Convection and the Earths Mantle
The main heat transfer mechanism in the Earth's mantle is convection, a thermally driven process where heating at depth causes material to expand and become less dense, causing it to rise while being replaced by complimentary cool material that sinks. This moves heat from depth to the surface in a very efficient cycle, since the material that rises gives off heat as it rises and cools, and the material that sinks gets heated only to rise again eventually. Convection is the most important mechanism by which the Earth is loosing heat, with other mechanisms including conduction, radiation, and advection. However, many of these mechanisms work together in the plate tectonic cycle. Mantle convection brings heat from deep in the mantle to the surface, where the heat released forms magmas that generate the oceanic crust. The midocean ridge axis is the site of active hydrothermal circulation and heat loss, forming black smoker chimneys and other vents. As the crust and lithosphere move away from the midocean ridges, it cools by conduction, gradually subsiding (according the square root of its age) from about 1.5-2.5 miles (2.54.0 km) below sea level. Heat loss by mantle convection is therefore the main driving mechanism of plate tectonics, and the moving plates can be thought of as the conductively cooling boundary layer for large-scale mantle convection systems.
The heat transferred to the surface by convection is produced by decay of radioactive elements, producing isotopes such as uranium 235, thorium, 232, and potassium 40, remnant heat from early heat-producing isotopes such as iodine 129, remnant heat from accretion of the Earth, heat released during core formation, and heat released during impacts of meteorites and asteroids. Early in the history of the planet at least part of the mantle was molten, and the Earth has been cooling by convection ever since. Estimating how much the mantle has cooled with time is difficult, but reasonable estimates suggest that the mantle may have been up to a couple of hundred degrees hotter in the earliest Archean.
Cross section of Earth showing possible modes of mantle convection
The rate of mantle convection depends on the ability of the material to flow. The resistance to flow is a quantity measured as viscosity, defined as the ratio of shear stress to strain rate. Fluids with high viscosity are more resistant to flow than materials with low viscosity. The present viscosity of the mantle is estimated to be 1020-1021 Pascal seconds (Pa/s) in the upper mantle and 1021-1023 Pa/s in the lower mantle, values sufficient for allowing the mantle to convect, and complete an overturn cycle once every 100 million years. The viscosity of the mantle is temperature dependent, so the mantle may have been able to flow and convectively overturn much more quickly in early Earth history, making convection an even more efficient process and speeding the rate of plate tectonic processes.
Current ongoing debate and research concerns the style of mantle convection in the Earth. The upper mantle is relatively heterogeneous and extends to a depth of 416 miles (670 km), where there is a pronounced increase in seismic velocities. The lower mantle is more homogeneous, and extends to a region known as D" (pronounced dee-double-prime) at 1,678 miles (2,700 km), marking the transition into the liquid outer core. One school of mantle convection thought suggests that the entire mantle, including both the upper and lower parts, convects as one unit. Another school of thought posits that the mantle convection consists of two layers, with the lower mantle convecting separately from the upper mantle. A variety of these models, presently held by the majority of geophysicists, holds that there is two-layer convection, but that subducting slabs are able to penetrate the 670-kilometer discontinuity from above, and that mantle plumes that rise from the D" region can penetrate the 670-kilometer discontinuity from below.
The shapes that mantle convection cells take include many possible forms that are reflected to a first order by the distribution of subduction zones and midocean ridge systems. The subduction zones mark regions of downwelling, whereas the ridge system marks broad regions of upwelling. Material is upwelling in a broad planiform cell beneath the Atlantic and Indian Oceans, and downwelling in the circum-Pacific subduction zones. There is thought to be a large plumelike "superswell" beneath part of the Pacific that feeds the planiform East Pacific rise. Mantle plumes that come from the deep mantle punctuate this broad pattern of upper-mantle convection, and their plume tails must be distorted by flow in the convecting upper mantle.
Real data on a cutaway of Earth showing movement of deep slabs of rock in mantle. Sinking slabs are blue, mantle is yellow, and rising molten rock is red. The sinking slabs, including one (at upper left) descending from the Caribbean, are up to 930 miles (1,500 km) across and penetrate up to 1,800 miles (2,900 km) to the D" region at the core-mantle boundary. The deep slabs can be detected by measuring the arrival times at points around the world of seismic shear waves produced by earthquakes. These waves travel faster through dense, cool rock than warm rock. (Steve Grand, Texas University/Photo Researchers, Inc.)
The pattern of mantle convection deep in geological time is uncertain. some periods such as the Cretaceous seem to have had much more rigorous mantle convection and surface volcanism. More or different types or rates of mantle convection may have helped to allow the early Earth to lose heat more efficiently. some computer models allow periods of convection dominated by plumes, and others dominated by overturning planar cells similar to the present Earth. some models suggest cyclic relationships, with slabs pooling at the 670-kilometer discontinuity, then suddenly all sinking into the lower mantle, causing a huge mantle overturn event. Further research is needed on linking the preserved record of mantle convection in the deformed continents to help interpret the past history of convection.
Convergent plate margin processes structural, igneous, metamorphic, and sedimentological processes that occur in the region affected by forces associated with the convergence of two or more plates are grouped under the heading of convergent plate margin processes. Convergent plate boundaries are of two fundamental types, subduction zones and collision zones. subduction zones are in turn of two basic types, the first of which is found where oceanic lithosphere of one plate descends beneath another oceanic plate, such as in the Philippines and Marianas of the southwest Pacific. The second type of subduction zone forms where an oceanic plate descends beneath a continental upper plate, such as in the Andes of south America. The southern Alaska convergent margin is particularly interesting, as it records a transition from an ocean/continent convergent boundary to an ocean/ocean convergent boundary in the Aleutians.
Arcs have several different geomorphic zones defined largely on their topographic and structural expressions. The active arc is the topographic high with volcanoes, and the backarc region stretches from the active arc away from the trench, and it may end in an older rifted arc or continent. The forearc basin is a generally flat topographic basin with shallow to deep-water sediments, typically deposited over older accreted sediments and ophiolitic or continental basement. The accretionary prism includes uplifted, strongly deformed rocks scraped off the downgoing oceanic plate on a series of faults. The trench may be several to six miles (up to 10 or more kilometers) deep below the average level of the seafloor in the region and marks the boundary between the overriding and underthrusting plate. The outer trench slope is the region from the trench to the top of the flexed oceanic crust that forms a several hundred to one-thousand-foot (few hundred-meter) high topographic rise known as the forebulge on the downgoing plate.
Trench floors are triangular shaped in profile and typically are partly to completely filled with grey-wacke-shale turbidite sediments derived from erosion of the accretionary wedge. They may also be transported by currents along the trench axis for large distances, up to hundreds or even thousands of miles (thousands of kilometers) from their ultimate source in uplifted mountains in the convergent orogen.
Flysch is a term that applies to rapidly deposited deep marine syn-orogenic clastic rocks that are generally turbidites. Trenches are also characterized by chaotic deposits known as olistostromes that typically have clasts or blocks of one rock type, such as limestone or sandstone, mixed with a muddy or shaly matrix. These are interpreted as slump or giant submarine landslide deposits. They are common in trenches because of the oversteepening of slopes in the wedge. sediments that get accreted may also include pelagic sediments initially deposited on the subducting plate, such as red clay, siliceous ooze, chert, manganiferous chert, calcareous ooze, and windblown dust.
The sediments are deposited as flat-lying turbi-dite packages, then gradually incorporated into the accretionary wedge complex through folding and the propagation of faults through the trench sediments. subduction accretion is a process that accretes sediments deposited on the underriding plate onto the base of the overriding plate. It causes the rotation and uplift of the accretionary prism, which is a broadly steady-state process that continues as long as sediment-laden trench deposits are thrust deeper into the trench. Typically new faults will form and propagate beneath older ones, rotating the old faults and structures to steeper attitudes as new material is added to the toe and base of the accretionary wedge. This process increases the size of the overriding accre-tionary wedge and causes a seaward-younging in the age of deformation.
Parts of the oceanic basement to the subducting slab are sometimes scraped off and incorporated into the accretionary prisms. These tectonic slivers typically consist of fault-bounded slices of basalt, gabbro, and ultramafic rocks, and rarely, partial or even complete ophiolite sequences can be recognized. These ophiolitic slivers are often parts of highly deformed belts of rock known as mélanges. Mélanges are mixtures of many different rock types typically including blocks of oceanic basement or limestone in muddy, shaly, serpentinitic, or even a cherty matrix. Formed by tectonic mixing of the many different types of rocks found in the forearc, mélanges are one of the hallmark rock units of convergent boundaries.
Major differences in processes occur at Andean-style compared to Marianas-style arc systems. Andean-type arcs have shallow trenches, fewer than 3.7 miles (6 km) deep, whereas marianas-type arcs typically have deep trenches reaching 6.8 miles (11 km) in depth. most Andean-type arcs subduct young oceanic crust and have very shallow-dipping subduction zones, whereas marianas-type arcs subduct old oceanic crust and have steeply dipping Benioff zones. Andean arcs have back-arc regions dominated by foreland (retroarc) fold thrust belts and sedimentary basins, whereas marianas-type arcs typically have
back-arc basins, often with active seafloor spreading. Andean arcs have thick crust, up to 43.5 miles (70 km), and big earthquakes in the overriding plate, while Marianas-type arcs have thin crust, typically only 12.5 miles (20 km), and have big earthquakes in the underriding plate. Andean arcs have only rare volcanoes, and these have magmas rich in sio2 such as rhyolites and andesites. Plutonic rocks are more common, and the basement is continental crust. Marianas-type arcs have many volcanoes that erupt lava low in silica content, typically basalt, and are built on oceanic crust.
Many arcs are transitional between the Andean or continental-margin types and the oceanic or Marianas types, and some arcs have large amounts of strike-slip motion. The causes of these variations have been investigated and it has been determined that the rate of convergence has little effect, but the relative motion directions and the age of the subducted oceanic crust seem to have the biggest effects. In particular old oceanic crust tends to sink to the point where it has a near-vertical dip, rolling back through the viscous mantle and dragging the arc and forearc regions of overlying Marianas-type arcs with it. This process contributes to the formation of back arc basins.
Much of the variation in the processes that occur in convergent margin arcs can be attributed to the relative convergence vectors between the overriding and underriding plates. In this kinematic approach to modeling convergent margin processes, the under-riding plate may converge at any angle with the overriding plate, which itself moves toward or away from the trench. since the active arc is a surface expression of the 68-mile (110-km) isobath on the subducted slab, the arc will always stay 68 miles (110 km) above this zone. The arc therefore separates two parts of the overriding plate that may move independently, including the frontal arc sliver between the arc and trench and the main part of the overriding plate. The frontal arc sliver is in most cases kine-matically linked to the downgoing plate and moves parallel to the plate margin in the direction that contains the oblique component of motion between the downgoing and overriding plate. Different relative angles of convergence between the overriding and underriding plate determine whether or not an arc will have strike-slip motions, and the amount that the subducting slab rolls back (which is age-dependent) determines whether the frontal arc sliver rifts from the arc and causes a back arc basin to open or not. This model helps to explain why some arcs are extensional with big back arc basins, others have strike-slip dominated systems, and others are purely compressional arcs. Convergent margins also show changes in these vectors and consequent geologic processes with time, often switching quickly from one regime to the other with changes in the parameters of the subducting plate.
The thermal and fluid structure of arcs is dominated by effects of the downgoing slab, which is much cooler than the surrounding mantle and cools the forearc. Fluids released from the slab as it descends past 68 miles (110 km) aid partial melting in the overlying mantle and generate the magmas that form the arc on the overriding plate. This broad thermal structure of arcs results in the formation of paired metamorphic belts, where the metamorphism in the trench environment grades from cold and low-pressure at the surface to cold and high-pressure at depth, whereas the arc records low- and high-pressure high-temperature metamorphic facies series. One of the distinctive rock types found in trench environments is the unusual high-pressure, low-temperature blue-schist facies rocks in paleosubduction zones. The presence of index minerals glaucophane (a sodic amphibole), jadeite (a sodic pyroxene), and lawson-ite (Ca-zeolite) indicate low temperatures extended to depths of 12-20 miles (20-30 kilometers) (7-10 kilobars [kb]). Since these minerals are unstable at high temperatures, their presence indicates they formed in a low-temperature environment, and the cooling effects of the subducting plate offer the only known environment to maintain such cool temperatures at depth in the Earth.
Forearc basins may include several-kilometer-thick accumulations of sediments deposited in response to subsidence induced by tectonic loading or thermal cooling of forearcs built on oceanic lithosphere. The Great Valley of California is a forearc basin that formed on oceanic forearc crust preserved in ophiolitic fragments found in central California, and Cook Inlet in Alaska is an active forearc basin formed in front of the Aleutian and Alaska range volcanic arc.
The rocks in the active arcs typically include several different facies. Volcanic rocks may include subaerial flows, tuffs, welded tuffs, volcaniclastic conglomerate, sandstone, and pelagic rocks. Debris flows from volcanic flanks are common, and abundant and thick accumulations of ash deposited by winds and dropped by Plinian and other eruption columns may be present. Volcanic rocks in arcs include mainly calc-alkaline series, showing an early iron enrichment in the melt, typically including basalts, andesites, dacites, and rhyolites. Immature island arcs are strongly biased toward eruption at the mafic end of the spectrum, and may also include tholeiitic basalts, picrites, and other volcanic and intrusive series. More mature continental arcs erupt more fel-sic rocks and may include large caldera complexes.
Relative motion vectors in arcs. Changes in relative motions can produce drastically different arc geology. Vu = velocity of underriding plate; Vo = velocity of overriding plate; Vb = slip vector between overriding and underriding plates; Vg = velocity of sinking; Vr = velocity of rollback. Note that Vu sin a = velocity of downdip component of subduction, and Vr = Vg cot 8.
Snow-covered Mount Fuji in Japan—a classical, active convergent margin volcano (AP images)
Back arc and marginal basins form behind exten-sional arcs, or may include pieces of oceanic crust trapped by the formation of a new arc on the edge of an oceanic plate. Many extensional back arcs are found in the southwest Pacific, whereas the Bering sea, between Alaska and the Kamchatka peninsula, is thought to be a piece of oceanic crust trapped during the formation of the Aleutian chain. Extensional back arc basins may have oceanic crust generated by seafloor spreading, and these systems closely resemble the spreading centers found at divergent plate boundaries. The geochemical signature of some of the lavas show some subtle and some not-so-subtle differences, however, with water and volatiles being more important in the generation of magmas in back arc suprasubduction zone environments.
Compressional arcs such as the Andes have tall mountains, reaching heights of more than 24,000 feet (7,315 m) over broad areas. They have little or no volcanism but much plutonism, and typically have shallow dipping slabs beneath them. Andean-type compressional arcs are characterized by thick continental crust with large compressional earthquakes, and show a foreland-style retroarc basin in the back arc region. some compressional arc segments do not have accretionary forearcs but exhibit subduction erosion during which material is eroded and scraped off the overriding plate, and dragged down into the subduction zone. The Andes show remarkable along-strike variations in processes and tectonic style, with sharp boundaries between different segments. These variations seem to be related to what is being subducted and plate motion vectors. In areas where the downgoing slab has steep dips the overriding plate has volcanic rocks; in areas of shallow subduction there is no volcanism.
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