Ch9_Circulation - Establishing Establishing the Circulation...

Info icon This preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
Establishing the Circulation 1 CHAPTER 9 ESTABLISHING THE CIRCULATION SUMMARY: Planetary differentiation created the large scale layers of the terrestrial planets billions of years ago, and these layers still exist today. Core, mantle, crust, and volatile rich exterior were a common heritage for Earth, Venus and Mars. Our conception of these layers as fixed and static is deeply ingrained in our daily experience. Rocks are solid, and while they can be broken, they do not flow. Missouri is far from the ocean and Manhattan is an island. Such facts are both concrete observation and common sense. Therefore when Alfred Wegener proposed early in the twentieth century that Africa and South America were once joined together and that continents move across the surface of the Earth, the idea was met with skepticism, harsh criticism, and even derision by the geological community. After World War II, a new generation of earth scientists began to explore the oceans. The gradual acquisition of new data revealed a remarkable symmetry to the Atlantic Ocean. Down the center ran the mid-Atlantic Ridge; depth of the ocean and thickness of sediment increased regularly from the ridge towards the continents. The symmetry extended to the pattern of magnetic anomalies, which soon became related to the periodic reversals in the Earth’s magnetic field that had been discovered on land. All these data could be explained by sea floor spreading, where new ocean crust was created at the ridge, and aged progressively as it spread away to both sides. Sampling confirmed this idea by showing that young volcanic rocks were recovered at the ridge axis and the oldest sediments were near the margins of continents. Global seismology then showed the complement to sea floor creation at ocean ridges—the ocean crust is recycled into the mantle at ocean trenches, and its return to the mantle can be mapped precisely by the distribution of earthquakes in locations such as Japan. The new theory of plate tectonics accounted for these observations by proposing that the Earth’s surface is made up of fixed plates created at ridges, destroyed at subduction zones, continually in motion. The plates consist of brittle lithosphere, and they float on top of the mobile asthenosphere flowing underneath. Continents drift not by plowing through the oceans as Wegener implausibly suggested, but as light rafts floating on top of the plates that are created and destroyed in the surrounding oceans. The continents are too light to be recycled, and hence continents preserve a much longer record of earth history than the continually recycled ocean floor. Mountain belts occur where plates collide. Earthquakes and volcanoes occur at the margins of plates where they spread apart, converge towards each other, or slide by one another. In a period of a few years during the mid 1960’s, our view of earth changed from a static surface with fixed and isolated continents and oceans to a surface in constant motion, with plates moving as fast as 20cm/year. Long-standing geological questions
Image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern