Unit: 4 The Forces Within Earth Earth Systems 3209 Unit: 4 The Forces Within Earth Reference: Chapters 4, 15, 16, 19; Appendix A & B

Evidence which Supports Plate Tectonics Unit 4: Topic 2.4 Evidence which Supports Plate Tectonics Focus on . . . outlining and describing current evidence which supports the theory of plate tectonics.

Evidence For Plate Tectonics The main evidence to support the idea of plate tectonics focuses on the different plate boundaries. The many different features seen at these boundaries provide overwhelming proof that the sea floor is indeed moving, in fact, it is being recycled! Evidence supporting Plate Tectonics Include: 1) Earthquakes and Volcanoes 2) Paleomagnestim A) Polar wandering B) Magnetic Reversals and Seafloor Spreading 3) Deep Ocean Drilling 4) Hot Spots Text Reference: Pages 521 – 527 and 540 - 547

Evidence For Plate Tectonics Earthquakes and Volcanoes Earthquakes and volcanoes do not occur randomly throughout the world, but occur in rather limited belts. These belts mark the location of Plate Boundaries. The largest active belt in the world surrounds the Pacific Ocean and is referred to as “The Pacific Ring of Fire.” 90% of all the world’s earthquakes occur there. Some of the more famous volcanoes are found surrounding the Pacific ocean. Mt. Fuji

Evidence For Plate Tectonics Earthquakes and Volcanoes These boundaries are areas where compressional forces cause tectonic plates to move toward one another and stress builds up. When the stress is to great, fractures (faulting) may occur within the tectonic plates and the plates may suddenly slip which results I an earthquakes. The boundaries are also places of high heat flow, which are characterized by molten rock rising to the surface and forming volcanoes. Example: Mount Saint Helens in USA.

Evidence For Plate Tectonics 2) Paleomagnetism (Fossil Magnetism) The permanent magnetism in rocks which indicates the direction of the magnetic field when the iron rich minerals became magnetized. The most persuasive evidence to support the Plate Tectonic Theory comes from the study of Earth’s magnetic field. The ideas of polar wandering and magnetic reversals on the ocean floor provide this evidence. Basaltic rocks contain iron-rich minerals which become magnetized in the direction of the magnetic field at the time when the rock solidified. If the rocks move the magnetism in the rocks retain its original magnetic alignment and “remember” the location of the magnetic poles at that time.

Evidence For Plate Tectonics 2A) Polar Wandering Is the apparent movement of the magnetic poles as outlined from studying the magnetism fossilized in successive basaltic lava flows ranging in age over millions of years. A plot of this magnetism showed that the magnetic pole appeared to change position considerably over the past 500 million years.

Evidence For Plate Tectonics 2A) Polar Wandering This was clear that either the magnetic pole had moved with time, an idea known as polar wandering, or the basaltic lava flows had moved, explained by continental drift. Plate Tectonic theory is believed to be the best explanation for polar wandering. If the magnetic poles remain stationary, then their apparent movement was caused by the drifting of continents.

Evidence For Plate Tectonics 2B) Magnetic Reversals and Seafloor Spreading Paleomagnetism also provided evidence for the Plate Tectonic Theory when scientist discovered that the magnetic field reverses polarity. Basaltic lavas solidifying during a time of reverse polarity would display opposite magnetism as rocks forming today (normal polarity). Rocks with magnetism the same as our present magnetic field is said to have normal polarity, while rocks with opposite polarity is said to have reverse polarity. Convection Currents Asthenosphere Normal Polarity Reverse polarity

Evidence For Plate Tectonics 2B) Magnetic Reversals and Seafloor Spreading This alternating magnetic polarity can be seen in: 1) successive lava flows making up a volcano; 2) the basaltic rock making up the ocean floor. At oceanic ridges the plates move apart and new basaltic rock is added to each plate. The magnetism of these basaltic rocks appears to alternate to produce identical magnetic patterns on both sides of oceanic ridges. This proved to be the strongest evidence to support seafloor spreading and therefore Plate Tectonics. Convection Currents Asthenosphere Normal Polarity Reverse polarity

Evidence For Plate Tectonics 3) Ocean Drilling From 1968 to 1983, the Deep Sea Drilling Project collected convincing evidence confirming the seafloor spreading idea and the Plate Tectonic theory. Drill core samples of the ocean floor and sediments on the ocean floor were collected with increasing distance from ocean ridges. Younger Older

Evidence For Plate Tectonics Ocean Drilling When the oldest sediment from each drill site was plotted against the distance from the ocean ridge, it was noted that the age of the sediment increased with increasing distance from the ridge. This evidence also confirmed the idea that the ocean basins are relatively young, because no sediment older than 180 million years was found. Continents were dated to be 4.6 billion years.

Evidence For Plate Tectonics 4) Hot Spots Mapping of the seafloor in the Pacific revealed a chain of volcanoes and seamounts that extend from the Hawaiian Islands to the Midway Islands and continue north to the Aleutian trench of the coast of Alaska. Scientist proposed that a plume of magma presently exist beneath Hawaii and the Pacific plate moved over this stationary magma chamber. Older Asthenosphere Hot Spot

Evidence For Plate Tectonics 4) Hot Spots This confirmed that the tectonic plates do move in relation to Earth’s interior thereby supporting the theory of Plate Tectonics. Radioactive age dates of the seamounts and volcanic islands confirm that the age increases the farther away you go from Hawaii, and the hot spot. Older Asthenosphere Hot Spot

Example 1: Based on the information below, which is true of letters “X” and “Y”? (A) different age, different polarity (B) different age, same polarity (C) same age, different polarity (D) same age, same polarity What do apparent polar wander paths obtained by paleomagnetism tell us? (A) the earth’s magnetic poles have moved (B) the earth’s magnetic poles have reversed (C) the earth’s rotational axis has moved (D) the lithospheric plates have moved

Example 2: Which plate tectonic evidence is described in the information below? (A) deep-ocean drilling (B) hot spots (C) paleomagnetism (D) Wadati-Benioff Zone What contributes to the reverse magnetic patterns found on the ocean floor near ridges? (A) alternating iron and nickel deposits (B) alternating bands of basaltic and granitic rocks (C) alternating magnetic polarity of Earth’s magnetic field (D) alternating magnetic attraction of Mars and Venus on the Earth As igneous rocks cool at mid oceanic ridges, the iron-rich minerals align to the Earth’s poles.

Your Turn . . . Take the time and complete the following questions . . . (Solutions to follow) Question: With the aid of a labelled diagram, explain how hot spots can be used to support the Theory of Plate Tectonics.

Solutions . . . Question: With the aid of a labelled diagram, explain how hot spots can be used to support the Theory of Plate Tectonics. Answer: The hotspot can prove the idea of plate tectonics because we know there are areas of hot spots in Earth’s interior. They burn their way through the crust. If the crust didn’t move then there would be one very large shield cone above the hot spot. Instead, we see a chain of volcanic islands that are increasing in age as we move away from the hotspot. The differing ages show the direction that the plate is moving. It moves in the direction that the islands get older.

Summary . . . Overview of Points covered: Evidence supporting Plate Tectonics Include: 1) Earthquakes and Volcanoes 2) Paleomagnestim A) Polar wandering B) Magnetic Reversals and Seafloor Spreading 3) Deep Ocean Drilling 4) Hot Spots