Download Earth Science Chapter 20: Mountain Building Chapter Overview

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.