Transcript The Earth
The Earth The Theory of Plate Tectonics • The theory that Earth’s lithosphere is divided into tectonic plates that drift on the athenosphere. • Divided into seven to eight main plates and several smaller ones. • The plates move by convection currents that either push the plates apart, together or make them slide against one another. Divergent Plate Boundaries • Oceanic: Where two oceanic plates move apart – Magma from the mantle flows up into the space between the plates and the magma hardens to form new oceanic crust. – This process is called sea-floor spreading. – Causes the oceans to widen!! – Long, continuous chains of underwater volcanoes form along the boundaries and are called midocean ridges. *What happens to the ocean during a divergent boundary? *What forms on each sides of the divergent boundary? *What new is being formed where the magma hardens? Note: The Mid-Atlantic Ridge is created by this and moves North America and Europe further apart. Divergent Plate Boundaries • Continental: Where two continental plates move apart – Since continental crust is MUCH thicker, it doesn’t split fully causing a big ridge. – The ridge can allow rivers to form in it and as it continues to split can allow ocean waters to flow in as well. – The Red Sea was formed when two continental plates diverged. *What would fill in the rift valley above? *Why didn’t the plates split all the way? *The faulted valleys will cause what phenomenon? Convergent Plate Boundaries • Oceanic = 2 plates made of oceanic crust collide – One plate is pushed under the other – Subduction zone – region where one plate slides under another plate – A deep canyon (trench) forms under the ocean where the plates meet – The plate sinks into the hot mantle and melts to form magma – Creates a chain of undersea volcanoes – As they keep erupting and forming new land, they will rise above the ocean floor and form volcanic islands – The Aleutian Islands formed this way. • What happens to the plate that slides underneath? • What is eventually formed after years of underground volcanic eruptions? Convergent Plate Boundaries • Continental-Oceanic Boundaries = 1 oceanic plate collides with 1 continental plate – The oceanic plate slides under the continental plate due to the increased density – This also forms a trench and volcanic mountains form next to the trench on the continental plate – The western coast of South America have many volcanoes formed this way. • Which plate slides under the other? • What is formed on the continental plate? Convergent Plate Boundaries • Continental = 2 continental plates collide – Creates the highest mountains in the world! – Folded mountains – the rock layers are squeezed together causing folds in the crust – The plates buckle and thicken. – Created: The Himalays, the Alps, The Ural Mountains and the Great Smoky Mountains • What are these types of continental mountain ranges called? • What makes these mountain ranges so special? Transform Boundaries • Two plates slide against each other • Cause the most earthquakes! • When they slide, pressure builds up and some rock breaks or slips and releases energy in the form of seismic waves. • Seismic wave – a wave of energy that travels away from the center of an earthquake in all directions • Fault – The break or crack along Earth’s surface where movement occurs. • Transform boundaries causes the most…? • What is a fault? Hot Spots • Hot Spot – an area of volcanic activity (usually in the middle of the plate) – Stays in the same place even though the plate moves – Results in a chain of volcanoes some of which are no longer active – Created the Hawaiian Islands – Due to a hot spot in the Mantle Ring of Fire • Ring of Fire – zone of frequent volcanic eruptions and earthquakes encircling the Pacific Ocean basin. – Contains more than 75% of all volcanoes on Earth – Located along the boundary of the Pacific Plate Mostly caused by subduction of plates resulting in melting magma. Question 1 1. True or False: Earthquakes can occur at any type of plate boundary. Question 1 Answer • True! Faults can occur anywhere there is a crack and pressure is built up releasing energy and causing seismic waves. Question 2 2. At which location are earthquakes mostq likely to occur? A. At a boundary where plates push together B. At a boundary where plates pull apart C. At a boundary where plates slide by each other D. In the middle of the plate Question 2 Answer • Transform plate boundaries (where two plates slide against one another) cause the most earthquakes because they cause the most faults. Question 3 3. A chain of mountains is located near the western coast of a continent next to a plate boundary. Which sentences describes how the mountains most likely formed? A. As a continental plate and an oceanic plate slide past each other, a fault formed, and mountains were exposed by earthquake movement. B. As a continental plate and an oceanic plate collided, part of the oceanic plate subducted, and volcanoes erupted, forming the mountains. C. As two oceanic plates collided, one of the oceanic plates subducted, and the other plate rose to the surface, forming mountains. D. As two oceanic plates moved apart, new seafloor was produced, forming a ridge. Question 3 Answer • B! When a continental and oceanic plate collides, the oceanic plate subducts below the continental and volcanic mountains are created on the continental plate. Question 4 4. What is the region where one plate slides under another plate called? A. An earthquake zone B. A hot spot C. A volcano D. A subduction zone Question 4 Answer D. When one plate slides under another, it is called a subduction zone. Question 5 5. Which landforms is most likely to form where two oceanic plates push together? A. Folded mountain B. Mid-ocean ridge C. River valley D. Volcanic island Question 5 Answer D. One oceanic plate subducts under the other and melts. This produces magma and many undersea volcanoes. When these break the surface of the water, volcanic islands are formed. Question 6 6. Where does sea-floor spreading occur? A. Where two oceanic plates collide B. Where an oceanic plate and a continental plate collide C. Where two oceanic plates pull apart D. Where two continental plates pull apart Question 6 Answer C. As two oceanic plates pull apart, magma is exposed creating new sea floor and the ocean expands. Question 7 7. A hot spot can produce: A. B. C. D. A chain of islands A mid-ocean ridge A trench A tectonic plate Question 7 Answer A. A hot spot will produce a chain of islands. As a plate moves over a hot spot, it will form volcanoes, keep moving and then form another.