Geology - Mountain Building - Quick Guides | Geology

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Study GuideGeology–Mountain Building1. Features of Mountain BeltsWhat Are Mountain Belts?Mountain belts are long chains of mountains that usually form near the edges of continents. They canstretch forthousands of kilometers in lengthand behundreds of kilometers wide. Most of themrunparallel to coastlines, where tectonic plates interact.One of the longest mountain belts on Earth is theAmerican Cordillera, which follows the westernedge of bothNorth America and South America. It includes many steep and rugged mountainranges.1.1 Mountain Height and AgeIn general,taller and steeper mountains are younger, whilelower and smoother mountains areolder. This happens because older mountains have had more time to be worn down byweatheringand erosion.For example:•TheRocky Mountainsare steep and rugged because they are relatively young.•TheAppalachian Mountainsare lower and rounded because they are very old.Most mountain ranges go through a long cycle:1.They are uplifted by tectonic forces.2.They erode down to lower elevations.3.They may be uplifted again before finally becoming stable.1.2 Major Mountain Ranges in the United StatesThe United States has several important mountain systems, including:•TheAppalachian Mountains

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Study Guide•TheRocky Mountains•TheOzark Mountains•Many mountain ranges along theWest CoastFossils and rock-age studies show that theAppalachian and Ozark Mountainsare among theoldest mountain ranges in the country. Their rounded shapes reflect millions of years of erosion.1.3 Cratons: The Stable Heart of ConƟnentsWhat Is a Craton?Billions of years ago, the interior of North America was once mountainous and tectonically active.Over time, erosion flattened this region into a nearly level surface called apeneplain.Acratonis a part of a continent that has beentectonically stable for hundreds of millions ofyears. Cratons form thestrong, stable coreof continents and are made mostly ofplutonic andmetamorphic rocks.1.4 Sedimentary Cover and ConƟnental GrowthThe craton acts like abasement, on top of which layers of sedimentary rock accumulate. In thecentral United States, about2,000 meters of sedimentary rockwere deposited inshallow oceansduring the Paleozoic Era.Continents grow larger through a process calledaccretion, where:•Sediments,•Volcanic arcs, and•Crustal fragmentsare added to the edges of cratons duringplate collisions.These collisions often lead to mountain building.

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Study Guide1.5 Rock Types Found in MountainsFolded Sedimentary RocksMost mountain ranges are made of sedimentary rock layers that have been folded and broken byintense tectonic forces. These layers can become up to five times thicker than the original sedimentlayers found in stable cratons.Because mountain belts often form nearsubduction zones, much of this sedimentary material:•Originally formed in the ocean,•Was scraped off tectonic plates, and•Became part of anaccretionary wedgethat was pushed onto the continent.1.6 Folding, ThrusƟng, and CompressionMountain-building forces are extremelycompressional. As a result:•Wide sedimentary basins are squeezed intonarrow mountain ranges.•Rock layers are bent intotight folds, includingoverturnedandrecumbent folds.Many mountain belts includefold-and-thrust belts, where large sheets of rock are pushed forwardand stacked on top of one another alonglow-angle thrust faults.1.7 Later Stretching and Fault-Block MountainsAfter uplift, the stress in a mountain belt often changes from compression totension. This stretchingcauses:•Normal faulting•Formation offault-block mountains, includinghorsts and grabensThese faults help the crust adjust to the stress created by vertical uplift.

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Study Guide1.8 Metamorphism and Mountain CoresDeep Changes Inside MountainsThecore of a mountain rangeusually contains the mostintensely metamorphosed rocks. Theserocks:•Started as sedimentary or volcanic rocks,•Were buried deep in the crust,•Were changed by heat, pressure, and deformation.Because the original rock features are often destroyed, geologists commonly identify these rocks asschistorgneiss.1.9 Magma and Crustal ThicknessSome of the most extreme metamorphic rocks, calledmigmatites, are found in mountain cores.Large underground magma bodies, known asbatholiths, also form during mountain building due topartial meltingof the crust.Thecontinental crust beneath mountain ranges is thickerthan beneath stable cratons. Youngermountain ranges usually havethicker crustthan older ones.Over time, the crust adjusts and stabilizes throughisostatic adjustment, similar to how icebergs floathigher when they are thicker.1.10 AcƟvity in Young vs. Old Mountains•Youngmountain beltsare tectonically active and experiencemore earthquakes andvolcanic activity.•Older mountain rangesare more stable and much less active.This difference reflects how recently the mountains were formed and how much tectonic energy is stillpresent.

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Study Guide2. Types of MountainsEven though every mountain range is unique, geologists group mountains intofour main structuraltypesbased on how they form. A single mountain range can includemore than one typeofmountain, so these categories often overlap.The four main types are:•Upwarped mountains•Volcanic mountains•Fault-block mountains (horst and graben)•Folded (complex) mountains2.1 Upwarped MountainsUpwarped mountains form when large areas of the Earth’s crust arepushed upward in a broadarch. In some cases, they also form when there ismajor vertical movement along a steep (high-angle) fault.Key features of upwarped mountains:•Broad, dome-like shape•Morerounded peaks

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