Ch 20 isostasy, orogeny, mountain building Name ________________________________ 8th Grade Earth Science Mrs. Krempa Date & Period _________________________ Use your textbook pages 561 - 576 to answer the following questions. 1. Topography: _____ variation in elevations of the crust ________________________________________ What does a topographic map show? _____ an area’s hill & valleys _______ 2. Isostasy: the displacement of the Mantle by Earth’s continental & oceanic crust in a state of equilibrium a. The crust and mantle are in equilibrium when the downward force of gravity on the crust is balanced (equal) to the upward force of buoyancy (float) on the mantle. (caused by the displacement of the mantle by the mass of the crust) b. A good example of this concept (equilibrium) is when people get in and out of a small boat. Explain: People get in a boat? Boat sinks deeper / displaces more water. People out of boat? Boat rises / displaces less water (people’s mass now out of boat). Similar to sinking and rising of Earth’s mantle depending on how much crust is sitting on it. Mountains = a lot of displacement, areas where crust is thin = less displacement 3. What is a “root”? (last paragraph on pg. 563) thickened areas of continental material that extend into the mantle below Earth’s mountain ranges 4. What purpose do mountain roots serve? Counterbalance the enormous mass of the mountain range above Earth’s surface 5. Isostatic rebound: slow process of the crust rising as the result of the removal of overlaying material a. In the process, mountains are _____ eroded ______ over hundreds of millions of years, while the crust below them ______ rises ___________. (weight of mountain being taken off of crust, crust can now expand back up – isostatic rebound!) 6. Orogeny: _______ all processes that form mountain ranges ____________________________________ 7. Divergent-Boundary Mountains a. These underwater volcanic mountains form a continuous chain the snakes along Earth’s ocean floor b. As a result of the decrease in density of the new oceanic crust as it cools, contracts and subsides, the crust stands highest ________ where the ocean crust is the youngest__________ and makes the underwater mountain chains have _________ gently sloping sides___________ 8. Uplifted Mountains a. Form when: _____ large regions of Earth have been slowly forced upward as a unit ___________ b. Have rocks that undergo: ____ less deformation than other building processes ___________ c. Are the result of (popular hypothesis): a part of the upper mantle (lithosphere) cools, becomes dense & sinks into the underlaying mantle. This “chunk” of cooled mantle provides buoyancy & vertical lift for the crust on top – making a “bulge”. d. Examples: ____ Sierra Nevada Mountains (in CA), Colorado Plateau______________________ 9. Fault-Block Mountains a. Form when: faults lift & stretch the crust and large pieces of crust are dropped downward due to tensional forces Explain: When Earth’s crust is stretched… ____ a series of normal faults can form _______ b. Are not necessarily associated with ____ plate boundaries __________________ c. Examples: Basin & Range Province (SW USA & Norther Mexico) Grand Teton Mountains (WY) d. Some of these mountain ranges consist of hundreds of nearly parallel mountains separated by normal faults .