+ Evidence for Plate Tectonics Chapter 9.4

+ Continental Drift - Review Wegener - Continental drift hypothesis 4 Strong Pieces of Evidence: Continental puzzle Matching Fossil records Matching mountain ranges Altered Ancient climates Why wasn’t his hypothesis accepted?

+ Continental drift hypothesis  The Main objection to Wegener's proposal was its inability to provide believable mechanism for movement of continents.  He felt the earths spin or ocean currents caused the continents to plow through the ocean floor similar to the way an iceberg moves.

+ Theory of Platetectonics Theory states: The earths lithosphere is fragmented into a dozen or more large and small plates which ride a top the asthenosphere. Earth largest plate is the Pacific plate. Oceanic plates like this one are created at divergent boundaries and recycled at subduction zones. The water and carbon compounds they pull down with them increases the fluidity of the magma and leads to chains of volcanoes. The Asthenosphere exists beneath the lithosphere and is hotter, weaker and more fluid than lithosphere which allows the lithosphere to move.

+ 4 Key Pieces of Evidence give rise to Plate Tectonics Theory  1) Ruggedness and youth of the ocean floor  2) Tectonic & Volcanic activity along trenches and mountain ranges  3) Paleomagnetism pattern on either side of divergent boundaries  4) Hot Spots & seafloor spreading

+ 1. Roughness and youth of the ocean floor Before the 19th century the ocean floor was thought to be flat. The development of echo-sounding methods or SONAR around 1955 allowed scientists to study the ocean floor in great detail. These maps showed underwater mountains chains in middle of oceans.

Evidence emerges as Technology improves  Sonar uses sound waves to measure water depth by measuring the time it takes for sound waves to travel from the device and back to a receiver.

 2/3 of the planet is covered with oceans and oceanic crust is also 2/3 of the crust on earth.  In the Search for new oil deposits people began drilling deep into the oceanic crust.  Analysis of the sediment layers showed that all oceanic crust is less than 200 million years old and that only there is only very young sediment layers near the mid-ocean ridges. Relative youth of the ocean floor from deep Ocean Drilling Ships drilling for oil found that the amount and maximum age of sediment decreased as they got closer to the mid-ocean ridges indicating the ocean floor had just formed at these locations.

 Also the oceanic crust was much younger than the continental crust.

Magnetism

+ 2. Earthquakes and volcanic activity along plate boundaries Seismographs are tools used to measure the shacking of the Earth and can be used to locate the epicenter of an earthquake. Our Knowledge was greatly advanced in 1960’s from the world wide standardized seismograph network or WWSSN. This was established to help monitor activities related to the 1963 treaty banning above ground testing of nuclear weapons.

+ Major Tectonic Plates

+ Mapping Earthquakes and Volcanoes taught us that most (90%) occur in narrow bands at Plate boundaries. Earthquake locations for events between 1965 and The red dots are shallow earthquakes, the green are intermediate depth, and the blue and purple are deep.

+ Triangles represent the location of a recently active volcanoes. Some volcanic regions such as the Hawaiian Islands are isolated but most correspond well to the map to Plate boundaries.

+ Earthquake patterns at subduction zones showed use the outline of the descending oceanic plate.

+ 3. Repeated changes of Earth’s magnetic field  A magnetometer is a device that can detect small changes in magnetic fields, similar to a compass.  In the 1960’s scientists using magnetometers on military subs began noticing odd magnetic stripped patterns in the ocean floor.

Magnetism  It was known that rocks like basalt containing iron-bearing minerals provide a record of Earth’s magnetic field as iron they solidify.  Surprisingly these patterns of magnetism in the rocks indicated that earths magnetic field had reversed many times in the geologic past.

Magnetism  The normal and revered polarity of the seafloor forms a series of stripes that run parallel to ocean ridges.  Additionally the same mirror image pattern of paleomagnetism was seen on either side of a mid- ocean ridge giving more evidence of Seafloor Spreading.

4. Seafloor Spreading  An American scientist named Harry Hess proposed the theory of seafloor spreading which states that new oceanic crust is formed at ocean ridges and destroyed at deep-sea trenches.  Convection currents in the mantel drive Magma toward surface along the ocean ridge where it cracks and spreads apart.

Seafloor Spreading  Seafloor spreading was the missing piece that Wegener could have used to complete his model of continental drift if only the technology had been available.  Continents are not pushing through ocean crust, as Wegener proposed; they ride with ocean crust as it slowly moves away from ocean ridges.

4. Hot Spots  Maps made by sonar show that some volcanic island chains form as a series of progressively older islands.  Radiometric dating of the rocks later confirmed this progressively older trend of the Hawaiian Islands as you move west.

 Seismographs are able to plot the locations of earthquakes and from an enormous magma hot spot underneath the newest of the volcanic islands.  The Hawaiian Islands are more evidence showing that the plate underneath the hot spot is moving NOT the hot spot as was previously thought.