Stress Compressional Tensional Shear
Folding • Folds are formed by compression resulting in shortening and thickening of the crust. • Parts of a fold: 1) limbs - the two sides of a fold. 2) axial plane - an imaginary surface that divides a fold as symmetrically as possible, with one limb on either side of the plane. 3) fold axis - a line drawn along the points of maximum curvature of each layer. Also defined as the intersection of the axial plane with the beds. In some folds, the axis is completely horizontal. In more complex folding, the fold axis is often inclined at an angle known as the plunge - a plunging fold. http://pirate.shu.edu/~schoenma/mountains.htm
Types of fold • anticline - formed by upfolding, or arching, of rock layers. It is a structure in which the oldest strata are found in the center. 2) syncline - a downfold, or trough, with youngest strata in the center. Symmetrical when the limbs on either side of the axialplane diverge at the same angle and asymmetrical when they do not. Asymmetrical fold is said to be overturned if one limb is tilted beyond the vertical. Some folds are said to be plunging since the axis of the fold penetrates into the ground.
Fold mountains Formation: • two plates converge • two continental plates or • a continental and an oceanic plate. Usually found along continents’ edges • When the plates converge, the accumulated layers of rock crumple as sedimentary rocks are forced upwards into a series of folds young fold mountains (10 to 25 million years), (Rockies and Himalayas) old fold mountains (over 200 million years), (Urals and Appalachians)
The French Alps http://www.deskpicture.com/DPs/Places/FrenchAlps.jpg
Overturned Folding www.indiana.edu/~g103/G103/week9/wk9.html
Faulting Faults are fractures in the crust along which displacement has taken place. Faults can occur as single discrete breaks, or as part of a large fault system (the San Andreas fault has displacement of hundreds of kilometers and consist of many interconnecting fault surfaces). These so called fault zones can be several kilometers wide. Sudden movements along faults are the cause of most earthquakes.
Formation of Block Mountains by: TENSION COMPRESSION • Reverse faults • Rock layers are compressed • Central block is pushed upwards • Overhanging escarpment • Normal Faults • Rock layers are pulled apart • Side blocks are lowered relative to the central block Examples: Death Valley, California (USA), Black Forest (Germany) Examples: Sierra Nevada (USA), Flinders Range (Australia), The Tetons (USA)
Formation of Rift Valleys by: TENSION COMPRESSION • Normal Faults • Rock layers are pulled apart • Central block is lowered • Example: Great Rift Valley, East Africa • Reverse Faults • Rock layers are compressed • Side blocks are pushed up • Overhanging escarpment • Example: Great Glenn Fault, Scotland
Strike-Slip Faults • Dominant displacement horizontal and parallel to the strike of the fault plane. • Rather than a single fracture along which movement takes place, large strike-slip faults consist of a zone of roughly parallel fractures up to several kilometers in width. • They DO NOT produce a fault scarp, like the normal and reverse faults • Many major strike slip faults cut through the lithosphere and accommodatemotion between two large crustal plates - transform fault. • Examples: San Andreas Fault, Anatolian Fault (Turkey)
New Madrid http://www.showme.net/~fkeller/quake/images/tennesseeguy.wordpressblog.jpg California
http://pirate.shu.edu/~schoenma/mountains.htm Strike Slip fault http://www.geo.arizona.edu/~cparkjr/hebgen/page2.html
The Great Rift Valley System(Africa) • +9656 km crack (fissure) in the earth's crust, • stretching from Lebanon to Mozambique. • It divides Kenya into two segments. • formed by violent subterranean forces that tore apart the earth's crust. • forces caused huge chunks of the crust to sink between parallel fault lines and force up molten rock in volcanic eruptions. • this process, called rifting, is still in progress • many active and semi-active volcanoes; and hotsprings are along the Rift.
Stages of Formation • (1) The RIFT VALLEY stage involves early graben formation prior to continental splitting. This stage may be associated with domal uplift caused by uprise of hot upper mantle material - but this uplift is not ubiquitous and may be connected with underlying mantle hotspots. Example: African Rift Valley. • (2) The YOUTHFUL stage, lasting about 50 my after the onsett of seafloor spreading, while the thermal effects are still dominant. This stage is characterised by rapid regional subsidence of the outer shelf and slope, but some graben formation may persist. Example: Red Sea. • (3) The MATURE stage during which more subdued regional subsidence may continue. Example: most of the present Atlantic continental margins. • (4) The FRACTURE stage when subduction starts and terminates the history of the continental margin
The continent of Africa is thought to have been split by a series of rift valleys in various states of development. Those in East Africa are still in thick crust. Those in West Africa are associated with thick oil-bearing sediments. In the Red Sea area the rifting has gone so far as to form a narrow ocean. In the south-east Madagascar has been completely separated from Africa by rifting.
R e f e r e n c e s • http://geology.rutgers.edu/~feigy/LectureNotes/12StructuralGeol.pdf • http://www.berghuis.co.nz/abiator/patana/6t/index/humanities/mountains/geofaulting/flash/main.swf • Chong, Marianne. Aspects of Physical Geography • Waugh, David. Integrated Approach