Section 3 THEORY OF PLATE

TECTONICS

THE PLATE TECTONIC THEORY

¡ Earth’s crust is constantly being created and destroyed

¡ The theory of plate tectonics states that Earth’s surface is made of rigid slabs of rock, or plates, that move with respect to each other

THE EARTH’S INTERIOR

¡ Asthenosphere: Plastic-like (semi-fluid) layer of the upper mantle

¡ Lithosphere: Consists of the crust and the rigid upper mantle

Ø Scientists realized that large pieces of Earth’s sur face moved about like rafts on a river.

Ø These “rafts” are pieces of lithosphere called lithospheric plates.

Ø There are two kinds of lithospheric plates: Ø Oceanic plates and Continental plates.

MOVING PIECES OF THE LITHOSPHERE

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Himalyan Mountains

PLATE BOUNDARIES

DIVERGENT

¡ Plates move away from one another (separate)

¡ Creates new crust

¡ Example: Mid-Atlantic Ridge

TYPES OF PLATE BOUNDARIES

CONVERGENT (Continental – Oceanic)

¡ Plates move/push together ¡ Oceanic plates will subduct (push under) the less

dense continental plates ¡ Disappearance of crust at deep ocean trenches ¡ Form volcanoes

¡ Example: Andes Mountains

TYPES OF PLATE BOUNDARIES

CONVERGENT (Oceanic – Oceanic)

¡ Plates move/push together ¡ The older, more dense, plate will subduct

beneath the newer plate ¡ Forms an island arc ¡ Form volcanoes

¡ Example: Mariana Islands

TYPES OF PLATE BOUNDARIES

CONVERGENT (Continental – Continental)

¡ Plates move/push together ¡ Form mountain ranges ¡ Cause earthquakes

¡ Example: Himalayan Mountains

TYPES OF PLATE BOUNDARIES

TRANSFORM

¡ Slide against one another in opposite directions, moving at different rates

¡ Cause earthquakes

¡ Example: San Andres Fault

TYPES OF PLATE BOUNDARIES

THE EARTH’S INTERIOR

¡ Inner Core: solid, dense, 6,000º C, Iron and Nickel

¡ Outer Core: liquid, dense, 3,700º-5,500ºC (increases with depth), Iron and Nickel

¡ Mantle: Solid, minerals rich in Iron, Silicon, Magnesium, 1,500º - 3,200º C

¡ Crust: upper layer, rigid, lighter rock, <1,000ºC

THE EARTH’S INTERIOR

¡ Where is the crust thickest? § Mountain Ranges.

¡ Where is the crust thinnest? § The Ocean Floor.

THE EARTH’S INTERIOR

¡ Why is the core so hot? § Still retains heat from original formation. § Radioactive materials releasing energy

as they decay. § Increased pressure from above.

THE EARTH’S INTERIOR

¡ What happens to rock density, going from the crust to the core? § It increases.

¡ What happens to temperature, going

from the crust to the core? § It increases.

¡ What happens to pressure, going from

the crust to the core? § It increases.

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

¡ All l iquids if not heated evenly from the base will display convection

CONVECTION

Ø Convection cells in Earth’s lower mantle drive the lithospheric plates on the sur face.

Ø Heated lower mantle material rises toward Earth’s sur face.

Ø Cooling makes the nearby material denser and it sinks deeper into the lower mantle.

Ø This sinking process is called subduction.

WHAT DRIVES LITHOSPHERIC PLATES?

Rising convection

current

Mid – ocean ridge

Lithosphere Convection cell in the

mantle

subduction

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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 sur face of the water to be pushed sideways.

¡ 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 sur face, and lateral movement starts at the base of the lithosphere

HEAT SOURCE?

CONVECTION CURRENTS ¡ Convection cells in the mantle move the plates.

§ Hot in the center, less dense magma rises up due to heat. § When the magma reaches the surface, it cools and sinks

back down creating a circular pattern of movement. ¡ This process happens continually.

¡ Hot – goes UP ¡ Cool – goes DOWN

Cool