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(destruction of crustal material)
a. subduction zone- a zone of plate convergence in which where an oceanic plate descends into the upper mantle beneath the overriding plate.
- where the plate descends at its lowest point, a "deep ocean trench" may result and may be up to 6-7 miles deep.
b. Oceanic-Continental Plate Convergence- ocean crust(basalt) is more dense than continental crust (granitic) and thus tends to sink more readily over semi-plastic aesthenosphere, thus oceanic crust downbends and subducted beneath continental crust, thin layer of mud/sediment on oceanic plate is scraped, folded and deformed at the interface of the overlying plate.
- subduction-related volcanism and volcanic arcs as oceanic slab descends into upper mantle, it is heated up to point that it will begin to melt, buoyant less dense magma may develop and accumulate until it rises into overlying continental crust, part of it will cool intrusively and part may extend to the surface of earth to form vocanoes.
a) volcanic arc - an arcuate chain of volcanoes on continental crust that result from subduction of oceanic crust beneath continental crust.
e.g. Cascade mountains in U.S., Andes in SAM, Sierra Mtns in CA are eroded core of volcanic arc.
c. Oceanic-Oceanic Convergence- one oceanic slab is subducted beneath another oceanic slab, similar to above only volcanic activity occurs on overriding oceanic slab, resulting in island arcs as opposed to volcanic arc. Volcanoes build up until they form islands at sea (e.g. Japan islands, mariannas islands)
d. Continental-Continental Convergence- case where 2 plates carrying continental crust collide, results in cont.-cont. collision (e.g. India to China resulting in Himalayas, or Europe and NAM ....Appalachians). Involves seafloor between continents being subducted until collision occurs, during collision sediment and rock strata are squeezed and deformed, and buckled into mountains. Results in complicated mess, with volcanics too.
- Transform Fault Boundaries: condition where plates slide horizontally past one another along a fault (or fracture along which there is movement) Crust is neither consumed nor destroyed. Transform faults connect convergent and divergent boundaries into a worldwide network of interconnected plate boundaries. Most transform faults are located along divergent boundaries, but some do intersect continents (e.g. San Andreas Fault in Ca).
IV. Plate Tectonics: Modern Version of Wegner's Continental Drift Hypothesis
A. Development of Modern Theory- during the period of mid-1900's many advances were made in technology regarding the study of the earth, particularly during after WWII. Many techniques were developed in which to map the ocean floor (out of search for German submarines with magnetometers and sonar) and to monitor seismic or earthquake activity around the world.
B. Evidence which turned the tide in support of Wegner's early ideas and the modern concept of plate tectonics
- Magnetism and Paleomagnetism (result of search for German submarines in WWII)
a. Basics of Magnetism
Earth has a magnetic field about it with a magnetic north pole and south pole similar to a bar magnet with lines of magnetic force flowing from North to south. Magnetic poles align with geographic n and s poles, a compass needle is a free magnet and will align itself with the earths north pole.
Paleomagnetism - iron-rich minerals such as magnetite (Fe3O4) act as tiny magnets, when these minerals cool from a magma there is a temperature at which they align with the magnetic field of the earth (curie point), and once totally coolen, they are frozen compass record of the earth's magnetic field. Paleomagnetism is hence fossil magnetism of the earth recorded in rock record. Thus if a rock moves after it has recorded the earths magnetism, it is then possible to reconstruct its original position.
b. Polar Wandering - a sequence of lava flows of similar age were studied in Europe and U.S. during 50 and early 60's. Their paleomagnetic signature was examined and the apparent magnetic north pole which these rocks suggested was plotted on a global map. It was found that the magnetic north poles were not located where they are today, so either the Poles have wandered from where they were, or the continents have drifted. When similar aged rocks from U.S. and Europe were examined through time, it was found that they exhibited separate polar wandering paths which converged together at at approximately 250 m.y., suggesting that the continents have migrated instead of the poles. Once the continents stop migrating then the poles matched up. If we reconstruct the continents across the Atlantic Ocean then they each have same pole orientation.
c. Magnetic Reversals
Following WWII, it was discovered that the orientation of the earth's magnetic poles have not remained constant, that they have reversed many times in the past, i.e. the magnetic north pole has switched to the south pole and vice versa.
Normal Polarity -So rocks have been found with paleomagnetism similar to todays polar arrangement termed "normal" polarity
Reversed Polarity- rocks which indicate magnetic north pole at current position of south magnetic pole
Magnetic reversals at oceanic spreading centers or ocean ridges- upon investigating paleomag across ocean spreading centers, it was discovered that patterns of normal and reversed polarity rocks of similar age are symmetrically arranged about a spreading center or oceanic ridge. suggests that new ocean crust is continually being formed at spreading centers and is symmetrically moved outward from center through plate movement.
- Evidence from Seismic Records of Earthquakes
a. the distribution of earthquake focii or origination points of earthquakes was examined around the world and at convergent plate boundaries or subduction zones. See
- or Mountains may form continuous chains extending for thousands of miles (e.g. western Cordillera region of SAM and NAM, near continuous mountain chain running from Tierra Del Fuego to Alaska).
- Mountains may be youthful (steep and of large relief) (e.g. Himalaya's) or mature (gently sloping and of relatively low relief) (e.g. Appalachian's).
B. Orogenesis: refers collectively to the processes that result in the elevation of land areas to form mountains.
C. Lecture examines the processes that result in mountain building, presented in the context of plate tectonic theory.
II. Crustal Uplift
A. Evidence that continental crust of earth has been uplifted in mountainous areas:
- presence of fossilized shells of sea critters found on top of mountains, suggests that between the time that the critters lived and the time they were found: that the animals died, were buried by sediment/mud, lithified into sed. rock, and subsequently deformed and uplifted into mountains.
- wave-cut terraces along recently uplifted portions of the coast of California. Ancient benches that were cut by wave erosion are now elevated hundreds of meter above sea level.
B. Isostasy- the concept of floating lithosphere in gravitational balance. I.e. density contrasts between less dense crust/lithosphere and more dense aesthenosphere results in the lithosphere "floating" on top of the aesthenosphere.
- mountainous areas are thickened portions of continental crust that extend deep into the aesthenosphere (roots of mountains).
- oceanic crust on the other hand is denser than continental crust and is thinner, thus owing to its relatively low isostatic position relative to the aesthenosphere.
- thus according to idea of isostacy, loading of crust will result in subsidence (volcanic piling, deposition of sediment in a basin, thrust loading), and unloading of the crust will result in uplift or bobbing up of crust (glacial melt, erosional denudation of landscape).
- Thus as Mountain Ranges undergo denudation, erosional unloading results in isostatic adjustment and continued uplift of the mountains, isostatic adjustments, and further erosion as mountainous crust is thinned and deepest portions of mountains are exposed through time.
III. ROCK DEFORMATION- process of rocks becoming physically deformed as they are subjected to stress
A. Plastic vs. elastic vs. brittle deformation of rocks: rocks may respond to stress in the form of folding like paper (plastic deformation) or fracturing into blocks (brittle deformation) or may deform elastically (i.e. given volume of rock will return to its original size and shape after
stress is removed)
- brittle deformation of rocks is rather easy to recognize, analogous to hitting concrete with sledge hammer. Conditions of stress result in fracturing or rupturing of rocks.
- elastic: stress is applied slowly under constant pressure, rocks return to original size and shape after stress is removed.
- plastic deformation: a set of conditions must be met before rocks will deform plastically
a. relative heat, constant pressure, and time
- Generally: as stress is applied to rocks at low temp, and low press, rocks will first deform elastically (with ability to return to original size and shape once stress is removed), once the level of stress exceeds the elastic limit of a given type of rock (i.e. the point or strentgh of a rock, with stress beyond which rock will fail), it will then either deform plastically or brittly.
IV. FOLDING OF ROCKS OR ROCK STRATA (LAYERS OF ROCKS)
A. Under components of horizontal stress: flat-lying layers of sedimentary/volcanic rocks may become bent into a series of folds (analogous to pushing and folding sheet of paper).
- folding process results in shortening and thickening the crust
B. Fold Types
- Anticlines-upfolded forms, results in older rocks becoming enclosed within younger strata
- synclines-downfolded forms, results in younger rocks becoming enclosed within older strata.
a. symmetrical folds - both limbs of the fold dipping at same angle away from fold axis
b. asymmetrical folds - both limbs of the fold not dipping at same angle away from fold axis
c. overturned folds - condition in which one limb of fold has been tilted beyond vertical
d. plunging folds- axis of fold is tilted
- Domes- more or less circular equivalent of anticline, oldest rocks exposed in center of dome
- Basin- more or less circular equivalent of syncline, youngest rocks exposed in center of dome
C. Outcrops Patterns Associated with Folded Rocks
plane.
a. Strike - the trend or compass direction of the line formed between the intersection of a horizontal plane with any inclined plane.
- Oblique-slip faults- faults which have both vertical and horizontal components of movement.
- Stress Regimes and Style of Faulting
a. Reverse/Thrust Faults- often associated with compression or squeezing of crustal blocks, rupture results when stress>strength of rocks. E.g. in association with convergent tectonic zones.
b. Normal Faults- associated with "pulling apart" or tensional forces exerted on crustal blocks. E.g. in association with rift zones or spreading centers in plate tectonics.
- Grabens- crustal block bounded by two inward-dipping normal faults, crustal block downdrops to form a graben.
- Horst- relatively uplifted crustal block flanked by two adjacent grabens.
B. Joints-in contrast to faults- fractures along which no appreciable movement has taken place.
- joints serve as a by-product, or structural features that accommodate stress during tensional and shear stresses associated with crustal movements.
- joints often occur in very low-stress regimes, with broad, gentle warping of earth's crust.
- joints often serve as sights of enhanced weathering processes, may result in streams and rivers following their trends. VI. MOUNTAIN TYPES
A. Folded Mountains (aka "complex mountains")
- Mountain relief a result of erosion and dissection of portions of the earth's crust that has been folded and thickened.
- Fold belts are also commonly associated with faulting, metamorphism, and igneous intrusion; although folding is the most conspicuous deformation style.
- E.g. Alps, Himalaya's, Appalachian Mountains
B. Fault-Block Mountains
- Associated with erosion and dissection of portions of the earth's crust that has been displaced and tilted along high-angle normal faults (in associatin ith tensional stresses)
