EARTHS CHARACTERISTICS

New Zealand Earthquake

• M 6.3 earthquake
• Shallow epicenter: 2-3

miles

• Close to Christchurch,

city of 370,000

• After shock to the

September 4th M 7.1

What is the composition of

lava formed on oceanic crust?

High percentages Fe, Mg

Low percentages Si, O

DESCRIBE A CONVERGENT
PLATE BOUNDARY

What is the source of the lava

that forms oceanic crust?

the mantle or asthenosphere

What is the composition of

lava associated with

continental crust?

High percentages Si, O
Low percentages Fe, Mg

The Theory of Plate Tectonics

• Based on two hypotheses

– Continental Drift
– Seafloor spreading

Explain the development of the
theory of plate tectonics in terms

of the scientific method.

The

Scientific
Method

One or more ideas, statements or questions;

Single or multiple hypotheses

Collection of

data

Observation of data or

information

Theory

Not accepted

Hypothesi

s

More Data

Continental drift hypothesis:

evidence?

• Proposed in 1911
• All evidence found on the continents
• Mechanism for moving the continents was

not known

• Alfred Wegener

proposed the
hypothesis of
continental drift in
1911

• He gathered

information from many
different sources and
used it as evidence for
his hypothesis

Continental Drift Hypothesis

• Fit of the continents
• Fossil evidence
• Ancient mountain ranges
• Past climate evidence

Continental Drift Hypothesis

Glaciers formed at the south pole

Swamps formed in tropical regions

• The

continental
drift
hypothesis
proposes that
the continents
were
assembled to
form the super
continent
Pangaea.

• Moved through

time

3:20

• Antonio Snider-Pelligrini

(1858), a geographer cut
out a map of Africa and
South America
suggesting they were
connected at one time

• Other physical evidence

based on observation
was used by Wegener

Fit of Continents

Fossil Evidence

• Similar terrestrial species were found on many

continents now separated by oceans.

• Information collected by paleontologists

Ancient Mountain Ranges

The same sequence of rocks is found in North America,
Great Brittan, and Norway. The pattern does not make
sense with the continents in their current configuration.

The Appalachian Mountains

Swamps

300 million years ago

Evidence of

Ancient Glaciers

•Glaciers carve the rock as they move.

•Scientists can determine the direction of movement (notice the
direction of movement noted in South America)

•As South America sits today, the pattern would not make sense.
(glaciers do not move from sea level to higher elevations)

Lack of mechanism to explain

continental movement.

• Evidence implied that the continents were at one

time assembled

• Wegener’s idea was not accepted by the

scientific community because he could not
explain how the continents moved

• With advances in technology, new information

was gathered in association with WWII

• What information do you know regarding

the seafloor that may be used to help
understand the hypothesis of seafloor
spreading?

Exploration of the Ocean Basins

Exploration of the Ocean Basins

• Seafloor features
• Composition of the seafloor
• Age of the seafloor
• Hot spots
• Seismicity
• Paleomagnetic pattern produced on the

seafloor

Exploration of Ocean Basins

Ocean basins contain a variety of landforms not discovered until WW II.

Support

• Echo sounding was used to map the seafloor

Seafloor features were
discovered using sonar.

4:26

Seafloor
features

• Mid-oceanic

ridges

• Trenches
• Volcanism

adjacent to
trenches

• Seamounts
• Fracture zones

Oceanic crust is composed of

volcanic rock.

• Basalt is volcanic rock with a mafic composition.
• Higher percentages of iron and magnesium, lower

percentages of silicon and oxygen.

Pillow basalts

Age of Ocean Crust

• Youngest at ridge
• Progressively older away from ridge
• Mirror image across the ridge

Seismicity

• Earthquakes are concentrated at what is

currently known as plate boundaries.

• Explain why there are broad bands of

seismicity associated with some areas.

Earth’s Magnetic Field

Bipolar:
acts like
a bar
magnet

Requirements for a magnetic

field

• An interior region of electrically conducting

fluid such as molten metal.

• Convection in that layer of fluid.
• At least moderately rapid rotation.

A compass has a different

declination depending on the
location on the Earth’s surface

Earth’s Magnetic Field

• Strength varies with location

– Red- stronger
– Blue weaker

Iron-rich rocks are magnetized as
they cool to the current magnetic

field.

• Basalt= iron rich volcanic rock; oceanic

crust

Paleomagnetic pattern across the

mid-oceanic ridge

Mirror image across the ridge and correlative with age

Positive
anomalies=
normal
polarities

Negative
anomalies =
reverse
polarities

Polar

Reversals

• Correlated to time
• Scientists dated

iron-rich volcanic
rocks on land

• Developed technology

to measure the
paleomagnetic
reading

Polar Reversals

• Timescale of polar

reversals

• Possibly related to

flow in the outer
core

Paleomagnetic Evidence

• Positive and negative recordings of the

oceanic crust create a symmetrical pattern
across the mid-oceanic trench

• In the 1963, Harry

Hess developed the
idea of seafloor
spreading to explain
the seafloor’s
formation

• Continental

movement is the
result of ocean floor
movement

Seafloor Spreading

Seafloor Spreading

• Mafic lava is extruded along the mid-oceanic ridge to

produce ocean floor.

• New material is added, the cooler material is split and is

“pushed” away from the ridge

Driving Mechanisms

Convection cells within
the mantle

Pushing at the
mid-oceanic ridges

Pulling at the subduction
zones

Push

Pull

Convection

The theory of plate tectonics

• Explains that the Earth’s lithosphere is

broken into distinct units that move as a
coherent package. Where the tectonic
plates meet, produces distinct physical
characteristics on the Earth’s surface.

The Scientific Method

Continental Drift

Hypothesis
Collection of

data

Observation of data or

information

Theory of Plate

Tectonics

Mechanism for plate

movement

Not accepted

Seafloor Spreading Hypothesis

Associated
with the
continents

Ocean
basins

key

Information to understand:

• The evidence used by Wegener to describe

the continental drift hypothesis.

• The evidence associated with technology that

was used to collect information about ocean
basins.

• The hypothesis of seafloor spreading
• The accepted tectonic plate driving

mechanism.

Earthquakes and volcanoes outline

the tectonic plate boundaries