Earth s Structure
The Four Layers The Earth is composed of four different layers. The crust is the layer that you live on, and it is the most widely studied and understood. The mantle is much hotter and has the ability to flow.
The Four Layers The outer core and inner core are even hotter with pressures so great you would be squeezed into a ball smaller than a marble if you were able to go to the center of the Earth!
The Crust The Earth's crust is like the skin of an apple and is broken into many pieces called plates. It is very thin in comparison to the other three layers.
The Crust The crust is only about 3-5 miles (8 km) thick under the oceans (oceanic crust) and about 25 miles (32 km) thick under the continents (continental crust). Continental Crust Oceanic Crust
The Crust The plates "float" on the soft, semi-rigid asthenosphere.
Lithospheric Plates
The Mantle The crust and the upper layer of the mantle together make up a zone of rigid, brittle rock called the Lithosphere.
The Crust The crust is composed of two rocks. - The continental crust is mostly granite. - The oceanic crust is basalt. Basalt is much denser than the granite. Because of this the less dense continents ride on the denser oceanic plates.
The Mantle The mantle is the largest layer of the Earth. The middle mantle is composed of very hot dense rock that flows like asphalt under a heavy weight.
The Mantle The movement of the middle mantle (asthenosphere) is the reason that the crustal plates of the Earth move.
The Mantle The crust and the upper layer of the mantle together make up a zone of rigid, brittle rock called the Lithosphere.
The Mantle The middle mantle "flows" because of convection currents which are caused by the very hot material at the deepest part of the mantle rising, then cooling and sinking again --repeating this cycle over and over.
The Outer Core The core of the Earth is like a ball of very hot metals. The outer core is so hot that the metals in it are all in the liquid state..
The Outer Core The outer core is composed of the melted metals of nickel and iron.
The Inner Core The inner core of the Earth has temperatures and pressures so great that the metals are squeezed together.
The Inner Core So much so that they are not able to move about like a liquid, but are forced to vibrate in place like a solid.
Wegner In the early 1900s, a German scientists by the name of Alfred Wegner became interested in the idea of Continental Drift while reading about different fossil locations around the earth.
Wegner Wegner hypothesized that, about 200 million years ago, there must have been one super continent at one time that he named Pangea.
Pangea Scientists use the similarity of rock types and fossil types that date to the same age to support their theory that the continents were connected to form a super continent.
Pangea Continental shelfs also support the idea of Pangea.
Pangea The break up of Pangea
Pangea Our final piece of evidence is the magnetic record of the ocean floor. This shows the pattern of reversal and we find a near perfect mirror image on opposing sides of the ridge.
Magnetic Reversal
Sea Floor Spreading
Magnetizing of Sea Floor
Plate Tectonics Why do the plates move? No single idea explains everything but we can identify several forces that contribute to the movement of the plates.
Plate Tectonics Slab pull is the sinking of the cooled dense oceanic plates which pulls on the rest of the plate dragging it down.
Plate Tectonics Ridge rises is the material deposited on the top of the ridge which slides downs from the rise pushing the plate downward.
Plate Tectonics Last, convection currents results in the movement within the mantle which functions as the driving force behind the motion of the plates.
Tectonic Plates Today, we know there are a dozen large lithospheric plates (smaller plates not shown). Some plates have continents; some don t. All are in motion.
Indo Australian Plate African Plate
Plate Tectonics Tectonic plates are moving at an average of 1-10 centimeters per year ( rate of fingernail growth). Modified from USGS Graphics
Plate Tectonics We appear to be headed for another super continent as North America, South America, Asia and Australia converge in the ever shrinking Pacific Ocean.
Plate Tectonics Notice that earthquakes & volcanoes (red) correlate with plate boundaries, and the deepest quakes (blue) are in subduction zones.
Plate Boundaries (Faults) There are three basic types of plate boundaries: 1. Divergent 2. Convergent 3. Transform
Divergent Plates A divergent boundary forms when plates are pulled apart. These occur at spreading ocean ridges and in continental rifts. Examples: Atlantic mid-ocean ridge Basin and Range, USA African Rift Valley Northern Red Sea
Divergent Plates Mid-Atlantic Ridge Spreading Center New ocean floor is being created by rising magma
Divergent Plates Divergent Boundaries usually create a Rift Valley. A rift valley is a narrow valley formed as the plates separate.
Convergent Plate Boundaries When plates move toward each other and one plate directly collides with another plate.
Convergent Plate Boundaries There are 3 types of convergent boundaries. 1. Ocean /Ocean (Marianas) 2. Continent/Continent (Himalayas) 3. Ocean /Continent (Cascades)
Ocean / Ocean Convergence Oceanic crust collides with another oceanic crust and one plate subducts. This has been known to produce a chain of islands from the resulting molten rock rising to the surface to create volcanic islands.
Oceanic/Continental Convergence Oceanic crust collides with continental crust: since oceanic crust is denser so it is forced under the continental crust (subducted). Subduction Zone: region along a plate boundary where one plate moves under another
Oceanic/Continental Convergence Deep Ocean Trench: usually forms along a subduction zone as oceanic crust moves down and melts, becoming part of the mantle material. Some magma rises through the continental crust and produces volcanic mountains
Continental / Continental Convergence Continental crust collides with continental crust: neither plate is subducted. Instead, colliding edges are crumpled and uplifted This produces large mountain ranges.
Transform Plate Boundaries In a transform plate boundary, Lithosphere plates are neither produced nor destroyed, two plates just slide past one another. This is also called a strike-slip fault. Strike-slip fault between two spreading ridges allows the two plates to move apart.
Transform Plate Boundaries Strike-slip fault between two spreading ridges allows the two plates to move apart. The San Andreas Fault in California is an example of this type of fault.