Mountain Building

1. Mountain Building

2. Vocabulary • Orogenesis • Oros—mountain • Genesis—to come into being • Lithosphere • The rigid outer layer of Earth, including the crust and upper mantle • Accretion • the increase in size of a tectonic plate by addition of material along a convergent boundary

3. Crustal Uplift • Evidence • Marine fossils often found in high elevations in mountains • Terraces hundreds of meters above sea level • Isostasy • A floating crust in gravitational balance • Example: blocks of wood floating in water • Mountain belts stand higher above the surface of the Earth and have roots that extend deeper into the supporting material below. • Crustal thicknesses for some mountain chains are twice as much as the average for the continental crust

4. Crustal Uplift • Crust beneath the oceans is thinner than that beneath the continents • Oceanic rocks are denser than continental rocks **adding weight to the crust makes it subside **when weight is removed, crustal uplifting occurs (ex.: cargo ship)

5. Crustal Uplift • Isostatic Adjustment • Ice Age glaciers added weight to the continents, making them downwarp by hundreds of meters • When glaciers melted, uplift occurred • Erosion of mountains causes uplift, also

6. Rock Deformation • Elastic deformation • When stress is applied, rocks bend, but will snap back if the stress is relieved • Plastic deformation • When the elastic limit is surpassed, rocks deform plastically or break (earthquakes) • They are permanently altered through folding and flowing

7. Rock Deformation • Folds • When flat-lying sedimentary and volcanic rocks are bent into a series of wavelike undulations • Example: pushing on one edge of a carpet until it folds • Anticline • Upfolding or arching of rock layers • Syncline • Downfolds, or troughs

8. Rock Deformation • Dome • When upwarping produces a circular or somewhat elongated structure • Basin • When downwarping produces a circular or somewhat elongated structure

9. Rock Deformation • Faults and Joints • Fractures in the Earth’s crust • Dip-slip faults • Vertical movement • Hanging wall—rock that is higher than the fault surface • Footwall—rock that is lower than the fault surface • Normal—hanging wall moves downward relative to the footwall • Reverse—hanging wall moves upward relative to the footwall • Thrust faults—have a very low angle

10. Rock Deformation • Strike-slip faults • The dominant displacement is along the strike or trend, of the fault (horizontal) • Transform faults—associated with plate boundaries • Oblique-slip faults—both vertical and horizontal movement • Tensional forces—pull the crust apart • Graben—central block bounded by normal faults; drop as the plates separate • Horsts—upfaulted structures that are adjacent to graben • Compressional forces—sections of crust are displaced toward one another

11. Rock Deformation • Joints • Fractures along which no appreciable displacement has occurred • Columnar joints form when igneous rocks cool and develop shrinkage fractures, producing elongated, pillarlike columns • Sheeting produces a pattern of gently curved joints that develop more or less parallel to the surface of large exposed igneous bodies.

12. Mountain Types • Fault-block mountains • Tensional stresses elongate and fracture the crust into numerous blocks. Movement along the fractures tilt the blocks producing parallel mountain ranges.

13. Mountain Types • Folded mountains (complex mountains) • Upwarped mountains • Caused by a broad arching of the crust or because of great vertical displacement along a high-angle fault • Volcanic mountains

14. Mountain Building • Convergent boundaries • Volcanic arcs are forming in most modern-day subduction zones • Aleutian-type subduction zones occur where two oceanic plates converge

15. Mountain Building • Andean type subduction zones • Passive continental margin—part of the same plate as the adjoining oceanic crust • Becomes active—subduction zone forms and the deformation process begins • The oceanic plate descends and becomes magma while there is an accumulation of sedimentary and metamorphic rocks along the subduction zone (accretionary wedge)

16. Mountain Building • Continents converge • Continental lithosphere is too buoyant to undergo subduction, a collision eventually results • Example: India colliding with the Eurasian plate

17. Mountain Building • Mountain Building and Continental Accretion • Smaller crustal fragments collide and accrete to continental margins • Example: mountainous regions rimming the Pacific • As oceanic plates move, they carry with them embedded oceanic plateaus or microcontinents • The upper portions of these thickened zones are peeled from the descending plate and thrust in relatively thin sheets onto the adjacent continental block. • This increases the width of the continent • Terrane—accreted crustal blocks