101 Chap2 Intro PlateTectonics

Introduction

Topics and Concepts

The vastness of time and scale in geology

The Scientific Method

The Law of Uniformitarianism

Age of the Solar System

The rates of most geological processes are

too slow to be easily observed by humans

This is the big difference between geology

and other sciences

Rates of most geologic processes range from

micrometers per year to centimeters per year

There are certain exceptions

such as earthquakes, landslides,

and volcanic eruptions

Barringer Meteor Crater

in Arizona

This feature, which is

about 2 kilometers

across, formed in less

than one minute about

50,000 years ago

Some Geologic Events are Rapid

The Grand Canyon in Arizona

It took more than 250 million years to deposit this sequence of rocks

The rocks at the bottom of the canyon are about 2 billion years old

Most Geologic Events are Slow

Geology is a Science (Just like chemistry and physics)

However, because of

the scale of the

problems considered,

both in size and time,

geologists face the

special challenge of

not being be able to

conduct experiments

in the typical

laboratory setting

Opinion is NOT part of the

Scientific Method

All science is based on

the assumption that the

natural world behaves

in a consistent and

predictable manner that

is comprehensible

through careful,

systematic study

The collection of scientific facts

through unbiased observations

and measurements

The development of one or more

working hypotheses or models

to explain these facts

and measurements

The development of

observations and experiments

to test the hypotheses

and measurements

The development of

observations and experiments

to test the hypotheses

The acceptance, modification or

rejection of the hypotheses or

models

Hypothesis Theory Law A hypothesis is an explanation initially offered

for a set of observations

When a hypothesis withstands many tests it

may be called a theory

A theory for which it seems there is no

sensible reasons to challenge is called a law

James Hutton

James Hutton (1726-

1797) is considered to

be the father of modern

geology

Born in Scotland,

schooled as both a

lawyer and a doctor, he

was a “gentleman

farmer” who spent most

of his free time pursuing

the study of geology

During a lifetime of research, James Hutton

set the groundwork for one of the most

important concepts in science

Law of Uniformitarianism

It can be summed up in a simple sentence:

“The present is the key to the past”

Hutton argued here that the

earth was shaped in the past

by the same forces that shape

it in the present, namely

erosion, sedimentation, and

gradual uplift

He maintained that rock formation was a cyclical

process, with mountains being eroded into the sea,

only to be consolidated and raised into new

mountains, which would in turn be eroded again

It was a view of earth-history that required a

vast expanse of geological time

The Earth could not be a mere 6,600 years old

Hutton was right

Science has

proven that the

Earth is very old

and the universe

is much older

Meteorites give us

access to debris left

over from the formation

of the Solar System

We can date meteorites

using radioactive

isotopes and their

decay products

The Age of the Earth

This radioactive dating has conclusively

proven that the Earth is just over

4,500,000,000 years old

It is difficult for the

human mind to grasp

such a large number or

to comprehend such a

staggering time span

So we will count to 4

billion 500 million at the

rate of one number per

second

4,500,000,000 seconds

75,000,000 minutes

4,500,000,000 seconds

75,000,000 minutes

1,250,000 hours

4,500,000,000 seconds

75,000,000 minutes

1,250,000 hours

52,083 days

4,500,000,000 seconds

75,000,000 minutes

1,250,000 hours

52,083 days

142 years 7 months 4 days

4,500,000,000 seconds

75,000,000 minutes

1,250,000 hours

52,083 days

If you start counting today

4,500,000,000 seconds

75,000,000 minutes

1,250,000 hours

52,083 days

Wednesday, August 22, 2012

4,500,000,000 seconds

75,000,000 minutes

1,250,000 hours

52,083 days

You will finish on

4,500,000,000 seconds

75,000,000 minutes

1,250,000 hours

52,083 days

You will finish on

Saturday, March 29, 2155

Test Question

1. How old is the Earth?

A. 4.5 trillion years

B. 4.5 billion years

C. 4.5 million years

D. 450,000 years

Chapter 2

Plate Tectonics: A Scientific Revolution Unfolds

Why is Mount Everest

over 29,000 feet high?

Why is it in Asia?

Why isn’t it in

Tennessee?

The unifying concept of the Earth sciences

The outer portion of the Earth is made up of about 20 distinct “plates” which move relative to each other

Plate tectonics explains:

Volcanoes

Earthquakes

Sea floor spreading

Mountain chains

Occurrence of same fossils on different continents

Plate Tectonics

Integrates evidence from many branches of science

First suggested based on evidence from cartography, geology and paleontology

Fully embraced after evidence from geophysical measurements

However, the proof of plate tectonics was a long rambling road

Plate Tectonics

300-400 years ago

(after accurate maps

became available) it

was noticed that the

coastline on both

sides of the Atlantic

could fit together

like a jigsaw puzzle

Atlantic Coastline

The fit was even

better when you

took the continental

shelf into

consideration

In 1858, Antonio

Snider-Pellegrini, an

American published

a sketch showing

this fit

Atlantic Coastline

Gondwanaland

In the 1890s, the Austrian

Geologist Eduard Suess

suggested that the present

day southern continents

once been formed in a

super continent which he

called Gondwanaland

The name comes from the

Gondwana region in India

Gondwanaland Gondwanaland included Antarctica, South

America, Africa, Madagascar, Australia and

Gondwanaland

Unfortunately, Suess

did not have any

evidence other than

the jigsaw puzzle fit

He could not explain

how Gondwanaland

split up into the

modern continents

However, he got to be

on a stamp

Continental Drift

Continental drift was proposed

by the prominent German

geologist Alfred Wegner in 1912

based on his observation of

drifting sheets of ice

The concept is that large-scale

horizontal movements of the

Earth’s continents are

responsible for the major

topographical features such as

mountains and ocean basins

Continental Drift Wegner called his super-continent “Pangaea”

He spent the next two decades, until his death,

promoting continental drift

Talk about an idea whose time

had not arrived!

Most scientist ridiculed Alfred

Wegener’s idea

Not even the public believed it

possible

No proof that continents move

No “driving mechanism” that

could move the continents.

“Why should the continents

move?”

Rejection of Continental Drift

Accumulating Evidence

Fossil record

Earth’s crust and mantle

Magnetism and the Earth’s

magnetic field

Paleomagnetism

Magnetic reversals

Polar wandering

The topography of the seafloor

Age of the seafloor

Seafloor spreading

The fossil record had revealed that the geology

and paleontology matched on opposite sides of

the Atlantic Ocean

Fossil Record

In fact, there are matching fossil records that

span across all of the continents

Without continental drift, this is hard to explain

Fossil Record

The Earth’s crust and uppermost mantle are solid, somewhat brittle and elastic

This outmost solid layer is called the lithosphere

It is thinnest under the oceans, averaging 50 kilometers (km) thick

The lithosphere under the continents can be up to 100 kilometers thick

Crust and Mantle

The layer below the lithosphere is called the asthenosphere and it extends to a depth of 300 kilometers

The rock is not rigid and can flow plastically due to the high temperature and moderate pressures

The existence of the plastic asthenosphere made plate motion more feasible

Crust and Mantle

Most iron-bearing minerals are at least weakly magnetic

Each magnetic mineral has a Curie temperature, the temperature below which it remains magnetic

Above the Curie temperature the mineral is not magnetic

The Curie temperature varies from mineral to mineral, but it is always below the melting temperature of the mineral

Magnetism

Test Question

The temperature point below which iron minerals

remain magnetic and above which they do not remain

magnetic is called the?

A. Wegener temperature

B. Magnetic pole temperature

C. Hawkins temperature

D. Curie temperature

Earth’s Magnetic Field

The simple presence of iron in the Earth’s core

is not enough to account for the Earth’s

magnetic field

The high temperatures in the Earth’s core are

far above the Curie temperature for any

magnetic mineral

The Earth has a magnetic

This why a compass points

to the north

Earth’s Magnetic Field

It is believed that the Earth’s magnetic field

originates in a layer called the outer core

The outer core is composed of a metallic fluid

consisting mainly of iron

This metallic fluid is

in motion and the

convection currents

act like a giant

dynamo, converting

mechanical energy

into magnetic energy

A hot magma is not magnetic

As a magma cools and solidifies, the iron-bearing minerals (such as the various ferromagnesian silicates) crystallize

Eventually, the minerals cool below the Curie temperature and the iron-bearing minerals become magnetic

Paleomagnetism

Like tiny compass needles, these magnetic minerals align themselves parallel to the lines of force of the Earth’s magnetic field

This remnant magnetism, which is also called paleomagnetism, points to the north pole like a sign post

But...

Paleomagnetism

Magnetic Reversals

About a century ago, a sequence of lava flows

were found in France where some of the flows

had the north and south poles reversed

Therefore, the north pole and south pole must

have repeatedly swapped positions

Magnetic Reversals

These magnetic reversals have occurred

though out the history of the Earth

They occur on an irregular basis ranging in

time from tens of thousands of years to

millions of years

Magnetic Striping on Seafloor

In the 1950s, the Atlantic

seafloor was found to

consist of alternating

stripes of normal and

reversely magnetized

Topography of the Atlantic

Also, in the 1950s, it

was discovered that an

underwater mountain

range ran north-south

in the middle of the

north Atlantic Ocean

The Mid-Atlantic Ridge

rises as high as 2

kilometers (1.5 miles)

above the abyssal

Topography of the Atlantic

It was eventually discovered that all of the oceans

have mid-oceanic ridges

In the 1960s, samples were

collected from the Atlantic

seafloor using special ships

with drill rigs

The rocks of the Atlantic

seafloor were discovered to

be basalt

Basalt contains radioactive

isotopes (such as U235)

which can be dated

Atlantic Ocean Sea Floor

It was discovered that the

youngest rocks of the

Atlantic Ocean seafloor are

found along the mid-oceanic

And that farther you move

away from the ridge, the

older the rocks become on

either side of the ridge

Seafloor Spreading The oldest oceans on Earth are only 180 million

years old, a fraction of the Earth’s total age

Therefore, the oceans are, relatively speaking,

young geologic surface features

Seafloor Spreading

By 1962, Harry Hess at Princeton

University and a Naval Reserve

Rear Admiral, and Robert S. Dietz

had coined the term seafloor

spreading

And in 1963, the team of F. J. Vine

and D. H. Matthews (and

independently L. W. Morley)

proposed that seafloor spreading

could explain the observed

magnetic reversal striping on the

Atlantic and Pacific seafloors

Seafloor Spreading

The Mid-Atlantic

Ridge is a great

fault zone where

hot magma rises

up, cools and

solidifies, forming

new basalt

Seafloor Spreading

The basalt becomes

magnetized in the

prevailing direction

of the Earth’s

magnetic field

Plates that Move

In 1965, the Canadian

Geologist, J. Tuzo Wilson

proposed the concept that

the crust of the Earth is a

mosaic of interacting plates

He called his hypothesis

plate tectonics

Plates that Move

Wilson explained that these

plates move relative to each

The continents ride on these

plates

Geologic features, such as

mountains, volcanoes and

earthquakes occur along the

plate boundaries

Plates that Move

There are 20 large and several micro tectonic plates

Plate movement is best described as chaotic

Each plate moves at a different speed and in

a different direction

Plates Move Slowly

Measured using GPS data

Most plates move from

about 15 to ~100

millimeters (1/2 to 4 inches)

per year

Or about the thickness of a

fingernail in one day

However, the Nazca Plate is

moving at ~150 mm (~4

inches) per year

Plates Move Slowly

Plate Boundaries The red dots show that most major earthquakes

occur along plate boundaries

Plate motion causes earthquakes

The black triangles show that volcanoes

commonly occur along plate boundaries

Plate Boundaries

3 Types of Plate Boundaries

Wilson proposed that tectonic plates interact in

three different ways along their boundaries

1. Transform 2. Divergent 3. Convergent

Test Question

The previous slide is what I call a “key slide”

The entire rest of today’s lecture is based upon

the material contained in the previous slide

So yes, this is an obvious test question

Divergent plate boundaries usually start

within continents

They can grow to become ocean basin

Divergent Plate Boundaries

Rifting occurs

where tensional

forces thin the

crust, magma

ascends and

volcanoes form

The crust is pulled

apart, forming a

valley

A long, narrow lake or sea forms

Eventually, an

expansive ocean

basin and ridge

are created

Can see the spreading

on the island of Iceland,

which sits on top of the

mid-Atlantic Ridge

Note that uprise and

decompression of the

underlying asthenosphere

results in magma

formation

The crust responds by

brittle fracture

Continental Rifts

Early rift sediments are

downfaulted into the

developing rift (which is

called a graben)

Erosion takes place on

the sides of the rift valley

Continental Rifts

Two examples are the

East African and Rio

Grande Rift Valleys

This can be the beginning

of ocean formation

(although it may not get

that far)

Rifting often begins at a

triple junction

Continental Rifts

East African Rift Zone

Perhaps the most interesting and spectacular

plate tectonic rift zone on the land surface

Ruwenzori Ol Doinyo Lengai

The East African Rift has a north-south

orientation with the African Plate moving to

the west and the Somali Subplate moving to

the east

A triple junction,

where three plates are

pulling away from one

another: the Arabian

Plate, and the two

parts of the African

Plate (the Nubian and

the Somalian) splitting

along the East African

Rift Zone

The Rio Grande Rift Valley

extends north from Mexico,

near El Paso, Texas

through New Mexico into

central Colorado

The rift is now essentially

quiet tectonically, but

significant deformation and

faulting with major offsets

was responsible for the

formation of the rift during

the past 30 million years

Rio Grande Rift

The geology has been described as a "taffy-like"

thinning of the lower crust, with upper crust

faulting in many places, to produce the rift valley

Rio Grande Rift

“Two features of the region most clearly exemplify the

Southwest's geologic-climatic-demographic circumstances.

These features constitute two "natural geographic corridors"

or focal areas concerning the relationship between people and

the Southweses physical environment: Arizona's Mogollon

Rim Corridor and New Mexico's Rio Grande Rift Corridor”

Rio Grande Rift

The Earth is maintaining a constant diameter

New crust is created at mid-oceanic ridges

The old crust sinks back into the Earth at

subduction zones along convergent plate

boundaries

Is the Earth Expanding?

Convergent Boundaries

There are three types of convergent plate

boundaries

Oceanic–Oceanic Japanese Islands

Oceanic–Continent Andes Mountains

Continent–Continent Himalaya Mountains

Convergent Boundaries First we need to talk about the density of the rock

The continental crust is lighter and has an

average density of 2.8 g/cm3

The oceanic crust is heavier and has an average

density 3.2 g/cm3

Complicating things is that old oceanic crust is

more dense than young oceanic crust

Oceanic - Oceanic

When two oceanic plates

collide under the ocean, one

plate slides underneath the

other at a subduction zone

Oceanic - Oceanic A deep trench forms along

the subduction zone

This is caused by the plate

sinking into the interior of

the Earth

Oceanic - Oceanic

Magma rises up along the subduction zone

creating volcanoes and forms volcanic island

arcs such as the Japanese Islands

Oceanic - Oceanic What determines which oceanic plate sinks?

The older, more dense oceanic plate will sink

beneath the younger, less dense oceanic plate

Oceanic - Continent

When an oceanic plate

collides with a continental

plate, the oceanic plate

slides underneath the

continental plate

Oceanic - Continent

The heaver, more dense

oceanic plate sinks beneath

the lighter, less dense

continental plate

Oceanic - Continent

A deep trench forms along

the subduction zone

This is caused by the

oceanic plate sinking into

the interior of the Earth

Oceanic - Continent

The lighter continental

crust is pushed up and

forms a mountain range

Oceanic - Continent

Magma rises up along the subduction zone creating

volcanoes, which adds to the size of the mountains

and creates a continental volcanic arc

Andes Mountains

The Andes Mountains

stretch over 5500

miles along the Pacific

side of South America

Andes Mountains

The Andes Mountains

were and still are being

created by the Nazca

Plate colliding with the

South America Plate

Note that the continent

of South America is

essentially riding on

the South America

Most continental tectonic plates are formed of rock with essentially the same density

So, when two continental plates collide, one does not sink beneath the other

Instead, the two plates literally smash into each other

The continental crust is deformed and uplifted, creating mountains such as the Himalaya Mountains

Continent - Continent

50 to 90 million years ago the continent of

India was an island a couple thousand miles

south of Asia

The continent of India is

riding on the Indian Plate

The Indian Plate has been

moving north for tens of

millions of year

Starting around 40-50 million

years, India began to ram into

the continent of Asia

As India smashed into Asia, the continental

crust of both India and Asia were pushed up

and deformed, creating what is currently the

greatest mountain range on Earth, the

Himalayas

Why is Mount Everest

over 29,000 feet high?

Because the continent

of India collided with

the continent of Asia

By the way, Mount

Everest, the highest

mountain on Earth, is

composed of

limestone rock, which

was created at the

bottom of the ocean

Transform Plate Boundaries

A transform fault plate boundary

occurs when two plate slide past

each other in opposite directions

The most famous

example is the San

Andreas Fault Zone

in California

The portion of

California in blue is

heading northwest

to Alaska

This is the most

studied fault zone

in the world

San Andreas Fault Zone

A driving force for plate tectonics has not been definitely identified

Each plate weighs trillions upon trillions of tons

Thought to be caused by the convection of the mantle

Friction at base of the lithosphere transfers energy from the asthenosphere to the lithosphere

Convection may have overturned the asthenosphere 4 to 6 times

What Causes the Plates to Move

Convection in the Mantle

Hot mantle material rises at the spreading mid-

oceanic ridges

Some of the magma erupts, but most spreads out

under the lithosphere and drags the crust along

Eventually, the slowly cooling material sinks

back into the mantle

An alternate hypothesis is that the convection

process may involve two convection layers

Or perhaps the convection process behaves

in a chaotic manner

Moving Continents

It is interesting to

speculate on how

the Earth looked as

the continents were

moved around...

50 Million Years A.D.

Or what the Earth will look like in 50 million years

250 Million Years A.D.

Or in 250 million years...

Chapter 3

Matter and Minerals

101 Chap2 Intro PlateTectonics

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