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Transcript
Earth Science Chapter 20: Mountain Building Chapter Overview Section 1 – Crust – Mantle Relationships 1. Earth’s Topography • When Earth’s topography is plotted on a graph, the bulk of Earth’s elevation is clustered around two ranges: 0 to 1 kilometer above sea level and 4 to 5 kilometers below sea level • Elevation is partially determined by the difference in density between oceanic crust and continental crust and partially determined by the thickness of the crust 2. Isostasy • The displacement of the mantle by Earth’s continental and oceanic crust is a condition of equilibrium called isostasy. The crust and mantle are in equilibrium when the force of gravity on the mass of crust involved is balanced by the upward force of buoyancy • Gravitational and seismic studies have detected thickened areas of continental material, called roots, that extend into the mantle below Earth’s mountain ranges 3. Isostasy and Erosion As peaks are eroded, mass decreases, and the roots of the mountains become smaller. A balance between erosion and the decrease on the size of the root will continue for hundreds of millions of years until both the mountains and their roots disappear. This slow process of the crust’s rising as the result of the removal of overlying material is called isostatic rebound Section 2: Orogeny 1. Mountain Building at Convergent Boundaries The processes that form all mountain ranges are called orogeny. Orogeny results in broad, linear regions of deformation known as orogenic belts, which are usually associated with plate boundaries. The greatest variety and tallest of these belts are found at convergent boundaries • Oceanic-Oceanic Convergence When an oceanic plate converges with another oceanic plate, one plate descends into the mantle to create a subduction zone. As parts of the subducted plate melt, magma is forced upward to form a series of volcanic peaks called an island arc complex • Oceanic-Continental Convergence When an oceanic plate converges with a continental plate, the descending oceanic plate force the edge of the continental plate upward. This uplift marks the beginning of orogeny. In addition to uplift, compressive forces may cause the continental crust to fold and thicken. As the crust thickens, higher and higher mountains form. • Continental-Continental Convergence Because of its relatively low density, continental crust is not subducted. Instead, the energy associated with the collision is transferred to the crust causing it to become highly folded and faulted. Compressional forces break the crust into thick slabs that are thrust onto each other along low-angle faults, which can double the thickness of the deformed crust Section 3: Other Types of Mountain Building 1. Divergent Boundary Mountains Ocean ridges are regions of very board uplift that seems to be related to the rising convection cells that form deep in the mantle beneath these ridges. • Ocean-Ridge Rocks Ocean ridges are composed mainly of igneous rocks. The magma pushes through dikes and erupts onto the seafloor to form igneous rocks called pillow basalts 2. Uplifted Mountains Some mountains form when large regions of Earth have been slowly forced upward as a unit; these mountains are called uplifted mountains. The cause of large-scale regional uplift is not well understood. Regional uplifts can form broad plateaus; erosional forces eventually carve these plateaus to form mountains, valleys, and canyons 3. Fault-Block Mountains Fault-block mountains form when large pieces of crust are tilted, uplifted, or dropped downward between large faults.