******************************************************** TECTONICS ******************************************************** ********************* AGE OF ANTARCTICA ********************* __________ QUESTION: How old is Antarctica? ANSWER from Lisa Gahagan on January 20, 1995 The oldest rocks on Antarctica are nearly 4 billion years old. The continent itself, as an entity, first came into being when Pangea broke up about 175 million years ago. *************************** IS ANTARCTICA AN ISLAND? *************************** __________ QUESTION: Is Antarctica considered to be an island? ANSWER from April Whitt on January 10 Yes, Antarctica is an island and a continent. Sort of like Australia, but with more ice and less beer. *************** EARTHQUAKES *************** __________ QUESTION: What earthquake activity there has ever been in Antarctica? And what effect would the miles of ice have on earthquake damage? ANSWER from Lisa Gahagn on February 1, 1995 Antarctica is unusual in that there are very few earthquakes there. Of all the seven or nine continents, or of all the 7 or 25 plates (depending on how you count), it has the fewest earthquakes, and it has none of the big, damaging kind. As to the effect of ice, it wouldn't change much. Ice is a rock like any other. Most rocks melt into liquid if you get them hot enough, for ice "hot enough" is 32 degrees F. Thus the iciness of Antarctica is not significant, at least from an earthquake point of view. _________ QUESTION: What are the chances of an earthquake occurring in Antarctica? ANSWER from Sridar Anandakrishnan on January 19, 1995 There are very few earthquakes in Antarctica. It is one of the questions we are trying to answer out there. There are numbers of plates boundaries and we have always been astonished that we haven't seen more earthquakes. We have wanted to see them, we have tried to record them, but Antarctica is a real puzzle because there are very few earthquakes. There should be many more considering the type of plate boundaries there are, and the type of continental structures there are, but there aren't that many. We are trying to work it out. But now it is still a puzzle. __________ QUESTION: Are there Earthquakes in Antarctica? ANSWER from Tom Stevens in February, 1995 I've never felt an earthquake in Antarctica, but they do occur. There are seismic stations set up in different parts of Antarctica which not only help measure earthquakes in Antarctica, but help in measuring and pin-pointing earthquakes throughout the world. ******************** MOUNTAIN RANGES ******************** __________ QUESTION: What plates are responsible for the building of the Transantarctic Mountain Range? ANSWER from Lisa Gahagan on January 23, 1995 The Transantarctic Mountains are an unusual intraplate mountain belt wholly within the Antarctic Plate. They are probably related to thermal uplift along the edge of a rift system developed within the plate during the fragmentation of the Gondwana supercontinent (southern part of Pangea). __________ QUESTION: What forces created the mountain ranges that stretch across the continent? ANSWER from Deane Rink: Mountain ranges are usually created by one of two processes; either they arise when one plate dives underneath another in an area called a subduction zone, or they are created from chains of undersea volcanoes spewing out lava that eventually builds up till it breaks through to above the surface of the water and just keeps building. The Himalayas are an example of the former, and the Hawaiian Islands are an example of the latter. Antarctic mountains have been created by both processes. ANSWER from Lisa Gahagan: We do not know what forces created the mountain ranges that stretch across the continent. In fact there was a meeting this past spring just to discuss what we do know about the Transantarctic mountains and what we might study in order to understand them. Some mountain ranges such as the Andes that stretch along the western margin of South America are easy to understand; we know from studying earthquakes that occur beneath them that the ocean crust west of South America is being subducted beneath South America. The wet sediments are dragged down, promote melting of the crust and are recycled as they erupt as volcanoes along the mountains of Peru, Chile and Argentina among other places. On the other hand, the Himalayas, the highest mountains on Earth, are formed because the Indian subcontinent is moving northwards and colliding with Eurasia. The two continents are crumpling as they collide and part of India appears to even be diving beneath Eurasia. Unfortunately, along the Transantarctic Mountains there are no earthquakes and little evidence to explain what forces may have created the mountains or are working even now to uplift them. Ian Dalziel points out that the Transantarctic Mountains follow the ancient edge of the Pre-Cambrian shield of East Antarctica. They reflect tectonic activity along the Pacific margin and uplift during the rifting and fragmentation of Gondwana. In addition to the old phase of uplift, there appears to be a much younger phase of uplift that is occurring now. Mt. Erebus and some of the other very young volcanoes along the western Ross Sea are evidence that tectonic activity is presently occurring. The activity is strikingly linear and may represent a rift similar to the East African Rift which is thousands of kilometers long but only a few tens of kilometers wide. Other people want the present uplift of the Transantarctic Mountains to be caused by a mantle plume or hotspot similar to what is forming the volcanoes of Hawaii and Iceland. While there may be a hotspot under the eastern margin of Marie Byrd Land that has also resulted in the sub-ice volcano that was mentioned far to the south, it would be unbelievably large if the same hotspot was also the cause of the uplift of the Transantarctic Mountains along the western Ross Sea. The thing that makes geology and Earth Sciences so exciting and fun is that we do not have all the answers and there are a lot more neat places to do work and to find answers to questions that no one else has ever understood. It's a blast to get paid to go to all these great places and not have to wait until we are retired to be able to afford the time and money to visit these places. ****************** PLATE TECTONICS ****************** __________ QUESTION: How does such a giant land became separated as one land, and how did it become the coldest land on Earth? ANSWER from Lisa Gahagan January 23,1995 Antarctica became separated from the Pangea supercontinent as a result of convection of the Earth's mantle. It is the coldest continent because it was isolated from the other continents by oceans during the break-up of the Pangea supercontinent. __________ QUESTION: How do you know what shape the continents were before Pangea? Can scientists predict where the continents will move in the future? ANSWER from Lisa Gahagan on January 23, 1995 We know the basic shapes of the ancient plates from the distribution of old Precambrian and Paleozoic rocks versus the younger terranes accreted to their margins. Yes, one can make predictions into the future - for example the continued northward motion of the Salinian block of North America to the west of the San Andreas fault, and the likelihood that North America will collide with Asia to form "Amasia" as the North Atlantic continues to open and the Pacific to close. Also, Australia is even now colliding with Southeast Asia and will, if present activity continues, become amalgamated with it in a few tens of millions of years. __________ QUESTION: What kind of plate boundaries exist near the Antarctic continent, and do they account for the formation of the volcanoes that exist there? ANSWER from Lis Gahagan on January 4, 1994: Most of the plate boundaries with the Antarctic plate are seafloor spreading centers which are usually 1000s of kilometers from the continent itself. Along the western side of the Antarctic Peninsula, seafloor subduction either stopped recently (~4 million years ago) or slowed dramatically. There are a few active volcanoes along the northern Antarctic Peninsula that are the result of this plate boundary. Young volcanoes in Marie Byrd Land (MBL), offshore MBL, and the probable sub-ice volcano found by airborne geophysics are probably the result of a "hotspot" similar to the force that produces the volcanoes in the Hawaiian Islands. These two hotspots are not related to plate boundaries. The last group of young and active volcanoes along the western margin of the Ross Sea and including Mt. Erebus near McMurdo Station are a special group. Some people want them to be a result of the Marie Byrd Land hotspot. If they are, then this would be one of the largest hotspots known anywhere with perhaps the smallest amount of accompanying volcanism. On the other hand, some of us feel that the remarkable linearity of the western Ross Sea volcanoes, starting with Mt. Erebus and trending generally northward to the Balleny Islands at 67=B0S, 163=B0E are more representative of an East African Rift style, long, linear rift. If it is a long linear rift, then these volcanoes may be along a new plate boundary. The chemistry of the Balleny Island volcanics is similar to that of the Hallett Volcanic province, a region of active volcanoes a few hundred kilometers due north of Mt. Erebus. __________ QUESTION: Can you calculate if Antarctica will ever move enough so that it will completely abandon the South Pole? ANSWER from Lisa Gahagan on December 14, 1994: Plate tectonic researchers have various methods to study how plates move relative to each other and relative to the north and south poles. One method is to compare how plates move relative to hotspots such as Hawaii. A hotspot is an area where hot material from the mantle rises to the Earth's surface. Hotspots generally persist for millions of years while moving very little, if at all. Two scientists, Duncan and Richards, compared the past movement of different plates relative to hotspots. We can take their model for how Antarctica moved in the past 10 million years and use it to predict how Antarctica may move in the future. If we do this, our calculations show that Antarctica will move completely off the South Pole in 150 million years. This is a distance of over 2600 kilometers. This method assumes that the direction and rate of motion of Antarctica will remain constant which is probably not true. From studies of the past motion of Antarctica, we calculate that Antarctica was moving at more than 30 mm/year to the south during the Early Cretaceous (120 million years ago) relative to the South Pole. Today Antarctica is moving at less than 10 mm/year to the north. Because the direction and rate of motion has changed in the past, we cannot assume that they will remain constant in the future. Therefore, our calculation for the future movement of Antarctica is only a speculation. __________ QUESTION: What is the relationship between earlier theories of continental drift and plate tectonics, and are there any areas of conflict between them? ANSWER from Robin Holcomb on November 15, 1994: You asked first about the relationship between the theories of continental drift and plate tectonics. A simple answer is that the connection is historical; the theory of plate tectonics is a mechanistic theory that was developed in the late 1960s to explain observations of continental motion that had been systematically organized by Alfred Wegener into the theory of continental drift about 50 years earlier. The theory of continental drift had been rejected by much of the geological community because it included no reasonable mechanism to explain the large movements of continents that it invoked. The later theory of plate tectonics overcame the objections to continental drift by incorporating a reasonable mechanism for the motions. There is no inherent conflict between the two theories; the later one can be viewed as including or explaining the earlier one. There are apparent conflicts between the theories in certain details, but those conflicts are nonessential, arising from differences in knowledge when the theories were developed. That is to be expected, and there are even conflicts over details among various contemporary workers who accept the general theory of plate tectonics. __________ QUESTION: What do you know of a theory that suggests that Texas, as part of Laurasia, and Antarctica, as part of Gondwanaland, were once joined? What is the general reaction to this hypothesis by the scientific community? ANSWER from Robin Holcomb on November 15, 1994: You asked about a particular theory of plate reconstruction in which there were 2 supercontinents of Laurasia and Gondwanaland, with Texas occurring in a key location. I think that you may be referring to a particular hypothesis that was proposed in the following research paper: Edredge Moores (1991) "Southwest U.S.--East Antarctic (SWEAT) connection: A hypothesis"; Geology, vol. 19, p. 425-428. Some additional papers that developed the idea further are these: Ian W.D. Dalziel (1991) "Pacific margins of Laurentia and East Antarctica-Australia as a conjugate rift pair: Evidence and implications for an Eocambrian supercontinent"; Geology, v. 19, p. 598-601. P.F. Hoffman (1991) Did the breakout of Laurentia turn Gondwanaland inside out?"; Science, vol. 252, p. 1409-1412. Ian W.D. Dalziel (1992) "On the organization of American plates in the Neoproterozoic and the breakout of Laurentia"; GSA Today, vol. 2, p. 237-241. The basic idea that has been developed is that a single big supercontinent called Rodinia broke up late in the Precambrian to form two large continents called Laurentia and Baltica. Later, near the Precambrian/Cambrian boundary, these continents were reassembled -- and then split apart again later -- and reassembled, and split again. A complex history. I called Eldredge Moores (a professor at the University of California at Davis) to find out the current status of his particular idea. He said that several tests of the hypothesis have been proposed, and that the work done so far seems to support it fairly well.