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Geology - Mountain Building

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    Study Guide Geology – Mountain Building 1. Features of Mountain Belts What Are Mountain Belts? Mountain belts are long chains of mountains that usually form near the edges of continents. They can stretch for thousands of kilometers in length and be hundreds of kilometers wide . Most of them run parallel to coastlines , where tectonic plates interact. One of the longest mountain belts on Earth is the American Cordillera , which follows the western edge of both North America and South America . It includes many steep and rugged mountain ranges. 1.1 Mountain Height and Age In general, taller and steeper mountains are younger , while lower and smoother mountains are older . This happens because older mountains have had more time to be worn down by weathering and erosion . For example: • The Rocky Mountains are steep and rugged because they are relatively young. • The Appalachian Mountains are 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 States The United States has several important mountain systems, including: • The Appalachian Mountains

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    Study Guide • The Rocky Mountains • The Ozark Mountains • Many mountain ranges along the West Coast Fossils and rock - age studies show that the Appalachian and Ozark Mountains are among the oldest mountain ranges in the country . Their rounded shapes reflect millions of years of erosion. 1.3 Cratons: The Stable Heart of Con Ɵ nents What 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 a peneplain . A craton is a part of a continent that has been tectonically stable for hundreds of millions of years . Cratons form the strong, stable core of continents and are made mostly of plutonic and metamorphic rocks . 1.4 Sedimentary Cover and Con Ɵ nental Growth The craton acts like a basement , on top of which layers of sedimentary rock accumulate. In the central United States, about 2,000 meters of sedimentary rock were deposited in shallow oceans during the Paleozoic Era . Continents grow larger through a process called accretion , where: • Sediments, • Volcanic arcs, and • Crustal fragments are added to the edges of cratons during plate collisions . These collisions often lead to mountain building.

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    Study Guide 1.5 Rock Types Found in Mountains Folded Sedimentary Rocks Most mountain ranges are made of sedimentary rock layers that have been folded and broken by intense tectonic forces. These layers can become up to five times thicker than the original sediment layers found in stable cratons. Because mountain belts often form near subduction zones , much of this sedimentary material: • Originally formed in the ocean, • Was scraped off tectonic plates, and • Became part of an accretionary wedge that was pushed onto the continent. 1.6 Folding, Thrus Ɵ ng, and Compression Mountain - building forces are extremely compressional . As a result: • Wide sedimentary basins are squeezed into narrow mountain ranges . • Rock layers are bent into tight folds , including overturned and recumbent folds . Many mountain belts include fold - and - thrust belts , where large sheets of rock are pushed forward and stacked on top of one another along low - angle thrust faults . 1.7 Later Stretching and Fault - Block Mountains After uplift, the stress in a mountain belt often changes from compression to tension . This stretching causes: • Normal faulting • Formation of fault - block mountains , including horsts and grabens These faults help the crust adjust to the stress created by vertical uplift.

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    Study Guide 1.8 Metamorphism and Mountain Cores Deep Changes Inside Mountains The core of a mountain range usually contains the most intensely metamorphosed rocks . These rocks: • 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 as schist or gneiss . 1.9 Magma and Crustal Thickness Some of the most extreme metamorphic rocks, called migmatites , are found in mountain cores. Large underground magma bodies, known as batholiths , also form during mountain building due to partial melting of the crust. The continental crust beneath mountain ranges is thicker than beneath stable cratons. Younger mountain ranges usually have thicker crust than older ones. Over time, the crust adjusts and stabilizes through isostatic adjustment , similar to how icebergs float higher when they are thicker. 1.10 Ac Ɵ vity in Young vs. Old Mountains • Young mountain belts are tectonically active and experience more earthquakes and volcanic activity . • Older mountain ranges are more stable and much less active. This difference reflects how recently the mountains were formed and how much tectonic energy is still present.

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    Study Guide 2. Types of Mountains Even though every mountain range is unique, geologists group mountains into four main structural types based on how they form. A single mountain range can include more than one type of mountain, so these categories often overlap. The four main types are: • Upwarped mountains • Volcanic mountains • Fault - block mountains (horst and graben) • Folded (complex) mountains 2.1 Upwarped Mountains Upwarped mountains form when large areas of the Earth’s crust are pushed upward in a broad arch . In some cases, they also form when there is major vertical movement along a steep (high - angle) fault . Key features of upwarped mountains: • Broad, dome - like shape • More rounded peaks

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