After formation: Earth's shaping

After Earth was originally formed, it has been shaped by numerous impacts by outside objects, such as asteroids and comets. Although Earth’s atmosphere incinerates many incoming objects, millions have managed to collide with its surface. Many impact sites are no longer visible to the naked eye due to erosion, but evidence for many existing craters remains. For example, Meteor Crater in Arizona is still intact, probably because it is very young – about 50,000 years old(1). This meteorite has been the subject of intense research, including analysis of minerals and materials within and surrounding the site. As research of other impact sites both on Earth and in the Solar System progresses, the predicted mechanisms for impact have been continually revised. High-resolution images of Jupiter’s moon Europa have been analyzed, and suggest that many craters are always not the primary result of an impact, as previously thought(2). Instead, they are formed from debris from a previous impact. Another long-held belief was that objects completely vaporize after colliding with Earth. However, while excavating a 145-million-year-old, 100-mile wide South African crater, a fossil meteorite was removed. Because this large remnant (about the size of a beach ball) was uncovered, “…computer models of large impacts might now need to be revised, to take into account conditions where some of the asteroid material endures,” according to Dr. Marco Andreoli, a geologist at the University of Witwatersrand(3). Increasing understanding of collisions allows astronomers to better understand the shaping of Earth’s surface.

Tectonics, and by proxy volcanoes, also contribute an enormous amount to the Earth’s surface. Because the temperature drastically increases as one moves toward the center of the Earth, convection occurs. Convection, in which warm air and materials rise, cool, sink and heat again, causes shifting in the Earth’s crust. Tectonics can be measured using seismographs, which tell us the activity that occurs beneath our feet. Plates, or segments of the Earth’s lithosphere, move and interact with each other. This can create both mountains and valleys, depending on the direction of the shifts. Plate tectonics can also result in earthquakes, which in themselves can cause many features, such as the movement of the continents. These main concepts can also explain volcanism, where hot and less dense molten rock, or magma, rises to the surface, where it is referred to as lava. Volcanoes play a major part in shaping the Earth’s surface, sometimes even by creating new pieces of land4. The Hawaiian Islands are direct results of volcanism. One major “hotspot” exists under these islands, which causes frequent flow, cooling and hardening of lava. Due to tectonics, plates slide over the hotspot. As these plates move, a chain of islands is created from the lava buildup(5).

Earth has been shaped by numerous meteor strikes, earthquakes and magma flows, but it has also been carved (and smoothed) by erosion. It has long been assumed that canyons and caves have been carved over time by the flow of water. The Grand Canyon, for example, is suspected to have formed from the Colorado River’s cutting power(6). Another form of erosion is wind erosion, which can, over time, both smooth out a landscape or create dunes. Loose pieces of sand or silt are transported by the wind; they can be picked up, then deposited somewhere else(7). Over millions or billions of years, erosion, as well as tectonics and extraterrestrial impacts, can have significant effects on Earth’s landscape.


REFERENCES:
1."Meteor Crater." Wikipedia, the free encyclopedia. 13 Apr. 2009. 14 Apr. 2009 .

2.Than, Ker. "Europa Images Challenge Crater Creation Theory." Space.com. 24 Oct. 2005. 6 Apr. 2009 .

3.Morelle, Rebecca. "Relic of ancient asteroid found." BBC NEWS News Front Page. 10 May 2006. 1 Apr. 2009 .

4.Bennett, Jeffery. The Cosmic Perspective: The Solar System. 5th ed. Benjamin Cummings.

5.Ruben, Ken. "The Formation of the Hawaiian Islands." Hawaiian Center for Volcanology. 4 Apr. 2005. School of Ocean and Earth Science and Technology. 29 Apr. 2009 .

6.Peterson, Joel L. "The mystery of the pre–Grand Canyon Colorado River—Results from the Muddy Creek Formation." GSA Today. Mar. 2008. The Geological Society of America. 29 Apr. 2009 .

7.Dutch, Steven. "Wind Erosion." University of Wisconsin - Green Bay. 3 Nov. 1999. University of Wisconsin - Green Bay. 29 Apr. 2009 .