Chapter 10 Plate Tectonics 10.3 The Changing Continents GEOLOGY Chapter 10 Plate Tectonics 10.3 The Changing Continents

10.3 The Changing Continents Objectives Identify how movements of tectonic plates change Earth’s surface. Summarize how movements of tectonic plates have influenced climates and life on Earth. Describe the supercontinent cycle.

Reshaping Earth’s Crust Cratons are large areas of stable rock older than 540 million years that exist on all continents today. Cratons that have been exposed at the Earth’s surface are called shields. Cratons are the cores around which the modern continents are formed. Rifting is the process by which Earth’s crust breaks apart and it can occur within continental crust or oceanic crust.

Reshaping Earth’s Crust Continental crust is thicker than oceanic crust and has a high silica content – making continental crust act as an insulator. Heat from the mantle builds up under the continent causing the lithosphere to become thinner and weaken. Eventually a zone of weakness breaks forming a rift.

Reshaping Earth’s Crust Continents can change shape by breaking apart and gaining material. A terrane is a section of geologic lithosphere that has a unique geologic history that is different from that of the surrounding lithosphere. Three characteristics can be used to identify terranes. Contains rocks and fossils that differ from those of surrounding terranes Major faults at the boundaries of a terrane Magnetic properties of a terrane are generally different from those surrounding it

Reshaping Earth’s Crust Terranes become part of a continent at convergent boundaries. Accretion is the process in which a terrane becomes part of another continent. When a terrane is scraped off of a subducting plate, it may form mountains or just add to the surface area of the continent. Terranes can be small volcanic islands, atolls, sea mounts, large chunks of continent.

Reshaping Earth’s Crust Large mountain chains form when large terranes collide, such as the Himalaya Mountains – when India began colliding with Asia about 45 million years ago.

Effects of Continental Change The past movements of tectonic plates have an impact on modern climate. Latitude and longitude (both determined by continental movement) of a continent have an effect on climate as well as ocean currents and proximity to other landmasses. Mountain ranges affect airflow and wind patterns as well as whether or not moisture makes it from one region to another. Shifts in ocean currents and air and moisture flow have a major impact on climate change.

Effects of Continental Change Most landmasses on Earth have been covered by glaciers at some time in the past. At one point, a global ice sheet formed when the continents were amassed near the South Pole.

Effects of Continental Change Populations of organisms are formed as mountain chains are uplifted and rift valleys form. New species may evolve when an existing species is separated into different populations. Isolation may also protect populations from predators or competitors.

The Supercontinent Cycle The supercontinent cycle is the process by which supercontinents form and break apart over millions of years. As continents move about through the process of plate tectonics, they converge every so often creating a cycle. The movement of continental plates toward convergent boundaries cause inevitable collisions. Over time the continents collide to form a giant supercontinent.

The Supercontinent Cycle Since continental crust does not subduct (it buckles up to form large mountain chains), convergent boundaries where continents collide often become geologically inactive and spreading centers must form elsewhere. This creates rifts elsewhere in the supercontinent. Pangaea is the supercontinent that formed about 300 million years ago and began to break up about 250 million years ago.

The Supercontinent Cycle The Appalachian Mountains of North America and the Ural Mountains of Russia formed during the collisions of continents during the formation of Pangaea. The Tethys Sea cut into the eastern edge of the supercontinent. The large, global ocean that surrounded the rest of Pangaea was Panthalassa. Pangaea broke up into two continents, Laurasia and Gondwanaland about 250 million years ago.

The Supercontinent Cycle The large east-west rift that split Pangaea sent Laurasia drifting and spinning slowly northward. Another rift (eventually becoming the North Atlantic Ocean) then separated Laurasia into North America and Eurasia. The Tethys Sea closed during the rotation of Laurasia and eventually became the Mediterranean. About 150 million years ago, a rift (eventually becoming the South Atlantic Ocean) developed between Africa and South America.

The Supercontinent Cycle Australia, India, and Antarctica then broke apart and drifted away. India would later collide with Eurasia (Asia) to form the Himalaya Mountains. Slowly, since the breakup of Pangaea, the continents have drifted to their current positions, sometimes picking up terranes along the way. With the knowledge of current tectonic movement and past patterns, we can predict what the next supercontinent will look like.

References Rift Formation - http://www-geol.unine.ch/Petrology/Research/theme1.html India/Asia Convergence - http://www-class.unl.edu/geol101i/15c_glaciers.htm Late Permian South Pole View - http://www3.interscience.wiley.com:8100/legacy/college/levin/0470000201/chap_tutorial/ch09/chapter09-5.html Paleomaps of Continental Movement - http://scotese.com/late.htm