7th Grade- Chapter 5 Notes

Chapter 5, Lesson 1 Notes

*In 1910, a German scientist named Alfred Wegener became curious about why some continents look as though they could fit together.

*Wegener’s hypothesis was that all the continents were once joined together in a single landmass and have since drifted apart.

*Wegener’s idea that the continents slowly moved over Earth’s surface became known as continental drift.

*According to Wegener, the continents were joined together in a super continent, or single landmass, about 300 million years ago.

*Wegener called the supercontinent Pangaea.

*Over tens of millions of years, Pangaea began to break apart.  The pieces of Pangaea slowly moved toward their present-day locations.  These pieces became the continents as formed today.

*Land features on the continents such as mountain ranges and coal fields provided Wegener with evidence for his hypothesis.

*Wegener could not provide a satisfactory explanation for the force that pushes or pulls the continents.  Because he could not identify the cause of continental drift, most geologists of his time rejected his idea.

Chapter 5, Lesson 2 Notes

*In certain places, the floor of the ocean appears to be stitched together like the seams of a baseball.

*Scientists found that the seams formed mountain ranges that ran along the middle of some ocean floors.

*Scientists called these mountain ranges mid-ocean ridges form long chains of mountains that rise up from the ocean floor.

*Mid-ocean ridges continually add new material to the ocean floor in a process called sea-floor spreading.

*Sea-floor spreading adds more crust to the ocean floor.  At the same time, older strips of rock move outward from either side of the ridge.

*The ocean floor eventually plunges into deep underwater canyons called deep-ocean trenches.

*At a deep-ocean trench, the oceanic crust bends downward.

*In a process taking tens of millions of years, part of the ocean floor sinks back into the mantle at deep-ocean trenches.

*The process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle again is called subduction.

*As subduction occurs, crust closer to a mid-ocean ridge moves away from the ridge and toward a deep-ocean trench.

*The processes of subduction and sea-floor spreading can change the size and shape of the oceans.

*An ocean surrounded by many trenches may shrink.

*An ocean that contains more ridges than trenches will probably grow larger.

Chapter 5, Lesson 3 Notes

*Earth’s lithosphere, its solid outer shell, is like an eggshell broken into pieces separated by cracks.  These pieces are called plates.

*Earth’s plates meet at boundaries.  Plates move apart, or diverge, from each other at a divergent boundary.  Plates come together, or converge, at a convergent boundary.

*Plates slip past each other along a transform boundary.

*In the mid-1960s, geologists combined what they knew about sea-floor spreading, Earth’s plates, and plate motions into a single theory called plate tectonics.

*The theory of plate tectonics states that Earth’s plates are in slow, constant motion, driven by convection currents in the mantle.

*Earth’s plates move because they are the top part of the large convection currents in Earth’s mantle.

*During subduction, gravity pulls denser plate edges downward, into the mantle.  Plates move very slowly- from about 1 to 12 centimeters per year.

*Earth’s plates can carry ocean floor, continents, or continents and oceans together.  So the movement of Earth’s plates has greatly changed the location of Earth’s continents, landmasses, and oceans.

*Faults-breaks in Earth’s crust where rocks have slipped past each other- form along plate boundaries.

*Plate movements produce changes in Earth’s surface and on the ocean floor.  These changes include the formation of volcanoes, mountain ranges, and deep-ocean trenches.

*Most divergent boundaries occur along mid-ocean ridges.  Where pieces of Earth’s crust diverge on land, many deep valleys called rift valleys have formed.

*Where two plates carrying oceanic crust meet at a trench, the plate that is denser sinks under the less dense plate.

*When two plates carrying continental crust collide, the collision squeezes the crust into high mountain ranges.

*Earthquakes often occur when two plates suddenly slip along a transform boundary.