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MOUNTAIN BUILDING

MOUNTAIN BUILDING. Chapter 20. Crust – Mantle Relationships. Mountains are spectacular features that can occur as individual peaks or immense ranges that cover many kilometers of landscape

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MOUNTAIN BUILDING

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  1. MOUNTAIN BUILDING Chapter 20

  2. Crust – Mantle Relationships • Mountains are spectacular features that can occur as individual peaks or immense ranges that cover many kilometers of landscape • How these geographic features are supported relies solely on the relationships between the Earth’s crust and the underlying mantle • 70% of the Earth’s surface is below sea level and 30% lies above the Earth’s surface

  3. Earth’s Topography • Most of the Earth’s topography follows a pattern of two modes of elevation: • 0 to 1 km above sea level • 4 to 5 km below sea level • These two modes reflect basic differences in density and thickness between continental and oceanic crust • Oceanic crust is made of basalt, which is more dense than continental crust (made of granite) • Although oceanic crust is more dense, the continental crust extends deeper into the mantle under mountain ranges due to the thickness of the mountain ranges • Likewise, the continental crust extends higher above the Earth’s surface due to its lower density reading than oceanic crust

  4. Continental Crust is less dense and thicker than oceanic crust, so it extends higher above Earth’s surface and deeper into the mantle

  5. ISOSTAsY • The displacement of the mantle by Earth’s continental and oceanic crust • The crust and mantle are in equilibrium with one another; meaning, the force of gravity on the mass of crust (mountain range) is balanced by buoyancy • A good model for isostasy is the water line of a boat when someone boards or leaves the boat

  6. Isostasy and Erosion • As the mountain peaks are eroded, mass decreases and the roots become smaller. The balance between erosion and the decrease in root size will continue for hundreds of millions of years until both mountains and roots disappear. (Ex. The Ozarks in Missouri) • The slow process of the crust’s rising due to removal of overlying material is called isostatic rebound. This is the reason that some mountain ranges, such as the Appalachian Mountains, still exist today!

  7. Appalachian Mountains The Appalachian mountains are the oldest mountain range on Earth and are believed to have once been connected to Europe as evidence shows similar geologic structures and rocks in this region

  8. Processes that build mountains • Mountains form as result of tectonic interactions. • Orogeny is the process that forms all mountain ranges, which results in broad, linear regions of deformation known as orogenic belts. • The greatest variety and tallest belts are found at convergent boundaries.

  9. Convergent – Boundary Mountains • At the convergent boundaries, two plates come together causing intense deformation of land. • Examples of deformation include folding, faulting, metamorphism, and igneous intrusion. • Each type of convergent boundary makes a different type of mountain range.

  10. Oceanic – Oceanic Convergence • When oceanic plates collide with one another, one goes down into the mantle to form a subduction zone • As parts of the subducted plate melts, magma is forced upward to form volcanic peaks referred to as an island arc

  11. Oceanic – Continental Convergence • This boundary is similar to oceanic-oceanic because the convergence of the two plates creates a subduction zone and trenches • However, major mountain belts can result from this convergent boundary • As the oceanic plate subducts it pushes up the continental plate, resulting in the beginning of orogeny • At these boundaries, compression causes the continental crust to fold and thicken

  12. Continental – Continental Convergence • Forms Earth’s tallest mountain ranges • Due to low density of the plates, neither subduct. Instead, both plates receive energy transfer from the collision. This causes the crust to become highly folded and faulted • Famous example = the Himalayas

  13. Other Types of Mountains • Divergent Boundary Mountains • Magma is less dense than surrounding material so it is forced upward and warms the overlying lithosphere. The lithosphere then bulges upward and is higher than the surrounding oceanic crust to form a gently sloping mountain range • Ocean ridge mountain ranges can be thousands of kilometers wide and are composed primarily of igneous rocks • Sometimes, magma pushes through plutons and erupts onto the seafloor to form igneous rocks called pillow basalts

  14. Nonboundary Mountains • Sometimes, mountains and peaks form in places far removed from tectonic boundaries • Three nonboundary types of mountains include: • Uplifted Mountains • Fault-Block Mountains • Volcanic Peaks

  15. Uplifted Mountains • Form when large regions of Earth have been slowly forced upward as a unit • The Adirondacks in New York

  16. Fault – Block Mountains • Form when large pieces of crust are tilted, uplifted, or dropped downward between large faults • Grand Tetons in Wyoming

  17. Volcanic Peaks • Volcanoes that form along oceanic-continental convergent margins are typically part of long mountain ranges • Volcanoes that form over hot spots are solitary peaks that form far from plate boundaries • Volcanic peaks of Hawaii formed over hot spots, such as Mauna Kea

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