Mod8_GEOL101_MountainBuilding

Mountain Building

Introduction

"Go tell it on the mountain" Mountain building, also known as orogeny, refers to the geological processes that create mountain ranges and modify the Earth's surface.

Key Concepts

• Orogenesis: The process that produces a mountain belt, typically associated with convergent plate boundaries. This involves the collision of tectonic plates (e.g., North American Plate and Eurasian Plate) that leads to the folding, faulting, and uplifting of the Earth's crust.

• Orogeny: Specific mountain-building events that occur at a distinct time and place, characterized by movements such as compressional folds, faults, metamorphism, and igneous activity that can result in significant geological features.

Why are Mountains High?

• Principle of Isostasy:

  • Isostatic Principle: The less dense crust floats atop denser mantle rocks due to gravitational equilibrium. This balance is crucial in understanding why mountains have significant elevations compared to surrounding landscapes.

  • Isostatic Adjustments: Changes in the mass of the crust (erosion or accumulation) necessitate adjustments to maintain equilibrium, involving processes like crust subsidence or rebound, resulting in gradual topographic changes.

Isostatic Adjustment

• Upon the release of compressional stress:

  • Erosion tends to dominate over uplift, leading to a gradual wearing down of mountainous regions while their buoyant roots rise toward the surface.

  • Deformation of the crust can also thicken it, causing lateral displacement of the mantle beneath, thereby influencing the overall elevation of mountain ranges.

Types of Mountains

• Collisional Mountains: Formed primarily from tectonic plate collisions, resulting in complex geological structures.

  • Examples: Fold and thrust belts such as the Himalayas (Asia) and the Rocky Mountains (North America), specifically seen in Colorado and Wyoming.

• Volcanic/Batholithic Mountains: Result from volcanic activity and large igneous intrusions, creating features like Mount St. Helens (Washington) and Mount Rainier (Washington).

• Block Mountains: Created from tectonic forces pulling the Earth’s crust apart, such as the Sierra Nevada mountains (California).

• Extensional Mountains: Formed via normal faults during rifting, resulting in mountain uplifts such as those seen in the Wasatch Range (Utah) and adjacent basins.

Major Mountain Belts

• Earth's Major Mountain Belts:

  • Include the Greenland Belt, Angara Shield, Baltic Shield, Canadian Shield, North Shield, Alps (Europe), Altay-Sayan (Mongolia), Tien-Shan (Central Asia), Caucasus (between Europe and Asia), Shan (Southeast Asia), Atlas Mountains (North Africa), Zagros Mountains (Iran), Himalayas, Indian Shield, Orinoco (South America), African Shield, Brazilian Shield, and the Australian Old Mountain belts.

• Young Mountain Belts: Formed within the last 100 million years, such as the Himalayas, still tectonically active today.

• Shields: Stable regions generally covered by sedimentary rock, with adjacent basins indicating varying resource distributions.

Building North America

• Major Geological Formations:

  • The Greenland Shield, Canadian North Shield, and buried Appalachian Mountains (eastern U.S.) witness ancient geological processes that shaped the continent.

• Precambrian Craton: The majority of North America is underpinned by ancient Precambrian rocks from geological collisions over immense periods, found in areas like the Canadian Shield.

• Rhodinia Supercontinent: Began breaking up around 600 million years ago, including land masses such as North America, South America, Africa, Florida, Baltica, Australia, and East Antarctica.

Evolution of North America

• Around 450 million years ago: The Great Ordovician Bank formed, characterized by extensive carbonate platforms in areas that are now parts of eastern North America.

• Eastern North America: Experienced geological events leading to rifting and continent-continent collisions, forming the North Atlantic Ocean.

Formation of the Appalachians

• Resulted from multiple geological events:

  • Initial collision with small crustal blocks (around 600 Ma and again at 450 Ma) leading to metamorphism and uplift of mountains, influencing places such as Virginia and West Virginia.

  • Acadian Orogeny: Involves collisions of microcontinents which caused thrust faults and granite intrusions, marked by significant formations in the Catskill Mountains.

  • Alleghenian Orogeny: Formed from substantial continental collision, resulting in intricate folding and thrust faults in the structures of the Appalachians, particularly visible in regions like Pennsylvania and Kentucky.

Summary of Appalachian Formation

• Transitioned from an initial rifting and passive margin to significant collisions with land masses, eventually leading to the rifting that formed the Atlantic Ocean. The Ouachita Mountains (Arkansas and Oklahoma) and Marathon Mountains (Texas) are examples of geological formations arising from collision and continental breakup, illustrating similar processes in North America's geological evolution.

Western US Geological History

• Involved initial rifting and passive margins, progressing to continental arcs, subduction zones, and collisions forming mountain ranges like the Rockies and Sierra Nevada.

• Sevier Orogeny: Characterized by thin-skinned deformation in regions like Utah and Nevada, where mountain-building processes are evident.

• Laramide Orogeny: Involved thick-skinned tectonics with significant reverse faults that uplifted mountains dramatically, influencing areas like Wyoming and the Black Hills of South Dakota.

Basin and Range

• Characterized by extensive block faulting and unique crustal extension, resulting in a landscape of alternating mountains (such as the Sierra Nevada) and basins (like the Great Basin). Key processes include thrusting and gravitational collapse, influencing geological changes throughout the region.

Conclusion

The complex mountain structures of North America elucidate the intricate tectonic activities and dynamic nature of Earth's crust, showcasing phenomena such as plate tectonics, collisional events, and isostatic adjustments, with visible effects across the continent.