GY 111 Lecture Notes D Haywick 2007 08 1 GY 111 Lecture Note Series Mountain Building 1 The Basics Lecture Goals A Distribution of mountain belts the return of plate tectonics B Isostacy C The Rock Cycle again or what goes around comes around the return of igneous sedimentary metamorphic rocks Reference Press et al 2004 the whole book Grotzinger et al 2007 the whole book A Distribution of mountain belts If you have ever looked at a decent map of the world one that shows topography you would have quickly recognized that mountains are neither randomly distributed nor do they occur as isolated features They occur in belts that are frequently pretty linear If you stripped the Earth of water something that the best maps also do you would have seen that most of the world s most impressive mountain belts occur beneath the surface of the ocean We didn t even know they existed before the 1940 s By now you should understand that all of these mountain belts are the result of plate tectonics The most impressive mountain belts occur where plates collide with one another The Cordilleran Mountains1 of western North America and the Andes of western South America are the result of oceanic continental plate collisions The Himalayas are the result of a continent continent collision India Asia Transform plate boundaries also result in impressive mountains as anyone in southern California Sierra Nevada Mountains or New Zealand Southern Alps would tell you Mid oceanic ridges like the Mid Atlantic Ridge or the East Pacific Rise as well as the divergent plate boundary in eastern Africa called the East African Rift are all mountain belts that are as much the result of heat thermal expansion as they are deformation All in all there are a lot of very impressive mountain belts on the surface of the Earth Sadly none of them are within driving distance to Mobile Yes I am aware that a 3 hour drive up Interstate 65 will get you to the Appalachian Mountains but I don t consider them overly impressive at least from a topographic point of view Consider the cross sectional cartoon of the Cordilleran Mountains and the southern Appalachians between Birmingham and Atlanta that occurs at the top of the next page 1 Many people call the Cordilleran Mountains the Rockies but this isn t entirely correct The Rocky Mountains are just one component or range of the Cordilleran Mountains Other ranges include the Cascades Washington State Oregon the Front Ranges Alberta the Coastal Ranges British Columbia and the Olympic Mountains Washington State GY 111 Lecture Notes D Haywick 2007 08 2 The Cordilleran Mountains are tall the highest mountains exceed 6500 m above sea level e g Mt McKinley in Alaska and rugged In contrast the Appalachian Mountains are relatively short most less than 2000m in height and rounded i e they have gentle slopes If you examine the mountains portrayed in the cartoon map of the Earth on the previous page you will see that some mountain belts are very rugged the Alps Andes Himalayas whereas others the Urals Tasmans are smooth The key to this character difference is the age of the mountain belts Those that are rugged like the Alps and Cordilleran Mountains are relatively young Those that are not like the Tasmans the Urals and the Appalachians stopped growing 100s of millions of years ago Since that time they have been at the mercy of chemical and physical weathering They have been eroding away and producing billions and billions of tons of sediment in the process You know the real mystery isn t why some mountain belts are rugged whereas others are not It s why mountains that stopped growing 300 million years ago Southern Appalachians or earlier e g the Urals formed about 500 million years ago still have any positive topographic relief at all You d think that in that time they would have been worn away to nothing They would have been were it not for an important mechanism known as isostacy B Isostacy Let s consider the Appalachian Mountains for a few minutes This mountain belt stretches from central Alabama all the way to Newfoundland see GoogleEarth image to right in eastern Canada2 If you compare the topography of the mountains in the south e g Alabama Georgia with the north Newfoundland you will see that the southern mountains are considerably more mountainous then their northern equivalents Once again the difference is due to age The forces that produced the northern portion of the Appalachians ended about 500 million years ago but continued deformation in the south lasted until about 315 million years ago The southern Appalachians have about another 185 million years before they are reduced to a peneplain Still even once this occurs there is considerable evidence left behind that there was a mountain belt there in the past Consider the following sketch of the structural elements in the Appalachian Mountains Even though the topographic relief is gone the faults folds and rocks that are produced during a mountain building episode are still there 2 and on to Ireland England and the rest of the United Kingdom via Labrador and Greenland but NOT Iceland GY 111 Lecture Notes D Haywick 2007 08 3 We can recognize the remains of mountain belts a couple of billions of years old even though a mountain has not rested there for more than 1 billion years Moreover some of the rocks now exposed at the Earth s surface contain minerals or feature suggesting that they originally formed well below the surface of the Earth possibly 20 km or more down How could those rocks now be at the surface of the Earth How could so much overburden have been removed If you were to draw a cross section through a major mountain belt like the Cordillera you would find that the positive relief of the mountains is mirrored in the subsurface by what are called continental roots These roots are composed of the same material that the mountains are namely continental crust We usually envision continental crust to be composed of granite 3 especially when we are discussing plate tectonics and subduction You should remember this from earlier lectures ocean crust basalt is more dense than continental crust granite which usually means that oceanic plate is subducted beneath continental plate in a convergent plate setting see cartoon to right But even though the continental crust is less dense than oceanic crust and the rest of the lithosphere it is resting on it is still pretty heavy If you pile a lot more of this crust on top of it e g by making