Ocean Ridges and Trenches

​Lesson 5.4: Sea Floor Spreading

Alfred Wegener laid the outline for the theory of Continental drift, but could not explain how the continents moved. In 1952, Marie Tharp and Bruce Heezen began collecting data from ships about the ocean floor. With this information, they found that the ocean floor varied in height, helping​ to give support the Wegener's initial theory.

Multiple Choice

The transfer of energy through empty space is called

• Conduction

• Radiation

• Convection

• Subduction

Multiple Choice

Any trace of an ancient organism that has been preserved in rock is called a

• Fossil

• Landform

• Landmass

• Continent

Lesson Objectives

• Define mid-ocean Ridges

• Describe how Sea-floor spreading affects the Earth's crust

• Define deep-ocean trenches

• Explain the process of subduction.​

​What are Mid-Ocean Ridges?

• In different areas of the Ocean floor, the floor showed signs of "seams" like those on a baseball

  • Curve along the the ocean floor

  • Form Mountain ranges

• Mid-ocean Ridges: long chains of mountains that rise up from the ocean floor​

Grade 8 Ohio | Lesion 5.4

How do we Study Mid-Ocean Ridges?

• In the 1990s, scientists used Sonar to map the ridges

  • Sonar uses sound waves to measure the distance to an object

  • Showed them to be the longest mountain range on Earth

Grade 8 Ohio | Lesion 5.4

Where are Mid-Ocean Ridges?

• Maps showed the ridges extend into all oceans

  • Under thousands of meters of water

  • ​The mid-ocean ridges are split by a steep valley

Grade 8 Ohio | Lesion 5.4

​What is Sea-Floor Spreading

• By the 1960s, geologists had begun to learn more about mid-ocean ridges

• Mid-ocean ridges add new material to the ocean floor

• Sea Floor Spreading: the process by which new material is added to the ocean floor by mid-ocean ridges​

  • A mid-ocean ridge forms in the oceanic crust

  • Molten material rises up, cools, and hardens

  • Over time, the strips of rock move outward from the ridge

Grade 8 Ohio | Lesion 5.4

​Evidence for Sea-Floor Spreading: Ocean Material

• In the central ridges, scientists found rocks shaped like pillows

  • This only happens when molten material hardens quickly

Grade 8 Ohio | Lesion 5.4

​Evidence for Sea-Floor Spreading: Magnetic Strips

• As molten material cools and hardens, magnetic material in it aligns with the direction of Earth's poles​ at times of eruption

  • Poles can reverse direction, so the magnetic stripes can indicate when molten material erupted

• Magnetic stripes are mirror images on either side of the ridge

​Evidence for Sea-Floor Spreading: Drilling Samples

• Rock Samples drilled up from the Ocean crust allowed scientists to compare the ages of samples from different locations

  • The farther from the ridge a sample was taken from, the older the rock

Grade 8 Ohio | Lesion 5.4

Multiple Choice

In Sea-floor spreading, new crust is added at a

• Magnetic Stripe

• Hot Spot

• Mid-ocean ridge

• Lava location

Multiple Choice

Which of the following is not evidence of sea floor spreading?

• Drilling Samples

• Magnetic Stripes

• Animal life on the Ocean Floor

• Hardened Ocean Material

​What Happens at Deep-Ocean Trenches?

• The ocean floor does not continue to get wider without stopping

  • Eventually, the ocean floor plunges into underwater canyons

• ​Deep-ocean trenches: deep underwater canyons where the oceanic crust bends downwards

  • The sinking into the mantle of crust takes tens of millions of years​

Grade 8 Ohio | Lesion 5.4

​The Process of Subduction

• Subduction: The process by which the ocean floor sinks beneath a deep ocean trench back into the mantle

• As new material on the ocean floor is created and cooled, it's density changes

  • Initially has a low density when hot, and the density increases as it cools

• While new material cools, it can collide with continental crust

  • Older crust is then pushed down towards the mantle​

Grade 8 Ohio | Lesion 5.4

​Subduction and Earth's Ocean

• Subduction and sea-floor spreading work together to change the size and shape of the ocean

  • They work together like a conveyor belt, replacing the ocean floor every 200 million years

• Size of the crust is determined by how fast crust is being created and destroyed in subduction

Grade 8 Ohio | Lesion 5.4

Multiple Choice

In seafloor spreading, molten material rises from the mantle and erupts

• along the edges of all the continents.

• along mid-ocean ridges

• in deep-ocean trenches

• at the north and south poles

Multiple Choice

Old oceanic crust is more dense than new oceanic crust because it is

• Hotter

• Cooler

• Taller

• Shorter

Multiple Choice

___________ is a device that scientists use to map the ocean floor.

• Sonar

• Conduction

• Crust

• Fossil