- To Accompany Discovery Techbook Mrs. Baker/cjcb

Plate Tectonics and Earth’s Internal

Structure

Part 1:

Earth’s Interior

Compositional layers of the Earth

Inner core Outer core Mantle Crust

◦ Compositional layers have different chemistries, e.g. aluminosilicate (Al, Si, O); iron- nickel alloy (Fe, Ni)

Layers of the Earth

Structural layers of the Earth

Inner core Outer core Mantle

◦ Mesosphere◦ Asthenosphere

Lithosphere

◦ Structural layers have different mechanical properties, e.g. liquid, solid, brittle, etc.

Layers of the Earth

Made up of the earth’s crust and upper portion of the mantle (which behaves more like the crust than the mantle below)

A structural layer of the Earth◦ Rocky and brittle◦ Cracks under pressure

Crust is made of continental crust and oceanic crust◦ Continental crust is older◦ New oceanic crust is constantly made at mid-ocean

ridges and is recycled at subduction boundaries

Lithosphere

Continental and Oceanic Crust differ by:

◦ Composition◦ Density◦ Thickness◦ Average age

Lithosphere cont’d

66% of the earth’s mass is here. Has two structural layers—asthenosphere

and mesosphere Asthenosphere is soft and putty-like. It

flows. The crust floats on this portion of the mantle.

At the mesosphere, the pressure is too great for the rock to “flow”.

Mantle

Almost all of the remaining 33% of earth’s mass is the core….almost entirely iron and nickel.

Outer core is liquid….how do we know this? Inner core is solid. Why?

Watch: “Determining the Structure of the Earth” video on the Engage! page

The Core

Part 2:

Continental Drift Hypothesis

Was just an idea….now it’s a sensation!

sid the sloth continental drift video - Bing Videos

Continental Drift

The idea that Earth’s surface is moving is not new.

In the early 1500’s, explorers using maps noticed how well the west coast of Africa and the east coast of South America fit together.

Early Ideas

1912 – Alfred Wegener proposed continental drift.◦ Used additional information such as the location of

similar fossils and similar rock formations on opposite sides of the ocean to support his hypothesis.

◦ Thought that the continents pushed through the rocks of the ocean floor

Continental drift – a hypothesis that Earth’s continents move on Earth’s surface.

Early Ideas

Wegener proposed that the continents came together to form a single large land mass called Pangaea about 300 million years ago.

How well can you do this?◦ “Earth’s a Puzzle” exercise

Continental Drift

Wegener’s theory of Continental Drift was not well-received at the time

◦ Wegener’s mechanism for continental drift wasn’t accepted. People just couldn’t believe that huge continents moved!

◦ Scientists of the time would “explain away” Wegener’s ideas and evidence.

Continental Drift

Part 3:

Evidence for Plate Tectonics

In the 1950s and 1960s, discoveries about earthquakes, magnetism, and the age of rocks on the ocean floor helped support some of Wegener’s ideas.

Scientists in the ’60s also proposed that the sea-floor is spreading: new ocean material is formed at ridges and then moves away towards subduction zones◦ The sea-floor is a conveyer belt

Explains why earthquakes and volcanoes are likely to occur in particular locations and how new crust forms along the ocean floor.

Theory of Plate Tectonics

Plate Tectonic Theory

So what latest and greatest evidence was used to develop “plate tectonic theory?”

Earthquakes do not occur randomly.

They occur primarily in concentrated belts.

Locations of Earthquakes & Volcanoes

http://www.pbs.org/wgbh/nova/teachers/activities/2515_vesuvius.html

Compare these two maps and what do you conclude???

Plate Boundaries Earthquake Activity

Volcanoes are also found concentrated along plate margins:

A bit harder to see, but different TYPES OF VOLCANOES are found in different places, too

You find mainly BLUE triangles along ocean ridges but RED, GREEN, and BLUE at other plate boundaries.

So what do all those BLUE, RED and GREEN triangles represent?

The different colors are for different types of volcanoes. One important fact is that the composition of magma is NOT the same for all volcanoes:

BLUE triangles have less viscous magma – think oozing like the Hawaiian Islands or Iceland

RED and GREEN triangles have more viscouslava, and so are more violent – think Mount St. Helens in Washington State

The earthquake and volcano belts mark the locations of plate boundaries.◦ Where two plates are pushing toward, pulling away, or

sliding past each other.◦ Strain builds up along plate boundaries, allowing

fractures to form earthquakes.◦ Areas of high heat flow volcanoes

Different depths of earthquakes and different types of volcanos are seen at different types of plate boundaries

Locations of Earthquakes & Volcanoes

But not all volcanoes are at plate boundaries – what about Hawaii??

Turns out, this is even MORE evidence for plate tectonics!

The islands get older the farther you move from the Big island of Hawaii (which is erupting now!) The trail of islands is thought to show plate movement.

The ocean floor gets progressively older farther from the oceanic ridges –

* evidence of sea-floor spreading!

BUT the oldest ocean floor is less than 300 million years old, compared with the oldest land rocks of over 4 billion years

• The ocean crust is disappearing

The ocean floor has “magnetic stripes” –recording past events of pole reversal

Some igneous rocks contain magnetic materials.

These magnetic minerals provide a record of the direction of Earth’s magnetic field at the time when the molten rock cooled.

Magnetism & Age of the Ocean Floor

Geologists found that at certain times, rocks reversed polarity.

This led to the discovery that at certain times, Earth’s current magnetic north became magnetic south.

Many magnetic reversals have occurred.

Magnetism & Age of the Ocean Floor

Mid-ocean ridge – long chain of volcanic mountains on the ocean floor with a deep central valley.

Magnetic reversals are recorded in bands of rock on either side of the ridge.

The center of the ridge (newest rocks) always shows the current orientation of the north and south poles.

Magnetism & Age of the Ocean Floor

Ridges represent boundaries where lithospheric plates are moving apart.

Newer rocks along a mid-ocean ridge are formed by hot, molten rock rising between the spreading plates.

As new rocks form, the older rocks move further from the ridge.

Magnetism & Age of the Ocean Floor

Part 4:

Types of Plate Boundaries:

Tectonic Plate Interactions

Layers of the Earth – Mr. Lee Lyric Video: http://www.bing.com/videos/search?q=Layers+of+the+Earth+Song&Form=VQFRVP#view=detail&mid=9F66B8A706BFB99772729F66B8A706BFB9977272

Divergent Boundaries Boundary between two lithospheric plates

that are moving apart.

Sometimes called spreading centers.

Divergent Boundaries Rift valleys – deep valleys at the center of a

mid-ocean ridge.

Rift – crack in the valley

Sea-floor spreading – molten rock forces its way upward through rifts.

Convergent Boundaries Boundary between two plates that are

moving toward each other (converging).

Two main types:◦ Subduction Boundaries◦ Collision Boundaries

Convergent Boundaries Subduction Boundary – when an oceanic

plate plunges beneath another plate.◦ Deep-sea trench – long deep trench that forms at

a subduction boundary.◦ Accompanied by the formation of a chain of

volcanic islands called a volcanic island arc.◦ Deep earthquakes occur at subduction zones

Convergent Boundaries Collision Boundary – the boundary that

forms when two continents collide and are welded into a single, larger continent.◦ Causes the crust at the boundary to be pushed

upward into a mountain range.◦ Shallow to medium depth earthquakes are

common

Transform Boundaries A boundary between two plates that are

sliding past each other. Fracture zones that offset the segments of a

mid-ocean ridge. Movement along these boundaries is not

uniform. Location of abundant shallow earthquakes

(San Andreas Fault)

Seafloor spreading centers “Symmetry” of landforms along either sides

of ocean ridges Fossil records matching up in distance

places We can now measure plate movement using

GPS. Plates each move at different rates◦ Average rate is a few centimeters per year

Evidence of Plate Tectonics

Causes of Plate Movement Mantle convection Ridge push Slab pull

Mantle Convection The process by which heat from Earth’s

inner and outer cores is transferred through the mantle.

The mantle may be moving the plates along with it as it convects.

Ridge Push The process by which the cooling, subsiding

rock exerts a force on spreading lithospheric plates that could help drive their movement.

Could also be called gravitational sliding.

Slab Pull The process by which the sinking edge of

the plate exerts a force on the rest of the plate.

Plate Tectonics: The changing Earth

The tectonic plates have moved extensively in the past:

http://education.nationalgeographic.com/maps/earths-tectonic-plates/

And they are still slowly moving…