- 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…