Geomorphology ch 5

Superplumes

A superplume occurs when a large mantle upwelling is convected to the Earth's surface. A superplume should not be confused with a hot-spot. Although similar, a superplume forms at the mantle-core boundary while a hot-spot occurs at the mantle-crust layer. Superplumes create cataclysmic events that affect the whole world when they explode. A superplume forms at the core-mantle boundary and confection forces the superplume through entire mantle until the superplume meets the crust. The world changes when the superplume explodes. Superplumes can occur anywhere, even in the middle of continental plates. In order to get a core-mantle upwelling to the Earth's surface, the thrust force of a downwelling must have occurred. The downwelling comes form a subduction zone that has cooled magma that has sunk into the earth. All this movement in the Earth's body causes plate tectinoc to go into full throttle. The Earth's fueling of tectonics forces an increase in oceanic crust formation. Superplumes increase world temperatures by the release of CO2 into the atmosphere. This phenonmenon is also called the greenhouse effect.

active plates

An active continental margin is found on the leading edge of the continent where it is crashing into an oceanic plate. An excellent example is the west coast of South America. Active margins are commonly the sites of tectonic activity: earthquakes, volcanoes, mountain building, and the formation of new igneous rock. places, as along the western edge of the American continents, continental margins coincide with plate boundaries and are active margins.

exotic or suspect terranes

As they may come from a different continent from the one they are attached to. Most of the western seaboard of North America appears to consist of these exotic terranes.

rugged mountainous areas

Below great escarpments, rugged mountainous areas form through the deep dissection of old plateaux surfaces. Many of the world's large waterfalls lie where a river crosses a great escarpment, as in the Wollomombi Falls, Australia.

intercontinental collision orogens.

Continent-continent collisions create intercontinental collision orogens. A splendid example is the Himalaya. The collision of India with Asia produced an orogen running over 2,500 km.

modified passive continental margin

Continent-island arc collisions occur when continents drift towards subduction zones connected with intra-oceanic island arcs. The continent resists significant subduction and a modified passive continental margin results. Northern New Guinea may be an example.

pull-apart basins

Convergent and divergent forces occur at transform margins. Divergent or transtensional forces may lead to pull-apart basins, of which the Salton Sea trough in the southern San Andreas Fault system, California, USA, is a good example

transverse orogens

Convergent or transpressional forces may produce transverse orogens, of which the 3,000-m San Gabriel and San Bernardino Mountains (collectively called the Transverse Ranges) in California are examples (Figure 4.12b). As transform faults are often sinuous, pull-apart basins and transverse orogens may occur near to each other. The bending of originally straight faults also leads to spays and wedges of crust.

structural landforms

Few landforms are purely tectonic in origin: exoge- nous forces - weathering, gravity, running water, glaciers, waves, or wind - act on tectonic landforms, picking out less resistant rocks or lines of weakness, to pro- duce structural landforms. An example is a volcanic plug, which is created when one part of a volcano is weathered and eroded more than another. A breached anticline is another example.

summary

Geological processes and geological structures stamp their marks on, or in many cases under, landforms of all sizes. Plate tectonic processes dictate the gross landforms of the Earth - continents, oceans, mountain ranges, large plateaux, and so on - and many smaller landforms. Diastrophic forces fold, fault, lift up, and cast down rocks. Orogeny is a diastrophic process that builds mountains. Epeirogeny is a diastrophic process that upheaves or depresses large areas of continental cores without causing much folding or faulting. The bound- aries of tectonic plates are crucial to understanding many large-scale landforms: divergent boundaries, convergent boundaries, and transform boundaries are associated with characteristic topographic features. Incipient divergent boundaries may produce rift valleys. Mature diver- gent boundaries on continents are associated with passive margins and great escarpments. Convergent boundaries produce volcanic arcs, oceanic trenches, and mountain belts (orogens). Transform boundaries produce fracture zones with accompanying strike-slip faults and other features. Plate tectonic processes exert an important influence upon such continental-scale landforms as mountain belts, but there is an important interplay between uplift, climate, and denudation.

obduction

In a few cases of continent-ocean collision, a slab of ocean floor has rather than underridden the continent. ex: Troödos Mountain region of Cyprus.

terranes

In breaking up, small fragments of continent sometimes shear off; these are called terranes. They drift around until they meet another continent, to which they become attached (rather than being subducted) or possibly are sheared along it.

modified continental- margin orogen.

Island arc-continent collisions occur where an island arc moves towards a subduction zone adjacent to the continent.

compound intra-oceanic island arc.

Island arc-island arc collisions are poorly understood because there are no present examples from which to work out the processes involved. However, the outcome would probably be a compound intra-oceanic island arc.

hot spot volcano

Mantle plumes are areas of hot, upwelling mantle. A hot spot develops above the plume. Magma generated by the hot spot rises through the rigid plates of the lithosphere and produces active volcanoes at the Earth's surface. As oceanic volcanoes move away from the hot spot, they cool and subside, producing older islands, atolls, and seamounts. As continental volcanoes move away from the hot spot, they cool, subside, and become extinct. Hot spots are places within the mantle where rocks melt to generate magma. yellowstone and hawaii.

orogenic mountain belt or orogen

Subduction of oceanic litho- sphere beneath continental lithosphere produces two chief features. First, it forms an oceanic trench, a dip- ping zone of seismic activity, and volcanicity in an orogenic mountain belt (or orogen) lying on the continental lithosphere next to the oceanic trench (as in western South America).

structural geomorphology

Such passive influences of geological structures upon landforms.

within- plate tectonics

Tectonic activity also occurs within lithospheric plates, and not just at plate edges. This is called within- plate tectonics to distinguish it from plate-boundary tectonics.

active

Where tectonic plates converge or slide past each other, the continental margins are said to be active.

lithosphere

The outer shell of the solid Earth - the lithosphere - is not a single, unbroken shell of rock; it is a set of snugly tailored plates (Figure 4.2). At present there are seven large plates, all with an area over 100 million km 2 . They are the African, North American, South American, Antarctic, Australian-Indian, Eurasian, and Pacific plates.

oceanic plates

The oceanic plates are linked into the cooling and recy- cling system comprising the mesosphere, asthenosphere, and lithosphere beneath the ocean floors. The chief cool- ing mechanism is subduction. New oceanic lithosphere is formed by volcanic eruptions along mid-ocean ridges. The newly formed material moves away from the ridges. In doing so, it cools, contracts, and thickens. Oceanic plates are formed by divergent plate boundaries. These zones, located along mid-ocean ridges, represent areas where the new oceanic crust is being created. As lava flows from these volcanic ridges, it quickly cools, forming extrusive igneous rock. Oceanic plates are mafic in nature, comprised of basalt rock. This oceanic rock is high in iron, magnesium and calcium content. More than half of this basalt rock is comprised of the minerals pyroxene and olivine. oceanic plates are much denser than continental plates. The average density of ocean plates is approximately 200 pounds per cubic foot. Oceanic plates are continually being renewed at divergent boundaries and recycled in subductive zones. As a result, the oldest oceanic rocks are less than 200 million years old. Oceanic plates cover approximately 71 percent of Earth's surface, they are much thinner than continental crust.

breakup unconformity

The palaeoplain at the new continental edge, which is created by the rifting, experiences downwarp- ing. Sea-floor spreading then favours the growth of a new ocean in which post-rift sediments accumulate as a wedge on the submerged palaeoplain to form a seawards-sloping basal unconformity. owing to its association with the fragmenting of a supercontinent

subduction zones

These are associated with earthquakes and volcanicity. Cold oceanic slabs may sink well into the mesosphere, perhaps as much as 670 km or below the surface. These boundaries mark the collision between two of the planet's tectonic plates. The plates are pieces of crust that slowly move across the planet's surface over millions of years. Where two tectonic plates meet at a subduction zone, one bends and slides underneath the other, curving down into the mantle. (The mantle is the hotter layer under the crust.) Oceanic crust is denser than continental crust. At a subduction zone, the oceanic crust usually sinks into the mantle beneath lighter continental crust.

microplate or platelets

These plates are often grouped with an adjacent major plate on a major plate map. For purposes of this list, a microplate is any plate with an area less than 1 million km2. Some models identify more minor plates within current orogens like the Apulian, Explorer, Gorda, and Philippine Mobile Belt plates.

transform plate boundaries (conservative)

Two plates sliding past each other forms a transform plate boundary. Natural or human-made structures that cross a transform boundary are offset—split into pieces and carried in opposite directions. Rocks that line the boundary are pulverized as the plates grind along, creating a linear fault valley or undersea canyon. As the plates alternately jam and jump against each other, earthquakes rattle through a wide boundary zone. In contrast to convergent and divergent boundaries, no magma is formed. Thus, crust is cracked and broken at transform margins, but is not created or destroyed.

intrusive forces

Volcanic force, are found within the lithosphere and produce such features as batholiths, dykes, and sills. The deep-seated, major intrusions - batholiths and stocks - result from plutonic processes, while the minor, nearer- surface intrusions such as dykes and sills, which occur as independent bodies or as offshoots from plutonic intrusions, result from hypabyssal processes.

convergent plate boundary

When two plates come together, it is known as a convergent boundary. The impact of the two colliding plates buckles the edge of one or both plates up into a rugged mountain range, and sometimes bends the other down into a deep seafloor trench. A chain of volcanoes often forms parallel to the boundary, to the mountain range, and to the trench. Powerful earthquakes shake a wide area on both sides of the boundary. If one of the colliding plates is topped with oceanic crust, it is forced down into the mantle where it begins to melt. Magma rises into and through the other plate, solidifying into new crust. Magma formed from melting plates solidifies into granite, a light colored, low-density rock that makes up the continents. Thus at convergent boundaries, continental crust, made of granite, is created, and oceanic crust is destroyed.

passive margin

Where continental margins lie inside plates, they are pas- sive margins. The break-up of Pangaea created many passive margins, including the east coast of South Amer- ica and the west coast of Africa. Passive margins are sometimes designated rifted margins where plate motion has been divergent, and sheared margins where plate motion has been transformed, that is, where adjacent crustal blocks have moved in opposite directions. Passive continental margins are found along the remaining coastlines. Because there is no collision or subduction taking place, tectonic activity is minimal and the earth's weathering and erosional processes are winning. This leads to lots of low-relief (flat) land extending both directions from the beach, long river systems, and the accumulation of thick piles of sedimentary debris on the relatively wide continental shelves. Again South America provides a great example. The Amazon River, whose source is in the Andes Mountains (the active margin) drains east across the interior of South America to the coast, where it enters the Atlantic Ocean and deposits the tremendous volume of sedimentary materials it eroded from the continent.

triple junctions

Y-shaped boundaries. name given to a place where three tectonic plates meet. There are roughly 50 plates on Earth with about 100 triple junctions among them.

wander- ing slivers go by several names

allochthonous terranes, displaced terranes, exotic terranes, native terranes, and suspect terranes.

elevation of orogens

appears crucially to depend upon the crustal strength of rocks. Where crustal convergence rates are high, surface uplift soon creates (in geological terms) an elevation of around 6 to 7 km that the crustal strength of rocks cannot sustain, although individual mountain peaks may stand higher where the strength of the surrounding crust supports them. However, in most mountain belts, the effects of denudation prevent elevations from attaining this upper ceiling. As tectonic uplift occurs and elevation increases, river gradients become steeper, so raising denudation rates. The growth of topography is also likely to increase precipittion (through the orographic effect) and therefore runoff, which will also tend to enhance denudation

continental flood basalts

are high volume eruptions that flood vast areas of the Earth, covering broad regions with flat lying lava surfaces. They are said to be the result of mantle convection through hot spots, which occur sporadically in time and place. In the USA we are familar with the Columbia River Flood basalts, which represent hundreds of black basalt lava layers which cover eastern Washington and parts of Idaho and Oregon. These occupy large tracts of land in far-flung places.

great escarpments

are highly distinctive landforms of many passive margins. They are extraordinary topographic features formed in a variety of rocks (folded sedimentary rocks, granites, basalts, and metamorphic rocks) and separate the high plateaux from coastal plains. is a major geological formation in Africa that consists of steep slopes from the high central Southern African plateau

strike slip fault zones

are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally. If the block opposite an observer looking across the fault moves to the right, the slip style is termed right lateral; if the block moves to the left, the motion is termed left lateral.

Marginal swells

are widespread asymmetrical bulges along continental edges that fall directly into the sea with steeper (2 ◦ ) slopes towards the coast. They develop after the formation of plateaux and major valleys.

anastomosing faults

branching and reconnecting. movement may produce upthrust blocks and down-sagging ponds (Figure 4.13). A change in the dominant direction of stress may render all these transform margin features more complex. A classic area of transform margin complexity is the southern section of the San Andreas fault system. Some 1,000 km of movement has occurred along the fault over the last 25 million years. The individual faults branch, join, and sidestep each other, producing many areas of uplift and subsidence.

Cratons

broad, central parts of continents. They are somewhat stable continental shield areas with a base- ment of Precambrian rocks that are largely unaffected by orogenic forces but are subject to epeirogeny. The main large-scale landforms associated with these areas are basins, plateaux (upwarps and swells), rift valleys, and intracontinental volcanoes.

Pacific-type margin

common around the Pacific Ocean's rim. called active margins.

4 types of collisions

continent colliding with another continent; an island arc colliding with a continent; a continent colliding with an island arc; and an island arc colliding with an island arc

wilson cycle

creation and breakup of supercontinents. named after the Canadian J Tuzo Wilson. starts with continent rifting and opening of a new ocean. If continents rift apart to form ocean basins, other oceans must close. This may be repeated throughout Earth history. Example: the IAPETUS ocean between England & Scotland in the Lower Palaeozoic, closed in the Caledonian; later opening of the Atlantic, almost in the same place. (1) Rifting of continents by mantle diapirism (2) Continental drift, seafloor spreading & formation of ocean basins (3) Progressive closure of ocean basins by subduction of ocean lithosphere (4) Continental collision and final closure of ocean basin

continental lithosphere

does not take part in the mantle-convection process. It is 150 km thick and consists of buoyant low-density crust (the tectosphere) and relatively buoyant upper mantle. It consists of about 50 km of crust and 100 km or more of uppermost mantle.

steady-state topography

ere changes in tectonic uplift rate are (geologically speaking) rapidly matched by adjustments in denudation rates, orogens seem to maintain a roughly steady-state topography.

plateaux

extensive area of flat upland usually bounded by an escarpment (i.e., steep slope) on all sides but sometimes enclosed by mountains. The essential criteria for plateaus are low relative relief and some altitude. Although plateaus stand at higher elevation than surrounding terrain, they differ from mountain ranges in that they are remarkably flat. most are erosion surfaces formed of uplifted palaeoplains. Ex:Africa.

Lowland or coastal plains

flat, low-lying land adjacent to a seacoast. One of the world's largest coastal plains is located in eastern United States. lie seawards of great escarpments. They are largely the products of erosion.

Continental-margin orogens

form when oceanic lithosphere is subducted beneath continental lithosphere. The Andes of South America are probably the finest example of this type of orogen. Indeed, the orogen is sometimes called an Andean-type orogen, as well as a Cordilleran-type orogen. Continental-margin island arcs form if the continental crust is below sea level. An example is the Sumatra-Java section of the Sunda Arc in the East Indies.

Intra-cratonic basins

form within stable continental interiors. A good example is the Western Canada Sedimentary Basin extending from the eastern side of the Rocky Mountains to central Manitoba. may be 1,000 km or more across.

morphotectonics

influence of tectonic processes on landforms, particularly at continental and large regional scales. refers to the study of short- and long-term superficial evidences of tectonic activity.

Steady-state margins produce two major landforms

intra-oceanic island arcs and continental-margin orogens

tectonic geomorphology

investigates the effects of active tectonic processes. faulting, tilting, folding, uplift, and subsidence-upon landforms. Tectonic geomorphology is the study of the interplay between tectonic and surface processes that shape the landscape in regions of active deformation and at time scales ranging from days to millions of years.

Mantle plumes

is a large column of hot rock rising through the mantle. The heat from the plume causes rocks in the lower lithosphere to melt. The largest (and most persistent) mantle plumes are presumed to form where a large volume of mantle rock is heated at the core-mantle boundary, about 1,800 miles below the surface, although smaller plumes may originate elsewhere within the mantle. Once the temperature increases sufficiently to lower the rock density, a column of the hotter-than-normal rock (perhaps 2,000 kilometers in diameter) starts to rise very slowly through the surrounding mantle rocks. Eventually, the rising column of hot rock reaches the base of the lithosphere, where it spreads out, forming a mushroom-shaped cap to the plume. The overlying lithosphere is pushed up and stretched out as the plume cap spreads. Heat transferred from the plume raises the temperature in the lower lithosphere to above melting point, and magma chambers form that feed volcanoes at the surface. This area is also known as a hot spot. Because the plume remains anchored at the core-mantle boundary, it does not shift position over time. So, as the lithospheric plate above it moves, a string of volcanoes (or other volcanic features) is created. cause of Hawaiian islands

Subduction

is deemed to create steady-state margins in the sense that oceanic crust is subducted indefinitely while a continent or island arc resists subduction. s a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced or sinks due to gravity into the mantle. Regions where this process occurs are known as subduction zones. Rates of subduction are typically in centimeters per year, with the average rate of convergence being approximately two to eight centimeters per year along most plate boundaries

epeirogeny

is the upheaval or depression of large areas of cratons without significant folding or fracture. The only folding associated with epeirogeny is the broadest of undulations. broad regional upwarp of the cratonic (stable interior) portions of continents. In contrast to orogeny, epeirogeny takes place over broad, nonlinear areas, is relatively slow, and results in only mild deformation. Phenomena accompanying epeirogeny include the development of regional disconformities that gently bevel underlying strata and the formation of regressive deposits if marine incursions have taken place.

basic landforms connected with convergent margins

island arcs and orogens. Their specific form depends upon (1) what it is that is doing the converging - two continents, a continent and an island arc, or two island arcs; and (2) whether subduction of oceanic crust occurs or a collision occurs.

volcanic forces

lead to the extrusion of magma on to the Earth's surface as lava and to minor intrusions (e.g. dykes and sills) into other rocks.

diastrophic forces

lead to the folding, faulting, uplift, and subsidence of the lithosphere. Diastrophic forces may deform the lithosphere through folding, faulting, uplift, and subsidence. They are responsible for some of the major features of the physical toposphere.

Orogeny

literally means the genesis of mountains, and when first used it meant just that. Later, it became associated with the idea of folding, and eventually it came to mean the folding of rocks in fold belts. As mountain building is not associated with the folding of rocks, it cannot be synonymous with orogeny. mountain-building event, generally one that occurs in geosynclinal areas. In contrast to epeirogeny, an orogeny tends to occur during a relatively short time in linear belts and results in intensive deformation. Orogeny is usually accompanied by folding and faulting of strata, development of angular unconformities (interruptions in the normal deposition of sedimentary rock), and the deposition of clastic wedges of sediments in areas adjacent to the orogenic belt. Regional metamorphism and magmatic activity are often associated with an orogenic event as well.

earths convention system

lithosphere may be regarded as the cool surface layer of the Earth's convective system. Convection is the process by which less dense material rises and more dense material sinks. The former are said to be more "buoyant" than the latter, and the vertical forces due to density difference are referred to as buoyancy forces. Rocks, water, and air—indeed, most materials—expand and thus become less dense as temperature increases, so convection is typically driven by temperature differences. In Earth's mantle hot rock rises and slightly cooler rock sinks. Convection drives our dynamic planet. The planet's liquid outer core convects, creating the Earth's magnetic field; the ocean convects, enabling exchange of CO2 with the atmosphere and transporting nutrients from depth that support important fisheries; and the atmosphere convects, acting in concert with the ocean to transport heat and moisture around the planet to create climate. Convection is the process by which less dense material rises and more dense material sinks. The former are said to be more "buoyant" than the latter, and the vertical forces due to density difference are referred to as buoyancy forces. Rocks, water, and air—indeed, most materials—expand and thus become less dense as temperature increases, so convection is typically driven by temperature differences. In Earth's mantle hot rock rises and slightly cooler rock sinks. Convection drives our dynamic planet. The planet's liquid outer core convects, creating the Earth's magnetic field; the ocean convects, enabling exchange of CO2 with the atmosphere and transporting nutrients from depth that support important fisheries; and the atmosphere convects, acting in concert with the ocean to transport heat and moisture around the planet to create climate.

Native terranes

manifestly relate to the continental margin against which they presently sit.

driving mechanisms

mantle convection may be the number one motive force, though this now seems unlikely as many spreading sites do not sit over upwelling mantle convection cells. Basaltic lava upwelling at a mid-ocean ridge may push adjacent lithospheric plates to either side. Or, as elevation tends to decrease and slab thickness to increase away from construction sites, the plate may move by gravity sliding. cold, sinking slab at subduction sites pulls the rest of the plate behind it.

hot spot trace(volcanic islands)

mantle plume stays in the same position while a plate slowly slips over it. In the ocean, this pro- duces a chain of volcanic islands

Fall Line

marks an increase in stream gradient and in places forms a distinct escarpment.

Suspect terranes

may be exotic, but their exoticism is not confirmable.

intra-oceanic arcs

ntra-oceanic arcs are magmatic arcs developed within ocean basins, built on oceanic derived crust. This oceanic crust is formed at either mid-ocean ridges or back-arc spreading centres, crust forming part of an oceanic plateau, crust formed by accretion of oceanic sediments in a subduction zone fore-arc or earlier intra-oceanic arc material. IOASs tend to form where older oceanic crust is subducted under younger oceanic crust. Relatively older oceanic crust is negatively buoyant. This causes the subducting plate to sink vertically in addition to subducting down-dip which causes roll-back of the trench and associated arc. This situation favours the development of convergent plate margins within the oceanic realm and thus IOASs.

another term for transform

oblique-slip faults

extrusive forces

occur at the very top of the lithosphere and lead to exhalations, eruptions, and explosions of materials through volcanic vents, all of which are the result of volcanic processes.

tectonic landforms

productions of earth interior processes without the intervention of the forces of denudation. they include volcanic cones, craters, fault scarps, and mountain ranges.

Continental rifting

occurs at sites where the con- tinental crust is stretched and faulted. The rift valley running north to south along much of East Africa is prob- ably the most famous example. Rifting is the process by which the continental lithosphere stretches. A Continental rift is the belt or zone of the continental lithosphere where the extensional deformation (rifting) is occurring. These zones have important consequences and geological features, and if the rifting is successful, lead to the formation of new ocean basins.

Divergent plate boundaries

occurs when two tectonic plates move away from each other. Along these boundaries, lava spews from long fissures and geysers spurt superheated water. Frequent earthquakes strike along the rift. Beneath the rift, magma—molten rock—rises from the mantle. It oozes up into the gap and hardens into solid rock, forming new crust on the torn edges of the plates. Magma from the mantle solidifies into basalt, a dark, dense rock that underlies the ocean floor. Thus at divergent boundaries, oceanic crust, made of basalt, is created.

Exotic or allochthonous terranes

originate from a continent different from that against which they now rest.

2 types of diastrophism

orogeny and epeirogeny

Atlantic-type margins

passive margin

Collisions

redeemed to occur when the continents or island arcs crash into one other but tend to resist subduction.

Intra-oceanic island arcs

result from oceanic lithosphere being subducted beneath another oceanic plate. The heating of the plate that is subducted produces volcanoes and other thermal effects that build the island arc. Currently, about twenty intra-oceanic island arcs sit at subduction zones. Most of these lie in the western Pacific Ocean and include the Aleutian Arc, the Marianas Arc, the Celebes Arc, the Solomon Arc, and the Tonga Arc. The arcs build relief through the large-scale intrusion of igneous rocks and volcanic activity. A deep trench often forms ahead of the arc at the point where the oceanic lithosphere starts plunging into the mantle.

tectonic predesign

several landscapes, patently of exogenic origin, have tectonic or endergenic features stamped on them. or under them. follows stress patterns in the lithosphere. The resulting landscape features are not fashioned directly by the stress fields. Rather, the exogenic processes act preferentially in conformity with the lithospheric stress (see p. 138). The conformity is either with the direc- tion of a shear or, where there is a free surface, in the direction of a principal stress.

cymatogeny

the arching, and sometimes doming, of rocks with little deformation over 10-1,000 km.

plume tectonics

the dominant style of convection in the major part of the mantle. Two super-upwellings (the South Pacific and African superplumes) and one super-downwelling (the Asian cold plume) appear to prevail

isostatic movements

the rebound of land after an ice sheet has melted

plutonic forces

which originate deep in the Earth, produce major intrusions (plutons) and associated veins