17-2 Seafloor Spreading

1. 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

2. 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 false Magnetometer: 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

3. Other technology Developments in sonar technology • Enabled scientists to measure water depth and map the topography of ocean floor

4. 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?

6. 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 ridge What 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

7. Ocean Rocks & Sediments Ocean 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

8. Magnetism Earth’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

9. Magnetic Polarity Time Scale Paleomagnetism: study of the history of Earth’s magnetic field How 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

11. 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

12. 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

13. Seafloor Spreading Theory that explains how new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches http://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

14. Seafloor Spreading

15. 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