Chapter 4: Plate Tectonics

The crust is the outermost layer of the Earth. 2 sections: 1. continental crust- makes up the continents, up to 70 km thick. 2. oceanic crust- under the ocean floor, up to 8 km thick

Just beneath the crust is a layer called the mantle Just beneath the crust is a layer called the mantle. The mantle is more dense than the crust. The rocks that make up the mantle are partially melted and flow like a very thick liquid. Because the crust is less dense, it floats on top of the mantle!

The prefix “iso” means equal The suffix “stasy” means standing still The balance between the downward force of the crust and the upward force of the mantle is called isostasy. The prefix “iso” means equal The suffix “stasy” means standing still

A fossil is the preserved remains of an ancient organism A fossil is the preserved remains of an ancient organism. Fossils give geologists an idea of the type and abundance of living organisms for a period of time

Based on fossil patterns uncovered worldwide, there is evidence that suggests that the continents were once a large landmass that broke up into large pieces, which have since drifted apart

The German Scientist Alfred Wegener came up with his Theory of Continental Drift. According to this idea, the continents were once joined together and have since drifted apart.

This single, giant landmass that was made of all the continents put together was called Pangaea.   The word Pangaea means “all Earth”

Evidence for Continental Drift Fossils: remains of the extinct plant Glossopteris have been found in Africa, Australia, India, and even Antarctica! This evidence suggests that these continents were once closer together and had similar climates

2. Rocks: rock formations and glacial deposits are very similar on the W side of Africa, and match up almost exactly with those on the E side of South America. 3. Shape of Continents: If you look closely at a map of the Earth, the continents look like pieces of a puzzle that would fit easily together.

So if we have all of this evidence showing that the continents were once joined together, how have they moved apart?   It wasn’t until the 1960’s that the technology arose that allowed us to scientifically examine the ocean floor.

This new technology showed that there is a large system of underwater mountains, called mid-ocean ridges, that run throughout the ocean floor. The mid-Atlantic ridge is the largest mountain chain in the world!

Ocean Floor Spreading: as either side of the ridge moves apart, lava escapes from the mantle and solidifies to form new rocks. This explains how the continents are able to drift!                                                                

Magnetic Evidence Some minerals have magnetic properties and align themselves with the Earth’s magnetic poles when they form. As these rocks move, their alignment with the poles moves. This provides a record of plate movement patterns and rates.

The process where the crust plunges back into the mantle Trenches: a deep, V-shaped valley on the ocean floor where old ocean floor sinks into the mantle.                                                                 The process where the crust plunges back into the mantle is called subduction. As the rocks are pushed deeper, they are melted by the interior heat of the Earth.

As new rocks are formed at mid-ocean ridges, older rocks are subducted into trenches. This keeps the Earth’s crust the same size!

Theory of Plate Tectonics: the Earth’s surface consists of several plates that are in constant motion. This idea links the theory of continental drift and ocean-floor spreading.   This theory is the modern version of Wegener’s ideas, and explains the formation, movement, collision, and destruction of the crust.

The outermost solid part of the Earth is called the lithosphere The outermost solid part of the Earth is called the lithosphere. This is made of a number of plates of crust floating on top of the mantle rock. Plates often contain both continental and oceanic crust. There are 7 major lithospheric plates.

The junction between 2 plates is called a plate boundary. 3 types: divergent boundary: plates move apart, creating new crust. These generally occur at mid-ocean ridges.

2. convergent boundary: plates are coming together, causing one plate to sink (subduct) into the mantle. Convergent boundaries will often have trenches.

3. Transform Boundary: Two plates grind horizontally past each other 3. Transform Boundary: Two plates grind horizontally past each other. No crust is created nor destroyed. Earthquakes are typical at these boundaries.

A convection current is the movement of materials caused by differences in temperature. As heat from the core rises to the crust, it pushes the plates outward, causing tectonic movement. Thus, it is believed that the movement of the tectonic plates is caused by convection currents.

As the rocks of the crust undergo stress, they slowly change shape and volume. This is the idea behind deformation! Deformation: the breaking, tilting, and folding of rocks due to stress.

3 Types of Stress 1. compression: rocks are squeezed together 2. tension: rocks are pulled apart 3. shearing: rocks are pushed in opposite directions, causing them to bend or break apart

1. an upward fold is called an anticline Any bend in a bed of rock is called a fold. There are 2 types of folds: 1. an upward fold is called an anticline 2. a downward fold is called a syncline

Sometimes stress causes rocks to break Sometimes stress causes rocks to break. A break or crack along which rocks move is called a fault. Rocks on one side of the fault slide past rocks on the other side.

4 Types of Faults: 1. Normal Fault: caused by tension, one side of the fault moves up, the other moves down.

2. Reverse Fault: caused by compression, one side of the fault moves up on the other side. 3. Thrust Fault: caused by compression, one side of the fault slides up over the other side, older rocks are pushed up on top of younger rocks.

4. Lateral Fault: caused by shearing, both sides of the fault slide horizontally past each other.

A rift valley forms when a section of rock between 2 normal faults slides down. Example: Death Valley in California (280 feet below sea level)

A large, flat section of land that is raised high above sea level is called a plateau. Example: Grand Canyon