Saturday, March 7, 2020
Geology and Topography of Antarctica
Geology and Topography of Antarctica Antarctica is not an ideal place for a geologist to work - it is widely considered one of the coldest, driest, windiest and, during winter, darkest places on Earth. The kilometers-thick ice sheet sitting on top of 98 percent of the continent makes geologic study even more difficult. Despite these uninviting conditions, geologists are slowly gaining a better understanding of the fifth-largest continent through the use of gravity meters, ice-penetrating radar, magnetometers, and seismic instruments. Geodynamic Setting and History Continental Antarctica makes up just a portion of the much larger Antarctic Plate, which is surrounded by mostly mid-ocean ridge boundaries with six other major plates. The continent has an interesting geologic history - it was part of the supercontinent Gondwana as recently as 170 million years ago and made a final split from South America 29 million years ago. Antarctica has not always been covered in ice. At numerous times in its geologic history, the continent was warmer due to a more equatorial location and differing paleoclimates. It is not rare to find fossil evidence of vegetation andÃ dinosaursÃ on the now-desolate continent. The most recent large-scale glaciation is thought to have begun around 35 million years ago. Antarctica has traditionally been thought of as sitting on a stable, continental shield with little geologic activity. Recently, scientists installed 13 weather-resistant seismic stations on the continent that measured the speed of earthquake waves through underlying bedrock and mantle. These waves change speed and direction whenever they encounter a different temperature or pressure in the mantle or a different composition in the bedrock, allowing geologists to create a virtual image of the underlying geology. The evidence revealed deep trenches, dormant volcanoes, and warm anomalies, suggesting that the area may be more geologically active than once thought. From space, Antarcticas geographic features seem, for lack of a better word, nonexistent. Underneath all of that snow and ice, however, lie several mountain ranges. The most prominent of these, theÃ TransantarcticÃ Mountains, are over 2,200 miles long and split the continent into two distinct halves: East Antarctica and West Antarctica. East Antarctica sits on top of a Precambrian craton, made up of mostly metamorphic rocks like gneiss and schist.Ã Sedimentary deposits from the Paleozoic to Early Cenozoic age lie above it.Ã Western Antarctica, on the other hand, is made up of orogenic belts from the past 500 million years. The summits and high valleys of the Transantarctic Mountains are some of the only places on the entire continent not covered in ice.Ã The other areas that are free from ice can be found on theÃ warmer Antarctic Peninsula, which extends 250 miles northward from West AntarcticaÃ towards South America. Another mountain range, theÃ Gamburtsev Subglacial Mountains, rises almost 9,000 feet above sea level over a 750-mile expanse in East Antarctica. These mountains, however, are covered by several thousand feet of ice. Radar imaging reveals sharp peaks and low valleys with topography comparable to the European Alps. The East Antarctic Ice Sheet has encased the mountains and protected them from erosion rather than smoothing them into glacial valleys. Glacial Activity Glaciers affect not only the topography of Antarctica but also its underlying geology. The weight of ice in West Antarctica literally pushes the bedrock down, depressing low-lying areas below sea level. Seawater near the edge of the ice sheet creeps between the rock and glacier, causing the ice to move much faster towards the sea. Antarctica is completely surrounded by an ocean, allowing sea ice to greatly expand in winter. Ice normally covers around 18 million square miles at the September maximum (its winter) and decreases to 3 million square miles during the February minimum (its summer).Ã NASAs Earth Observatory has a nice side-by-side graphic comparing the maximum and minimum sea ice cover of the past 15 years. Antarctica is almost a geographic opposite of the Arctic, which is an ocean semi-enclosed by landmasses. These surrounding landmasses inhibit sea ice mobility, causing it to pile up into high and thick ridges during the winter. Come summer, these thick ridges stay frozen longer. The Arctic retains around 47 percent (2.7 of 5.8 million square miles) of its ice during warmer months. The extent of Antarcticas sea ice has increased by approximately one percent per decade since 1979 and reached record-breaking levels in 2012 to 2014. These gains do not make up for diminishing sea ice in the Arctic, however, and global sea ice continues to disappear at a rate of 13,500 square miles (larger than the state of Maryland) per year.