Chapter 4, Lesson 2, Seafloor Spreading

Chapter 4, Lesson 2, Seafloor Spreading Slide 1 *What is the main idea? _______________ -Why did scientists investigate the seafloor? *New discoveries led to seafloor spreading as an explanation for continental drift. ___________________________________________________ -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.

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.

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.

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.

Chapter 4, Lesson 2, Seafloor Spreading Slide 5

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

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

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.

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.

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 and accept Wegener’s continental drift hypothesis.

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.

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.

Questions??? What type of igneous rock did scientists find on the seafloor? Which new technology allowed scientists to map the ocean floor? What are the mountains in the middle of the ocean called? What is it called when the magnetic field is pointed south? How were scientists able to determine the age of the seafloor?