Plate Tectonics The Dynamic Earth

The Dynamic Earth The Earth both inside and out is dynamic (always changing).

Studying the Earth’s layers Scientists, called geologist, study the Earth’s rock layers to determine the age of the rocks and to understand the layers of the planet beneath our feet.

Layers The order of the rock layers determines their age. The oldest rock layers are on the bottom. The newer layers are on the top of the older layers as long as the layers have not been disturbed. When forces disturb the layers, they are no longer neatly stacked so it is harder to date the layers.

Events that disturb the rock layers Faulting: a break in the Earth’s crust along which blocks of crust slide against each other.

Events that disturb the rock layers Intrusion: molten rock from the Earth’s interior that squeezes into existing rock and cools.

Events that disturb the rock layers Folding: happens when rock layers bend and buckle from Earth’s internal forces.

Events that disturb the rock layers Tilting: happens when rock layers slant without folding.

Label the layers (oldest layer=#1) C. 3 D. 2 E. 1 F. 4 A B F D E C

Geologic Change Most geologic change, such as the formation of mountains, canyons and oceans occurs gradually. For example: Mt. McKinley in Denali National Park in Alaska “grows” 1 inch per year.

The Grand Canyon The Grand Canyon is one of the best places to see Earth’s history recorded in rock layers. The walls are made from sedimentary rock. The oldest layers are 2 billion years old, which is almost ½ as old as the Earth. The Earth is about 4.5 billion years old!!!

Sudden changes Not all geologic changes occur gradually, however. There have been some catastrophic, or “sudden” changes in history of the Earth that have caused rapid extinction of some species. For example: asteroids and comets have struck the Earth creating large craters. An Asteroid impact may have led to the extinction of dinosaurs.

Continental Drift The continental drift hypothesis—continents have moved slowly to their current locations.

Continental Drift All continents were once connected as one large landmass now called Pangea.

Pangea

The land mass broke apart, and the continents drifted to their present positions.

Evidence of Continental Drift a. Puzzle-like fit of the continents b. Similar fossils have been found on different continents.

Evidence of Continental Drift c. Remains of warm-weather plants in Artic areas and glacial deposits in tropical areas suggest that continents have moved.

Evidence of Continental Drift Similar rock structures are found on different continents. Africa South America

At first, continental drift was not accepted because no one could explain how or why continents had moved.

Evidence of Continental Drift

Seafloor Spreading Using sound waves, scientist discovered a system of underwater mountain ranges called the mid-ocean ridges in many oceans.

Theory of seafloor spreading In the 1960s, Harry Hess suggested the theory of seafloor spreading to explain the ridges.

Seafloor Spreading 1. Hot, less dense material below Earth’s crust rises upward to the surface at the mid-ocean ridges.

Seafloor Spreading 2. Then, it flows sideways, carrying the seafloor away from the ridge.

Seafloor Spreading 3. As the seafloor spreads apart, magma moves up and flows from the cracks, cools, and forms new seafloor.

Evidence of seafloor spreading Youngest rocks are located at mid-ocean ridges. Reversals of Earth’s magnetic field are recorded by rocks in strips parallel to ridges.

Seafloor spreading

C A D B C. Molten rock flows onto the seafloor and hardens as it cools A. Hot, molten rock is forced upward toward the seafloor at a mid-ocean ridge. D. New seafloor moves away from the ridge, cools, becomes denser, and sinks. B. Molten rock pushes sideways in both directions as it rises, moving the mantle with it.

Theory of Plate Tectonics

Plate Movements 1. Earth’s crust and upper mantle are broken into sections. 2. The sections, called plates, move on a plastic-like layer of the mantle.

Plate Movements Lithosphere 3. The plates and upper mantle form the lithosphere. 4. The plastic-like layer below the lithosphere is called the asthenosphere. Lithosphere Asthenosphere

Plate Boundaries

Plate boundaries 1. Plates moving apart—divergent boundaries.

Plate boundaries 2. Plates moving together – convergent boundaries. a. Denser plates sink under less dense plates (subduction zones). b. Newly formed hot magma forced upward forms volcanic mountains.

Plates Collide Plates crumple up to form mountain ranges. Earthquakes are common.

Transform Boundaries When plates slide past one another, they are called transform boundaries. Sudden movements can cause earthquakes. This is an example of what happened in Haiti.

Convection currents in the Earth Convection inside the Earth is the cycle of heating, rising, cooling, and sinking of material inside the Earth. This is thought to be the force behind plate tectonics.

Features caused by plate tectonics 1. Faults and rift valleys

Features caused by plate tectonics 2. Mountains and volcanoes

Features caused by plate tectonics 3. Strike-slip faults are the cause of earthquakes.

Testing for plate tectonics Scientists can measure movements as little as 1 cm per year.