Section 17.2 Seafloor Spreading Objectives Summarize the evidence
Section 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. Section 17.2 Seafloor Spreading Mapping the Ocean Floor
Until the mid-1900s, many scientists thought that the ocean floors were essentially flat and that oceanic crust was unchanging and was much older than continental crust. Advances in technology during the 1940s and 1950s showed that all of these widely accepted ideas were incorrect.
Section 17.2 Seafloor Spreading How do we know what is on the ocean floor? We have mapped the magnetic fields and the topography of the ocean floor using sonar and
magnetometers. Section 17.2 Seafloor Spreading Tools to Map the Ocean Floor 1. magnetometer
can detect small changes in magnetic fields.generated by ocean floor rocks. 2. sonar can measure water depth and map the topography of the ocean floor. Section 17.2
Seafloor Spreading Ocean-Floor Topography These maps showed underwater mountain chains (ridges) and deep-sea trenches. Section 17.2
Seafloor Spreading Ocean-Floor Topography The deepest trench, the Marianas Trench, is more than 11 km deep. Mount Everest, the worlds tallest mountain, stands at 9 km above sea level, and could fit inside the Marianas Trench with six Empire State
buildings stacked on top. Section 17.2 Seafloor Spreading What evidence did these tools give us? Concept 1. The age of oceanic crust
increases with distance from a ridge. Proof: The sediment is thicker the closer you are to the continents. Section 17.2 Seafloor Spreading
What evidence did these tools give us? Concept 2: Magnetic north has changed over time. Proof: The magnetic record is in the rocks! The study of this is called Paleomagnetism. Section 17.2
Seafloor Spreading Magnetism Earth has a magnetic field generated by the flow of molten iron in the outer core. This field is what causes a compass needle to point to the North.
A magnetic reversal happens when the flow in the outer core changes, and Earths magnetic field changes direction. Section 17.2 Seafloor Spreading
Paleomagnetism What happens? 1. Lava comes up through the mid-ocean ridge. 2. Lava cools and the iron-bearing minerals crystallize.
3. As they crystallize, these minerals behave like tiny compasses and align with Earths magnetic field. Section 17.2 Seafloor Spreading
Magnetism What does this look like on the ocean floor? Section 17.2 Seafloor Spreading Seafloor Spreading
Seafloor spreading is the theory that explains how new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches. Harry Hess Section 17.2 Seafloor Spreading
How Seafloor Spreading Occurs 1. 2. 3. 4. Magma is hot, and rises from the mantle
It is forced toward a ridge The two sides of the ridge spread apart. When the magma solidifies, new ocean floor is added to Earths surface. Section 17.2 Seafloor Spreading
Result of Seafloor Spreading The cycle of spreading and the intrusion of magma continues the formation of ocean floor, The newest crust is by the mid-ocean ridge. The oldest crust is by the continents. Movie Clip
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