PLATE TECTONICS:

A Scientific Revolution

Unfolds

Earth’s continents are not static

They gradually migrate across the globe

Blocks of continental material collide, creating earth’s great mountain chains

Theory of Plate Tectonics – discovered in 1968 due to the use of advanced tools and equipped with more knowledge.

Alfred Wegener: “The Origin of

Continents and Oceans”

Formulated the: CONTINENTAL DRIFT THEORY

The inspiration for continental drift is believed to have

come to Wegener when he observed the breakup of

sea ice during a Danish-led expedition to Greenland.

This theory suggested that a single supercontinent

consisting of all Earth’s landmasses once existed.

Pangea – a giant landmass, a single supercontinent

Evidences

1st Evidence:

The Continental Jigsaw Puzzle

The seaward edge of its continental shelf, which lies submerged a few hundred meters below sea level, proved the existence of a single supercontinent.

2nd Evidence:

Fossils Match Across the Seas

Identical fossil organisms had been discovered in rocks from both South America and Africa.

Mesosaurus – an aquatic fish-catching reptile whose fossil remains are limited to black shales of the Permian period (about 260 million years ago) in eastern South America and southwestern Africa.

Glossopteris – seed fern fossil distributed and located within the continents closer to the south pole.

3rd Evidence:

Rock Types and Geologic Features

Wegener found evidence of 2.2-billion-yea1= old

igneous rocks in Brazil that closely resembled similarly

aged rocks in Africa.

“It is just as if we were to refit the torn pieces of a

newspaper by matching their edges and then check

whether the lines of print run smoothly across. If they do,

there is nothing left but to conclude that the pieces

were in fact joined in this way.”

4th Evidence:

Ancient Climates

Wegener learned that evidence for a glacial period that

dated to the late Paleozoic had been discovered in

southern Africa, South America, Australia, and India.

Plate Tectonics

Oceanographic Explorations led to the discovery of the

Oceanic Ridge System.

This led to the unfolding of a far more encompassing

theory than continental drift, known as plate tectonics.

The uppermost mantle and the overlying crust behave

as a strong, rigid layer, known as the lithosphere, which is

broken into segments commonly referred to as plates.

The lithosphere, in turn, overlies a weak region in the

mantle known as the asthenosphere (intense

temperature and pressure wherein rocks are very near

to their melting temperatures).

Earth’s Major Plates:

North American Plate

South American Plate

Pacific Plate

African Plate

Eurasian Plate

Australian-Indian Plate

Antarctic Plate

Plate Boundaries:

1. Divergent boundaries (constructive margins)— where two plates move apart, resulting in upwelling of hot material from the mantle to create new seafloor.

2. Convergent boundaries (destructive margins)—where two plates move together, resulting in oceanic lithosphere descending beneath an overriding plate, eventually to be reabsorbed into the mantle or possibly in the collision of two continental blocks to create a mountain system.

3. Transform fault boundaries (conservative margins)— where two plates grind past each other without the production or destruction of lithosphere.

4. Collision Boundaries – where two plates crash into each other forming mountain ridges.

Divergent Boundaries:

Oceanic ridges - elevated areas of the seafloor that are characterized by high heat flow and volcanism.

Seafloor spreading

Continental Rifting - occurs where opposing tectonic forces act to pull the lithosphere apart.

Convergent Boundaries:

Deep-ocean trenches - are the surface manifestations produced as

oceanic lithosphere descends into the mantle.

Oceanic-Continental Convergence - whenever the leading edge

of a plate capped with continental crust converges with a slab of

oceanic lithosphere, the buoyant continental block remains

"floating," while the denser oceanic slab sinks into the mantle.

Oceanic-Oceanic Convergence - where two oceanic slabs

converge, one descends beneath the other, initiating volcanic

activity by the same mechanism that operates at all subduction

zones.

Continental-Continental Convergence - results when one landmass

moves toward the margin of another because of subduction of the

intervening seafloor.

Transform Fault Boundaries:

Transform faults are part of prominent linear breaks in the

seafloor known as fracture zones.

Like the Mendocino Fault, most transform fault

boundaries are located within the ocean basins;

however, a few cut through continental crust.

Testing the Plate Tectonics

Model

Evidence: Ocean Drilling

Some of the most convincing evidence for seafloor spreading

came from the Deep Sea Drilling Project.

To accomplish this, the Glomar Challenger, a drilling ship capable of

working in water thousands of meters deep, was built.

One of the early goals was to gather samples of the ocean floor in

order to establish its age.

The Ocean Drilling Program, the successor to the Deep Sea Drilling

Project, employed a more technologically advanced drilling ship to

continue the work of the Glomar Challenger.

The youngest oceanic crust would be found at the ridge crest, the

site of seafloor production, and the oldest oceanic crust would be

located adjacent to the continents.

Evidence: Hot Spots

Mapping volcanic islands and seamounts (submarine volcanoes) in

the Pacific Ocean revealed several linear chains of volcanic

structures.

Hawaiian Island—Emperor Seamount chain, showed that the

volcanoes increase in age with increasing distance from the “big

island” of Hawaii.

Most researchers are in agreement that a cylindrically shaped

upwelling of hot rock, called a mantle plume, is located beneath

the island of Hawaii.

Hot spot, an area of volcanism, high heat flow, and crustal uplifting.

Evidence: Paleomagnetism

Anyone who has used a compass to find direction knows that

Earth’s magnetic field has a north and south magnetic pole.

Some naturally occurring minerals are magnetic and hence are

influenced by Earth’s magnetic field.

One of the most common is the iron-rich mineral magnetite, which is

abundant in lava flows of basaltic composition.

Rocks that formed thousands or millions of years ago and contain a

“record” of the direction of the magnetic poles at the time of their

formation are said to possess fossil magnetism, or paleomagnetism.

Apparent Polar Wandering – a strong evidence to Europe’s drifting

in relation to the poles and that either the magnetic poles had

migrated.

Magnetic Reversals and Seafloor

Spreading

How is Plate Motion Measured?

The amount of tectonic plate movement is measured

using Global Positioning System (GPS) satellites and a

network of GPS receivers. This technology allows

scientists to measure plate movements as small as a few

millimeters per year.

This network of satellites is more stable than the Earth's

surface, so when a whole continent moves somewhere

at a few centimeters per year, GPS can tell.

What Drives Plate Motions?

The plate tectonics theory describes plate motion and the role that

this motion plays in generating and modifying the major features of

Earth’s crust.

Plate-Mantle Convection - mantle convection is driven by a

combination of three thermal processes: ( l) heating at the bottom

by heat loss from Earth’s core; (2) heating from within by the decay

of radioactive isotopes; and (3) cooling from the top that creates

thick, cold lithospheric slabs that sink into the mantle.

Forces that Drive Plate Motion:

There is general agreement that the subduction of cold, dense slabs

of oceanic lithosphere is a major driving force of plate motion.

Slab pull, occurs because cold slabs of oceanic lithosphere are

more dense than the underlying asthenosphere and hence “sink

like a rock.”

Ridge push, this gravity-driven mechanism results from the elevated

position of the oceanic ridge, which causes slabs of lithosphere to

“slide” down the flanks of the ridge.

Models of Plate Mantle Convection:

Layering at 660 Kilometers - some researchers argue that

the mantle resembles a “giant layer cake" divided at a

depth of 660 kilometers.

Whole Mantle Convection - in which cold oceanic lithosphere sinks to great depths and stirs the entire

mantle.

Next Chapter . . . .

Earthquake andEarth Interiors

Chapter 8

Jamil Faisal Saro Adiong

Photo: http://newsinfo.inquirer.net/

Photo: http://www.geardiary.com/

Photo: Rappler

Photo: Al Jazeera

What is an Earthquake?

Earthquakes are natural geologic

phenomena caused by the sudden

and rapid movement of a large

volume of rock.

Points to remember:

Faults - the violent shaking and destruction caused by earthquakes are theresult of rupture and slippage along fractures in Earth’s crust

Focus - the origin of an earthquake occurs at depths between 5 and 700kilometers / the zone within Earth where the initial displacement occurs

Epicenter - the point at the surface directly above the focus

Seismic Waves - a form of elastic energy that causes vibrations in thematerial that transmits them

Photo: wordsmith.org

Photo: geopick.uncc.edu

Discovering the causes of Earthquakes

Earthquakes are produced by the rapidrelease of elastic energy stored in rock that has been deformed by differential stresses.

Aftershocks and Foreshocks

Aftershocks are the numerous small tremors that gradually diminish in frequency and intensity over a period of several months.

Because aftershocks happen mainly on the section of the fault that has slipped, they provide geologists with data that is useful in establishing the dimensions of the rupture surface.

In contrast to aftershocks, small earthquakes called foreshocks often precede a major earthquake by days or in some cases by several years. Monitoring of foreshocks to predict forthcoming earthquakes has been attempted with limited success.

Trivial Question:How often do earthquakes occur?

Thousands of earthquakes DAILY!Most of them are too small to be felt by people and many of them occur in remote regions. Their existence is only identified through sensitive seismographs.

Plate Tectonic Theory

Large slabs of Earth's lithosphere are in continual slow motion.

Seismology:The Study of Earthquake Waves

The study of earthquake waves, seismology dates back to attempts made by the Chinese almost 2,000 years ago to determine the direction from which these waves originated.

Seismographs

• Instruments that record earthquake waves. • Have a weight freely suspended from a support that is securely attached to bedrock.

Measuring the size of the Earthquakes

•Intensity - a measure of the degree of

earthquake shaking at a given locale based

on observed effects

•Magnitude - quantitative measurement of

ground motion

Destruction from Seismic Waves

•Intensity

•Duration of the shaking

•The nature of the material upon which the structure rests

•the nature of building materials and the construction

practices of the region

Trivial Question:I’ve heard that the safest place to be in a house during an earthquake is in a doorframe. Is that really the best place?

No!If you're inside, the best advice is to duck, cover, and hold. When

you feel an earthquake, duck under a desk or sturdy table. Stay

away from windows, bookcases, file cabinets, heavy mirrors,

hanging plants, and other heavy objects that could fall. Stay under

cover until the shaking stops. And, hold on to the desk or table: If

it moves, move with it.

Liquefaction:

In areas where unconsolidated materials are saturated with water, earthquake vibrations can turn stable soil into a mobile fluid, a phenomenon known as liquefaction

•Fire•Landslides•Tsunami

Earth's Interior

Forces Within the Earth's

Layered Structure

The heaviest materials (metals) would be in the center.

Lighter solids(rocks) would be in the middle and

liquids and gases would be on top.

Formation of Earth's Layered Structure

High-velocityimpact ofnebular debris and the decay ofradioactive elements caused the temperature of our planet to steadily increase

Melting produced liquid blobs of heavy metal that sank toward the center of the planet.This process occurredrapidly on the scale of geologictime and produced Earth's dense iron-rich core.

Three basic divisions of Earth's interior: (1)the iron-rich

core, (2)the thin primitive crust, and (3) Earth's largest

layer, called the mantle.

Probing Earth's Interior: "Seeing" Seismic

Waves

Earthquakes are large enough that their seismic wavestravel all the way through Earth and can be recorded onthe other side. This means that the seismic waves act like medical x-rays used to take images of a person's insides.

Interpreting the waves recorded on seismograms in order to identify Earth structure is challenging. Seismic waves donot travel along straight paths; instead, seismic waves arereflected, refiucted, and diffiacted as they pass through our planet.

Earth's Internal Structure

Earth's Crust - The crust, Earth's relatively thin, rocky outer skin, is comprised of two types: continental crust and oceanic crust

- oceanic crust is roughly 7 kilometers (4 miles) thick and composed of the dark igneous rock basalt.

- continental crust averages 35-40 kilometers (22-25 miles) thick but may exceed 70 kilometers (40 miles) in some mountainous regions such as the Rockies and Himalayas.

Earth's Internal Structure

Earth's Mantle -More than 82 percent of Earth's volume iscontained in the mantle, a solid, rocky shell that extends to a depth ofabout 2,900 kilometers (1,800 miles). The boundarybetween the crust and mantle represents a marked change in chemical composition. The dominant rock type in the upper mostmantle is peridotite.

Earth's Internal Structure

The uppermantle extends from the crust-mantle boundary down to a depth of about 660 kilometers (410 miles).

Upper mantle divided into two parts: top portion of the upper mantle is part of the stiff lithosphere, and beneath that is the weaker asthenosphere.

Lithosphere - (sphere ofrock) consists of the entire crust and uppermost mantle and forms Earth's relatively cool, rigid outershell.

Asthenosphere - a depth of about 350 kilometers (217 miles) comparatively weak layer.

Earth's Internal Structure

Earth's Core - The composition of the core is thought to be an iron-nickel alloy

with minor amounts of oxygen, silicon, and sulfur—elements that readily form

compounds with iron.

The core is divided into two regions: The outer core is a liquid layer 2,270

kilometers (1,410 miles) thick. It is the movement of metallic iron within this

zone that generates Earth's magnetic field. The inner core is a sphere with a

radius of 1,216 kilometers (754 miles)