Andean arcs have back-arc regions dominated by foreland retroarc fold thrust belts and sedimentary basins, whereas marianas-type arcs typically have.
Andean arcs have thick crust, up to 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. 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 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 miles kilometers 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.
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.
These have been modeled by the injection of hot fluids into the base of a tank of motionless fluid. This is called the plume mode of mantle convection. Numerical experiments show that mantle convection is controlled from the top by continents, cooling lithosphere, slabs and plate motions and that plates not only drive and break themselves but can control and reverse convection in the mantle Supercontinents and other large plates generate spatial and temporal temperature variations.
The migration of continents, ridges and trenches cause a constantly changing surface boundary condition, and the underlying mantle responds passively Plates break up and move, and trenches roll back because of forces on the plates and interactions of the lithosphere with the mantle. Density variations in the mantle are, by and large, generated by plate tectonics itself, for example through slab cooling, refertization of the mantle, continental insulation, and these also affect the forces on the plates.
Surface plates are constantly evolving and reorganizing although major reorganizations are infrequent. They are mainly under lateral compression although local regions having horizontal least-compressive axes may be the locus of dikes and volcanic chains. The mantle is generally considered to convect as a single layer whole mantle convection or, at most two the standard geochemical model. However, the mantle is more likely to convect in multiple layers as a result of gravitational sorting during accretion and the density difference between the mantle products of differentiation.
Cross section through the whole-mantle tomography model of Ritsema et al. The mantle may be divided into three or more chemically distinct layers. The core has low viscosity and high thermal conductivity so the base of the mantle is in contact with a stress-free isothermal bath.
The top boundary condition is plate tectonics. It is not an isothermal, stress-free, homogeneous, uniform boundary condition. If the plates are held together by lateral stress then the surface must be free to self-organize, a condition not yet allowed in any simulation. Reorganization means the ability to form new plate boundaries and generate new plates that are consistent with the ever-changing stress state of the lithosphere.
In the absence of plates or a high viscosity lid the mantle would experience Rayleigh-Benard convection. Above a critical Rayleigh number fluids spontaneously convect and self-organize.
Buoyancy of the fluid, which is dependent on the coefficient of thermal expansion expansivity and temperature fluctuations, drives the flow and the viscosity forces of the fluid dissipate the energy. Temperature-dependent viscosity, a semi-rigid lithosphere held together by lateral compressive stresses and buoyant continents and thick crust regions completely change this.
Gravitational forces on cooling plates cause them to move. Dissipation takes place in and between the plates, causing them to self-organize and to organize the underlying weaker mantle. Pressure decreases the expansivity and Rayleigh number so it is difficult to generate buoyancy or vigorous small-scale convection at the base of the mantle.
In addition, heat flow across the CMB is about an order of magnitude less than at the surface so it takes a long time to build up buoyancy. In contrast to the upper TBL, which involves frequent ejections of narrow dense slabs into the interior, the lower TBL is sluggish and does not play an active role in mantle convection. CMB upwellings are expected to be thousands of kilometers in extent and embedded in high-viscosity mantle. Lithospheric architecture and slabs set up lateral temperature gradients that drive small-scale convection.
This lateral temperature difference sets up convection. Shallow upwellings resulting from this mechanism are intrinsically three-dimensional and plume-like. Materials usually expand when heated. This causes them to rise when embedded in compositionally similar material.
Pressure drives atoms closer together and suppresses the ability of high temperatures to create buoyancy. This is unimportant in the laboratory but it also means that laboratory simulations of mantle convection, including theinjection experiments used to generate plumes, are not relevant to the mantle. Unfortunately, computer simulations are generally used to confirm the laboratory results and, when applied to the mantle, also ignore the effects of pressure on material properties.
In fact, the effects of temperature are also generally ignored except the effect of temperature on density. This is called the Boussinesq approximation. This works fine in the laboratory, but does not apply to the mantle. Lateral variations in temperature are what drives thermal convection.
Lateral variations in pressure are generally unimportant since the pressure in the material outside of the rising element is about the same as inside. But the increase in pressure with depth means that viscosity, thermal conductivity and expansivity change, making it harder for material to convect. The system responds by increasing the dimensions of the thermal instabilities in order to maintain buoyancy and to overcome viscous resistance. A closed or isolated system at equilibrium returns to equilibrium if perturbed.
In a tank of fluid, or the mantle, with a cold surface and a hot bottom, heat will flow by conduction alone unless the temperature gradient gets too large. The stable conduction situation is called equilibrium. A far-from-equilibrium dissipative system, provided with a steady source of energy or matter from the outside world, can organize itself via its own dissipation. It is sensitive to small internal fluctuations and prone to massive reorganization.
The fluid in a pot heated on a stove evolves rapidly through a series of transitions with complex pattern formation even if the heating is spatially uniform and slowly varying in time. The stove is the outside source of energy and the fluid provides the buoyancy and the dissipation via viscosity. Far-from-equilibrium self-organization and reorganization require an open system, a large, steady, outside source of matter or energy, non-linear interconnectedness of system components, dissipation, and a mechanism for exporting entropy products.
Under these conditions the system responds as a whole, and in such a way as to minimize entropy production dissipation. Certain fluctuations are amplified and stabilized by exchange of energy with the outside world. Structures appear which have different time and spatial scales than the energy input. Similar considerations apply to a fluid cooled from above. The cold surface layer organizes the flow and drives the convection. If the fluid has a strongly temperature-dependent viscosity, or if buoyant things are floating on top, only part of the surface layer is able to circulate into the interior.
If the upper mantle is near or above the melting point there are other sources of buoyancy and dissipation and the possibility of lubrication. Volcanic chains can form as a result of buoyant dikes breaking through the surface layer at regions of relative extension. Melts are predicted to pond beneath regions of lateral compression.
The idea that a deep TBL may be responsible for narrow structures such as volcanic chains is based on heating-from-below experiments and calculations, or injection experiments. The effects of pressure on thermal properties are not considered the Boussinesq approximation. In the Earth the effects of temperature and pressure on convection parameters cannot be ignored and these must be determined as part of the solution in a self-consistent way.
Furthermore, it is the cooling of the mantle that controls the rate of heat loss from the core. The core does not play an active role in mantle convection. The magnitude of the bottom TBL depends on the cooling rate of the mantle, the pressure and temperature dependence of the physical properties and the radioactivity of the deep mantle.
The local Rayleigh number of the deep mantle is very low. Chemical boundaries are hard to detect by seismic techniques but the evidence favors one such boundary near 1, km. The seismic boundary at an average depth of km is primarily due to a solid-solid phase change, in mantle minerals, with a negative Clapeyron slope. Instead, their crystal line structure changes in important ways. Rocks become much, much more dense. The transition zone prevents large exchanges of material between the upper and lower mantle.
Some geologists think that the increased density of rocks in the transition zone prevents subducted slabs from the lithosphere from falling further into the mantle.
These huge pieces of tectonic plates stall in the transition zone for millions of years before mixing with other mantle rock and eventually returning to the upper mantle as part of the asthenosphere, erupting as lava, becoming part of the lithosphere, or emerging as new oceanic crust at sites of seafloor spreading. Some geologists and rheologists, however, think subducted slabs can slip beneath the transition zone to the lower mantle.
Other evidence suggests that the transition layer is permeable , and the upper and lower mantle exchange some amount of material. It is not liquid, vapor , solid, or even plasma. Instead, water exists as hydroxide. Hydroxide is an ion of hydrogen and oxygen with a negative charge.
In the transition zone, hydroxide ions are trapped in the crystalline structure of rocks such as ringwoodite and wadsleyite. These minerals are formed from olivine at very high temperatures and pressure. Near the bottom of the transition zone, increasing temperature and pressure transform ringwoodite and wadsleyite. This allows the transition zone to maintain a consistent reservoir of water.
Subduction is the process in which a dense tectonic plate slips or melts beneath a more buoyant one. Most subduction happens as an oceanic plate slips beneath a less-dense plate. Along with the rocks and minerals of the lithosphere, tons of water and carbon are also transported to the mantle.
Hydroxide and water are returned to the upper mantle, crust, and even atmosphere through mantle convection, volcanic eruptions, and seafloor spreading. The lower mantle is hotter and denser than the upper mantle and transition zone. The lower mantle is much less ductile than the upper mantle and transition zone. Although heat usually correspond s to softening rocks, intense pressure keeps the lower mantle solid. Geologists do not agree about the structure of the lower mantle.
Some geologists think that subducted slabs of lithosphere have settled there. Other geologists think that the lower mantle is entirely unmoving and does not even transfer heat by convection. In still other areas, geologists and seismologist s have detected areas of huge melt. The iron of the outer core influences the formation of a diapir , a dome -shaped geologic feature igneous intrusion where more fluid material is forced into brittle overlying rock.
The iron diapir emits heat and may release a huge, bulging pulse of either material or energy—just like a Lava Lamp. This energy blooms upward, transferring heat to the lower mantle and transition zone, and maybe even erupting as a mantle plume.
At the base of the mantle, about 2, kilometers 1, miles below the surface, is the core-mantle boundary, or CMB. Mantle convection describes the movement of the mantle as it transfers heat from the white-hot core to the brittle lithosphere.
The mantle is heated from below, cooled from above, and its overall temperature decreases over long periods of time. All these elements contribute to mantle convection. Convection currents transfer hot, buoyant magma to the lithosphere at plate boundaries and hot spots. Earth's heat budget , which measures the flow of thermal energy from the core to the atmosphere, is dominate d by mantle convection. In this model, the mantle convects in a single process.
A subducted slab of lithosphere may slowly slip into the upper mantle and fall to the transition zone due to its relative density and coolness. Over millions of years, it may sink further into the lower mantle.
Some of that material may even emerge as lithosphere again, as it is spilled onto the crust through volcanic eruptions or seafloor spreading.
Layered-mantle convection describes two processes. Plumes of superheated mantle material may bubble up from the lower mantle and heat a region in the transition zone before falling back. Above the transition zone, convection may be influenced by heat transferred from the lower mantle as well as discrete convection currents in the upper mantle driven by subduction and seafloor spreading. Mantle plumes emanating from the upper mantle may gush up through the lithosphere as hot spots.
A mantle plume is an upwell ing of superheated rock from the mantle. As a mantle plume reaches the upper mantle, it melts into a diapir. This molten material heats the asthenosphere and lithosphere, triggering volcanic eruptions. The Hawaiian hot spot, in the middle of the North Pacific Ocean, sits above a likely mantle plume. As the Pacific plate moves in a generally northwestern motion, the Hawaiian hot spot remains relatively fixed.
Loihi, a mere , years old, will eventually become the newest Hawaiian island. Geologists think mantle plumes may be influenced by many different factors. Some may pulse, while others may be heated continually. Some geologists have identified more than a thousand mantle plumes. Until tools and technology allow geologists to more thoroughly explore the mantle, the debate will continue. The mantle has never been directly explored.
Even the most sophisticated drilling equipment has not reached beyond the crust. Drilling all the way down to the Moho the division between the Earth's crust and mantle is an important scientific milestone, but despite decades of effort, nobody has yet succeeded. In , scientists with the Integrated Ocean Drilling Project drilled 1, meters 4, feet below the North Atlantic seafloor and claimed to have come within just meters 1, feet of the Moho.
Many geologists study the mantle by analyzing xenoliths. Xenolith s are a type of intrusion—a rock trapped inside another rock. The xenoliths that provide the most information about the mantle are diamonds. Diamonds form under very unique conditions: in the upper mantle, at least kilometers 93 miles beneath the surface. Above depth and pressure, the carbon crystallizes as graphite , not diamond.
The diamonds themselves are of less interest to geologists than the xenoliths some contain. These intrusions are minerals from the mantle, trapped inside the rock-hard diamond.
0コメント