Crust-Mantle Relationships & Orogeny

Biblical Reference Your father’s blessings are greater than the blessings of the ancient mountains, than the bounty of the age-old hills. Genesis 49:26

Topography Topography is the variations in elevation of Earth’s crust. Topographic maps show differences in elevation on Earth’s surface.

Earth’s Topography When Earth’s topography is plotted on a graph, the is a distinct pattern in the elevations. Most of Earth’s elevations cluster around two ranges: 0 to 1 km above sea level 4 to 5 km below sea level

Earth’s Topography Graph The two main elevation ranges reflect the basic differences between continental and oceanic crust.

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

Isostasy The displacement of the mantle by Earth’s continental and oceanic crust is called Isostasy. The crust and mantle are in equilibrium when the downward force of gravity on the mass of crust is balanced by the upward buoyant force that results from the displacement of mantle by the crust.

Roots Continents and mountains “float” on the mantle, because they are less dense than the underlying mantle. Gravitational and seismic studies have found thickened areas of continental material, called Roots, that extend into the mantle below Earth’s mountain ranges. A mountain range needs large roots to counterbalance the enormous mass of the range above Earth’s surface.

Roots Scientists estimate that Mount Everest’s root is nearly 70 km thick.

Erosion & Isostasy The Appalachian Mountains formed hundreds of millions of years ago when the North American Continent collided with Europe and Africa. As the Appalachian Mountains rose above Earth’s surface, deep roots formed to buoyantly support them into isostatic equilibrium. As the peaks eroded, the mass decreased, and the roots rose and eventually eroded.

Isostatic Rebound A balance between erosion and decreased root size continues for hundreds of millions of years until the roots are exposed above the surface. The slow process of rising crust and the removal of overlying material is called Isostatic Rebound. Metamorphic rocks formed deep within the crust are able to rise to the surface in the Appalachian Mountains due to erosion and isostatic rebound.

Seamounts Seamounts are individual mountains created by hot spots under the ocean floor. The oceanic crust around these peak displaces the underlying mantle until equilibrium (isostasy) is achieved.

Orogeny Orogeny is the term used to describe all of the processes that form mountain ranges. It comes from the Latin roots “Oros” (mountain) and “Gen” (to make). The geologic term for mountain ranges is Orogenic Belts.

Convergent Boundaries Most of Earth’s mountain ranges formed along convergent plate boundaries.

Convergent Boundaries At convergent plate boundaries, compressive forces squeeze the crust and cause folding, faulting, metamorphism and igneous intrusions. Each type of convergent boundary creates a different type of mountain range.

Oceanic-Oceanic Convergence When two oceanic plates converge, they form individual peaks of a volcanic island arc like the Lesser Antilles.

Island Arc Complexes Island Arc Complexes partly composed of basaltic and andesitic magmas. Some large island arc complexes also contain sedimentary rocks formed from sediments of eroded island arc material.

Oceanic-Continental Convergence Oceanic and continental plates converge to form much larger and complicated mountain belts. At an oceanic-continental boundary, the descending oceanic plate forces the edge of the continental plate upward, marking the beginning of orogeny. At an oceanic-continental boundary, compressions causes continental crust to fold and thicken. Igneous and metamorphic activity is common.

Oceanic-Continental Convergence

Continental-Continental Convergence Because of its relatively low density, continental crust cannot be subducted into the mantle when two continental plates converge. Intense folding and faulting along continental plate boundaries produce some of the highest mountain ranges on Earth. Marine sedimentary rocks are often found at the summits of these ranges. The marine sediments existed in the basins between the continents before they collided.

Continental-Continental Convergence

The Appalachian Mountains Geologists have divided the Appalachian Mountains into several distinct regions. Each region is characterized by rocks that show different degrees of deformation.

Appalachian Mountain History Between 800 to 700 mya North America separated from Africa, forming the Atlantic Ocean and a shallow sea. A continental fragment was between the two divergent boundaries. About 700 to 600 mya, the plate motion reversed, forming a volcanic island arc east of North America.

Appalachian Mountain History About 400 mya, the fragment became attached to North America, thrusting metamorphic rocks over younger rocks to form the Blue Ridge Province. Between 400 to 300 mya, the island arc became attached to North America, forming the Piedmont Province and pushing the Blue Ridge Province further west.

Appalachian Mountain History Between 300 and 260 mya, the Atlantic Ocean closed, because Europe, Africa and South America collided with North America to form Pangaea. The extensive folding created the Valley and Ridge Province. When Continents Collide

Pop Quiz Which place on Earth is an example of Oceanic-Oceanic Convergence Zone and Island Arc? A. The Himalayas B. The Appalachians C. The Lesser Antilles D. The Andes

Pop Quiz Which feature normally associated with oceanic-continental convergence is NOT associated with continental-continental convergence? A. Faults B. Folds C. Mountains D. Volcanoes

Pop Quiz Which land masses collided to form the Himalayas? A. Africa and Asia B. North America and Europe C. South America and Africa D. Africa and Australia