Lecture 3-5 Plate Tectonics

7/30/2019 Lecture 3-5 Plate Tectonics

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Continental Drift & Plate Tectonics

Moving plates beneath our feet

The jigsaw puzzle-like fit of land masses. Coincidence?


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Theory of Continental Drift

The theory or hypothesis first postulated in

1855, restated in 1911, that the present

distribution of the continents is the result of

fragmentation of the pre-existing land masses

which have drifted apart, the intervening

hollows having become occupied by the

oceans.

Antonio Snider-Pellegrinis idea in 1811

Snider-Pellegrinis map

Snider-Pellegrini proposed that the continents broke apart catastrophically,

causing the Biblical Flood in the Book of Genesis. This idea was not widely

accepted because of the move towards scientific thinking and the need for

evidence.


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Alfred Wegener s idea in 1911

The idea of moving

continents resurfaced in

1911, restated by Alfred

Wegener (v'g-nr) (1880

1930), a german geophysicist

and geographer.

Wegener hypothesized that

all land masses were joinedtogether in one super

continent,Pangaea, around

300 million years ago.

His evidence:

1. Close geographic fit of

continents;

2. Distribution of fossils;

3. Distribution of rock

types and landforms

(geology)

4. Distribution of ancient

climatic (paleoclimate).


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Wegeners evidence fossils

If continents were separate, than how to explain:1. How species could cross/swim from one continent to another?

2. How species could live in very different climatic regions (glaciars, deserts,

forests)?

Continental drift offered the simplest explanation.

Wegener s evidence - geology

Wegener wondered: why were rock

types and structural landforms the

same although on different

continents? Why did mountain belts

always occur on the edge of

continents?


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Wegener s evidence - paleoclimate

Why do glacial landforms

occur on different

continents? Even on those

with hot climates today?

Hard to explain without

accepting continental drift.

Mechanisms for continental drift

1. Centrifugal forces caused by the rotation of the

earth (daya emparan)

2. Tidal forces generated by the gravitational pull of

the sun and moon (daya pasang surut)

However, Wegener realized these forces were

inadequate. This was the reason his theory was notwidely accepted. Dismissed as fairytale.


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Wegener perished in the

North Pole in 1930, while

trying to measure thethickness of the ice

sheet.

His ideas about

continental drift gained

wide acceptance only

much later after his

death.

Theory of Plate Tectonics

The theory that the lithosphere consists of several

plates which can move slowly on the astenosphere;

This theory is actually based on continental drift but

with new evidence, using newer technologies.

New evidence:

Ocean floor features (ridges and trenches)

Sea-floor spreading (perebakan dasar laut)


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New evidence for the Plate Tectonics theory

1. Ocean floor features (bathymetry) became

clearer with the use of sonar (SOund NavigationAndRanging)

An active sonar system

Oceanic ridge =permatang lautan

Oceanic trench = lurah lautan


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Oceanic features ridges and trenches

Evidence from the North Atlantic Ridge:

(1) Earthquakes occurred in a narrow band under the crest of the ridge;(2) The presence of deep valleys called rifts at the crest of the ridges.

(3) The rifts generated a large amount of heat (see next slide);

led to a seafloor spreading hypothesis.


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2. The seafloor spreading hypothesis

Age of marine rocks & magnetic polarities

The Mid-Atlantic Ridge moves at an average of 5 cm/year


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But what happens to old seafloor?

Benioff zone: a deep, active

seismic area in a subduction zone


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As new oceanic crust is created, the old ones are

destroyed at the other end of the plates. This iswhere the Benioff zone (or the Wadati-Benioff

zone) is located.

At the Benioff zone, oceanic crust slides down

under the continental crust and returns to the

mantle (subduction).

Because oceanic crust is cold and brittle, it

fractures as it descends back into the mantle. As

it fractures it produces earthquakes that get

progressively deeper (e.g. 600 700 km deep).

E.g. the Ring of Fire in the Pacific Ocean

Mechanism for plate tectonics =

convection currents? (arus perolakan)


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Major Plates

Laurasia:

North America

Eurasia (Europe &

North Asia)

Gondwanaland:

South America

South Africa

India

Australia

Antarctica

What about Malaysia?


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Landforms created by

PLATE TECTONICS

Types of plate boundaries


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A divergent boundary has a spreading center; also known

as a constructive margin because new sea floor is created.

Characterized by ocean ridges and sea floor spreading.

New sea floor

Ocean ridge

At the axis of the oceanic ridge


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(1)Mountain ranges

Oceanic trenches

High seismicity

Down-folding (subduction)

A convergent boundary results from colliding plates.

Also known as a destructive margin (remember subduction and

the Benioff Zone?).

There are 3 different types of convergence boundaries.

Up-folding (mountain-formation)

South America Plate vs Nazca Plate

The collision of the plates have created the Peru-Chile trench (8,000

m deep) and the volcanic Andes mountains (6,900 m high)


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(2)Undersea volcanoes ->

volcanic islands -> island arcs

(e.g. Solomon islands,

Japanese islands, Aleutian

islands, Indonesian islands)

Oceanic trenches

High seismicity

If two oceanic plates

collide, which will be

subducted? The older or

younger plate?

Also known as a collisionalboundary

(3)

Mountain ranges

Subduction does not occur. Why?


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At a transform boundary, plates move past one

another along strike-slip faults (sesar jurus gelincir)

Also known as a conservation

margin.

Earthquakes caused by

vibrations from plate

movements along the

boundary. E.g. California.


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Subduction zones are indicated by zones of great volcanic and

earthquake activity, as in the Ring of Fire in the Pacific Ocean

A special case ofhotspot formation

THE HAWAIIAN ISLANDS


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Hotspot

A hotspotis a stationary place where a magma

plume burns through the mantle to reach the

surface. As a plate moves over the hotspot, a series

of volcanic islands are formed.

Source of hotspot = radioactive decay?

Two points:

1. Hotspot is stationary2. Lithospheric plates move over the

hotspot

plume


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Formation of the Hawaiian Archipelago

41-Nihoa < f ! ! : tauelIsland

hu

Earth's crust

- PacificDrift-M odel illustrat ing th e ' Hot Spot' th eory

HotSpot

I t \

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Seamount Seamount

Seamount

Seamounts

(=guyots) have

very high

biodiversity

28 million years old 400,000 years old


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The Pacific Plate has moved over the Hawaiian hotspot for millions

of years, resulting in a trail of volcanic islands and underwatermountains (also known as the Hawaii-Emperor Seamount Chain).

Therefore, not all volcanoes occur at the boundaries of plates but

can also occur intraplate.

Documents

Lecture 3-5 Plate Tectonics