17-2 Seafloor Spreading Objectives Summarize the evidence that led to the discovery of seafloor spreading Explain the significance of magnetic patterns.

17-2 Seafloor Spreading

Objectives Summarize the evidence that led to the discovery of seafloor

spreading Explain the significance of magnetic patterns on the seafloor Explain the process of seafloor spreading

Main Idea: Oceanic crust forms at ocean ridges and becomes part of the seafloor

Mapping the Ocean Floor

Until mid-1900s, many thought ocean floors were flat and ocean crust unchanging & older than continental crust Advances in technology during 40s & 50s showed these widely

accepted ideas to be falseMagnetometer: a device that can detect small changes in magnetic fields Originally developed to detect magnetic fields generated by the

steel hulls of submarines Modifications were made to measure the magnetic field

strength of ocean-floor rocks

Other technology

Developments in sonar technology Enabled scientists to measure water depth and map the

topography of ocean floor

Ocean-Floor Topography

• Using maps from sonar & magnetometer data Scientists discovered vast mountain chains called ocean ridges

that run along ocean floors around Earth much like seams on a baseball

• more than 80,000 km long and up to 3 km above ocean floor!• Volcanoes quite common along these ridges Sonar also revealed deep-sea trenches• Narrow, elongated depression in seafloor• Deepest trench, Marianas Trench, in Pacific Ocean that is more than 11 km

deep…..Mount Everest stands 9 km! How could they have formed? What’s the source of volcanism associated w/ these mountains? What forces could depress Earth’s crust to such depths?

Ocean Rocks & Sediments

Ages of rock that make up seafloor vary, but these variations are predictable! Age of oceanic crust consistently increases with distance from

ridgeWhat was weird was that scientists also discovered even the oldest parts of seafloor are geologically young—around 180 million years• Why are ocean-floor rocks so young compared to continental

rocks? Some of which are at least 3.8 billion years old

Ocean Rocks & SedimentsOcean floor sediments are typically a few hundred meters thick This seems odd when you consider that large areas of

continents are blanketed with sedimentary rocks that are as much as 20 km thick….

Scientists found that thickness of sediments increases with distance from the ocean ridge

MagnetismEarth’s magnetic field Generated by the flow of molten iron in outer core Causes compass needle to point North

Magnetic Reversal Happens when flow in outer core changes Have occurred many times in Earth’s history

Magnetic Polarity Time Scale

Paleomagnetism: study of the history of Earth’s magnetic fieldHow can we study Earth’s magnetic field past? When lava solidifies, iron-bearing minerals such as magnetite

crystallize. As they crystallize, they behave like tiny compasses and align with Earth’s magnetic field

So if we sift through such minerals, we are able to tell the polarity of the Earth at the time of crystallization

This phenomenon allowed scientists to construct a magnetic polarity time scale

Magnetic Symmetry

• Regions of normal & reverse polarity form a series of strips across the ocean floor parallel to ocean ridges

Originally hypothesized because we knew oceanic crust is mostly basaltic rock, containing large amounts of iron-bearing minerals. Therefore, rocks on the ocean floor should show a record of magnetic reversals

• Ages & widths of the strips match from one side of the ridges to the other

Magnetic Symmetry• By matching magnetic patterns on seafloor with the known

pattern of magnetic reversals on land, we could determine the age of ocean floor from magnetic recording and create isochron maps of the ocean floor

Isochron: imaginary line on a map that shows points that have the same age—that is, they formed at the same time

Seafloor Spreading

Theory that explains how new ocean crust is formed at ocean ridges and destroyed at deep-sea trencheshttp://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_seafloorspreading.html Magma is forced upward toward surface of the crust along ocean ridge. As the

2 sides of the ridge spread apart, the rising magma fills the gap that is created. When the magma cools and solidifies, a small amount of new ocean floor is added to Earth’s surface

As spreading along ocean ridge continues, more magma is forced upward and solidifies

Cycle of spreading and intrusion of magma continues the formation of ocean floor, which slowly moves away from the ridge

Seafloor Spreading

Links to Wegener

• Seafloor spreading was the missing link Continents are not passing through ocean crust. In

fact, continents are more like passengers that ride along while ocean crust slowly moves away from ocean ridges!

https://www.youtube.com/watch?v=6CsTTmvX6mc