Sea-Floor Spreading

Learning Target I will explain the evidence for and process of sea-floor spreading.

Think-Pair-Share Where does magma from the Earth’s interior flow out onto the surface? What causes the magma to harden.

Deep Ocean Floor Near freezing No light Living things are scarce However, some areas are teaming with life – Off the coast of Mexico & South America Water sinks through cracks into the crust, gets heated, by the mantle, & spurted back into the ocean.

Deep Ocean Vents Bizarre creatures: Giant tube worms Giant clams Spider-like crabs White fish

Sonar Sonar – a device that bounces sound waves off underwater objects and then records the echoes of the sound waves.

Mid-Ocean Ridges Ocean floor has huge mountain chains Mid-ocean ridge – undersea mountain ranges Most ridges are far below water, but some rise above the water

Iceland

Ocean Floor

Sea-Floor Spreading Sea-floor spreading – the sea floor spreads apart along both sides of a mid-ocean ridge as new crust is added. The ocean floor moves like conveyor belts, carrying the continents with them.

Sea-Floor Spreading Starts at the mid-ocean ridge with a crack for magma to erupt through, older rock moves outward on both sides, & as the new material cools it forms a strip of solid rock in the center of the ridge.

Think-Jot-Share How does sea-floor spreading support the idea that continents drift? Why didn’t scientists know this in Wegener’s time? Is sea-floor spreading happening today? If so, why don’t we feel it?

Evidence for Sea-floor Spreading 1.Molten material 2.Magnetic stripes in the rock 3.Drilling samples

Evidence for Sea-floor Spreading Scientists found new material along mid-ocean ridges – Alvin: 4 km down in the ocean Strange rocks shaped like pillows or toothpaste squeezed from a tube Forms when molten material cools quickly after erupting under water Happened repeatedly

Evidence for Sea-floor Spreading Earth’s magnetic poles have reversed themselves many times throughout history – Last time 780,000 years ago Rock in the ocean floor lies in a pattern of magnetized stripes Stripes are a record of reversals in Earth’s magnetic field

Magnetic Stripes cont. Rocks contain iron As the molten rocked cooled, the iron lined up with the direction of Earth’s magnetic poles This locked the iron bits in place “magnetic memory”

Evidence for Sea-Floor Spreading 1968 Glomar Challenger – drilling ship – Drilled pipes 6 km into the ocean floor – Brought up sea-floor samples The farther away from the ridge the samples were taken, the older the rocks were Youngest rocks were in the center of the ridges

Think-Pair-Share What are the three types of evidence to support the theory of sea-floor spreading? Why does oceanic crust get older as it moves away from the ridges? What test could be done today to test the theory, that couldn’t be done in the 1960s?

Expanding Ocean? How can the ocean floor keep getting wider? The ocean floor plunges into deep ocean canyons called deep-ocean trenches. – Takes tens of millions of years – Most of the ocean floor is about 5 km deep – Trenches are more than 8 km deep

Deepest Trench The Mariana trench is the deepest trench in the world plunging to 6.8 miles deep.

Subduction Zones Subduction – the process by which ocean floor sinks beneath a deep-ocean trench and back into the mantle

Subduction Process As rocks moves away from the ridge, it cools & becomes more dense Gravity pulls the older, denser crust beneath the trench

Subduction & Earth’s Oceans Sea-floor spreading & subduction change the size and shape of the oceans The sea floor is renewed every 200 million years

Pacific Ocean Pacific Ocean covers almost 1/3 of the planet – Shrinking – The trenches swallow up more oceanic crust, then the ridges can produce – The ring around the Pacific has many trenches

The Atlantic Atlantic ocean is growing Only a few small trenches, but a large mid-ocean ridge The oceanic crust has nowhere to go, so the ocean is expanding It’s attached to continental crust, so it pushes the continents as it grows

Assignment 1.Along what feature of the ocean floor does sea-floor spreading begin? 2.What are the steps in the process of sea-floor spreading? 3.What three types of evidence supported the theory of sea-floor spreading? 4.How do rocks along the central valley of the mid-ocean ridge provide evidence of sea-floor spreading? 5.Where would you expect to find the oldest rocks on the ocean floor?