1. Chapter 4, Lesson 2, Seafloor Spreading

2. Chapter 4, Lesson 2, Seafloor Spreading Slide 1
*What is the main idea?
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-Why did scientists investigate the seafloor?
*New discoveries led to seafloor spreading as an explanation for continental drift.
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-Scientists wondered why rocks on the seafloor were so different from rocks on land.
-Through collecting seafloor samples, scientists knew that most rocks on the seafloor are made of basalt, an igneous rock that is made of highly dense minerals.
-By the 1950s, new technologies were being developed to explore the seafloor.

3. Chapter 4, Lesson 2, Seafloor Spreading Slide 2
*How did scientists map the seafloor?
*During World War II, a new method was developed for mapping the seafloor.
*This new method used technology called sonar.
*Sonar emits sound waves that bounce off the seafloor and back to a receiver that records the time it takes for the waves to return.
*Because scientists know the speed of sound waves in water, they can use the data to calculate the depth of water.
*With this new technology, the topography(mapping) of the seafloor was mapped.

4. Chapter 4, Lesson 2, Seafloor Spreading Slide 3
-What are mid- ocean ridges?
-Through use of sonar, scientists were able to find,hidden under ocean waters, the longest mountain ranges on Earth.
-In the middle of the seafloor, these mountain ranges are called mid-ocean ridges.
-Scientists discovered that there is more heat escaping from Earth at the mid-ocean ridges than at other locations in the oceans.
-The closer you move toward a mid-ocean ridge, the more heat flows from the mantle.

5. Chapter 4, Lesson 2, Seafloor Spreading Slide 4
*What is seafloor spreading?
*Harry Hess, an American geologist, studied the seafloor and proposed it was hot beneath the mid-ocean ridges because lava erupted there and made new seafloor.
*Seafloor spreading is the process by which new seafloor is continuously made at the mid-ocean ridges.
*Convection brings hot material to the mantle toward the surface, causing magma to form
*The magma flows out as lava through cracks, along the ridge, and cools to form new seafloor.

6. Chapter 4, Lesson 2, Seafloor Spreading Slide 5

7. Chapter 4, Lesson 2, Seafloor Spreading Slide 6
-What are magnetic polarity reversals?
-Whenever you use a compass, the north-seeking end of the needle points to Earth’s magnetic north pole.
-However, sometimes the magnetic poles reverse.
-When the magnetic field points north, it is called normal.
-When the magnetic field points south, it is called reversed.
-Scientists learned the ages of each of these reversals.
-Igneous rocks can record these reversals

8. Chapter 4, Lesson 2, Seafloor Spreading Slide 7
Normal magnetic field
Reversed magnetic field

9. Chapter 4, Lesson 2, Seafloor Spreading Slide 8
-What are magnetic reversals?
(continued)
-Tiny crystals, along a mid-ocean ridge, record the magnetic field orientation that existed when oceanic crust cools.

10. Chapter 4, Lesson 2, Seafloor Spreading Slide 9
*What are magnetic stripes on the seafloor?
*Scientist can measure Earth’s magnetic field with instruments called magnetometers.
*Using ships, planes, and satellites, magnetometers can move over the ocean and measure the strength of the magnetic field.
*They make a striped pattern when graphed because they contain alternating strips of rock with normal and reversed polarity.
*The seafloor is youngest at the mid-ocean ridge and the distance of a stripe and its age can be determined by calculating the speed of the seafloor movement.

11. Chapter 4, Lesson 2, Seafloor Spreading Slide 10
*What are magnetic stripes on the seafloor? (continued)
*By using magnetometers, scientists were able to find that the seafloor and continents move only centimeters per year.
*Scientists finally could understand accept Wegener’s continental drift hypothesis.

12. Chapter 4, Lesson 2, Seafloor Spreading Slide 11
-What is seafloor drilling?
-Scientists were able to determine the age of the seafloor using deep-sea drilling.
-They designed a boat, the Glomar Challenger, that could drill and collect samples from the seafloor.
-Scientists used drill pipes several kilometers long to cut through rock and bring up samples.
-The ages of the samples showed that the oldest rocks were farthest from the mid-ocean ridge, supporting the seafloor spreading hypothesis.

13. Chapter 4, Lesson 2, Seafloor Spreading Slide 12
*Summary
*By the 1950s, new methods and technologies, such as sonar, were being developed to map and explore the seafloor.
*When scientists mapped the topography of the seafloor they discovered underwater mountain ranges known as mid-ocean ridges.
*Harry Hess proposed the seafloor spreading hypothesis which is the process that new seafloor is continuously made at mid- ocean ridges.
*Seafloor spreading helped explain continental drift.