DOCSLIB.ORG

On Subducting Slab Entrainment of Buoyant Asthenosphere

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
On Subducting Slab Entrainment of Buoyant Asthenosphere doi: 10.1111/j.1365-3121.2007.00737.x On subducting slab entrainment of buoyant asthenosphere J. Phipps Morgan,1 Jo¨ rg Hasenclever,2,3 M. Hort,3 L. Ru¨ pke2,4 and E. M. Parmentier5 1Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USA; 2IfM-GEOMAR, Kiel University, Kiel, Germany; 3Institute of Geophysics, Hamburg University, Hamburg, Germany; 4Department of Physics, PGP, Oslo University, Oslo, Norway; 5Department of Geological Sciences, Brown University, Providence, RI, USA ABSTRACT Laboratory and numerical experiments and boundary layer upon the subduction rate and the density contrast and viscosity analysis of the entrainment of buoyant asthenosphere by of the asthenosphere, while the upper (cold) side of the slab subducting oceanic lithosphere implies that slab entrainment is may entrain as much by thermal ÔfreezingÕ onto the slab as by likely to be relatively inefficient at removing a buoyant and mechanical downdragging. This analysis also implies that lower viscosity asthenosphere layer. Asthenosphere would proper treatment of slab entrainment in future numerical instead be mostly removed by accretion into and eventual mantle flow experiments will require the resolution of 10– subduction of the overlying oceanic lithosphere. The lower (hot) 30 km-thick entrainment boundary layers. side of a subducting slab entrains by the formation of a 10– 30 km-thick downdragged layer, whose thickness depends Terra Nova, 19, 167–173, 2007 the Basin and Range (Goes and van 1975; Karato and Wu, 1993) than it Introduction der Lee, 2002) in the western U.S. does as cooler temperatures. As the Typical cartoons of mantle convection However, beneath stable continental uppermost suboceanic mantle is clo- show oceanic lithosphere moving in regions such as the Archaean shields sest to its solidus temperature, the concert with the underlying mantle in W. Australia (Gaherty et al., 1999) asthenosphere could simply be a con- flow (Fig. 1a). If, however, the and N. America (Goes and van der sequence of this effect, and the pres- asthenosphere layer – presumed to Lee, 2002) seismic wavespeeds are ence of any partial melt would underlie at least much of the oceanic significantly faster, implying the pres- enhance the reduction in viscosity lithosphere – is itself less dense and ence of much stronger (lithospheric) (Hirth and Kohlstedt, 1995) and seis- viscous than the underlying mantle, mantle between 60 and 250 km depths mic wavespeeds (Hammond and then the resulting asthenosphere cir- (see also Gung et al., 2003). The LVZ Humphreys, 2000). A higher water culation can be strongly altered in is an observed fact. Most researchers content has also been suggested to be ways that may have significant geolo- believe the LVZ also corresponds to a the origin for the LVZ (Karato and gical consequences (Fig. 1b,c). Here, lower viscosity region within EarthÕs Jung, 1998), and to lead to a viscosity we study the question of how effi- mantle. Rock-mechanics arguments, reduction in this region of the mantle ciently a subducting slab will entrain for example, imply the existence of a (cf. Hirth and Kohlstedt, 1996). buoyant asthenosphere near a subduc- low-viscosity zone at the shallowest We favour the hypothesis that tion zone, a question that leads to a hot mantle where competing tempera- EarthÕs asthenosphere forms as a sim- better understanding of the behaviour ture and pressure effects lead to a ple consequence of mantle plume up- of the asthenosphere–lithosphere sys- viscosity minimum (Weertman and welling – as hot buoyant plume tem in mantle convection. Weertman, 1975; Buck and Parmen- material naturally tends to rise, up- There exist several different views tier, 1986; Karato and Wu, 1993). welling plumes bring hot buoyant on what exactly is asthenosphere. Studies of the distribution of stresses in material to the ÔceilingÕ of the con- Seismic observations of the radial the oceanic plates require a low vis- vecting mantle that is formed by the Earth structure show a low-seismic- cosity asthenosphere of 1018–1019 Pa-s base of the mechanical lithosphere velocity (Dziewonski and Anderson, underneath the ocean lithosphere (Phipps Morgan et al., 1995a). In this 1981) and high attenuation (Widmer (Richter and McKenzie, 1978; Wiens case, asthenosphere material will actu- et al., 1991) layer between 100 and and Stein, 1985), consistent with the ally be hotter than the underlying 300 km depths. This layer with low 1018–1019 Pa s viscosities inferred mantle (at least in terms of its poten- seismic wavespeeds is now known to from post-glacial rebound at Iceland tial temperature; e.g. temperature cor- be well developed beneath the 60– (Sigmundsson and Einarsson, 1992). rected for adiabatic effects). If the 100 km-thick oceanic lithosphere Several potential mechanisms have asthenosphereÕs average temperature (Gaherty et al., 1999) and active been suggested to cause the formation is that of hot upwelling plumes, then young continental margins such as of a shallow asthenosphere layer with an observation of the deflection of the low wavespeeds, high attenuation and 410- and 660-km discontinuities be- Correspondence: J. Phipps Morgan, reduced viscosity. Silicate mantle close neath Iceland suggests that it may be Department of Earth and Atmospheric to its melting temperature will have 10–20% (i.e. 150–300°C) warmer Sciences, Cornell University, Ithaca, NY slower wavespeeds, higher attenuation than the underlying ambient mantle USA. Tel.: +1 607-821-1741; fax: +1 607- (Faul and Jackson, 2005), and lower (Shen et al., 1996). This would result 254-4780; e-mail: [email protected] viscosity (Weertman and Weertman, in a 0.5–1% reduction in density of Ó 2007 Blackwell Publishing Ltd 167 On subducting slab entrainment of buoyant asthenosphere • J. Phipps Morgan et al. Terra Nova, Vol 19, No. 3, 167–173 ............................................................................................................................................................. (a) viscous deformation mechanism in the uppermost mantle, dislocation creep of olivine at 1600 K, a 200 K increase in temperature leads to a >100-fold viscosity reduction. Numerical and laboratory experiments of asthenosphere (b) slab-entrainment We first discuss several numerical experiments that explore the entrain- ment surrounding a subducting slab. The key question we are interested in is how much asthenospheric material is downdragged by subducting slabs. (c) Focusing on the effects of subduction processes, we also exclude mantle plumes as part of the numerical mod- elling domain. In order to highlight the implications of a hot and depleted, hence weak and buoyant astheno- sphere layer we have simplified our setup for the numerical calculations in that phase transitions and melting pro- cesses are not considered. Instead, we begin with a preexisting 200 km-thick asthenosphere layer with a higher tem- perature and more depletion than the underlying mantle and ask how quickly Fig. 1 (a) Cartoon illustrating the ÔtypicalÕ connection of lithospheric plate motions with deeper mantle flow. (b) Cartoon illustrating the possible connection between plate it can be removed by plate subduction. motions and deeper flow when a buoyant low-viscosity asthenosphere decoupling Consider the buoyant astheno- zone is present. (c) A snapshot after 7 Ma from a numerical experiment discussed in sphere entrainment scenario sketched the text that shows the behaviour illustrated in (b). Here the white line marks the in Fig. 1(b) and Box 1. Intuition sug- boundary of asthenosphere material – a gradational asthenosphere–mesosphere gests a simple flow pattern may form, transition (centred around 300 km depth) was made by having the asthensosphere– and this pattern is in fact seen in the mesosphere interface conductively cool for 50 Ma in a static configuration before numerical and laboratory experi- surface plate motions were Ôturned onÕ. The oceanic lithosphere and slab are assumed ments. The moving plate tends to to move at 63 km Ma)1. White streamlines in the mantle wedge show the recirculation drag shallower asthenosphere towards that develops in this region, colours show asthenosphere and mesosphere speeds, and the subduction zone, which creates a arrows show instantaneous flow directions. (The experiments with a gradational relative pressure high that, in turn, asthenosphere–mantle interface evolve quite similarly to previous experiments in locally tilts the asthenosphere base which this transition was assumed to be discontinuous, as seen by comparing the and drives deeper lateral astheno- evolution of supplementary videos oceanside.mpg and oceanside_sharp.mpg.) The sphere flow away from the subduction asthenosphere return flow beneath the base of the oceanic lithosphere is in good zone as discussed in previous studies agreement with Schubert and TurcotteÕs (1972) simple boundary layer model, a (Schubert and Turcotte, 1972; Phipps boundary layer theory for the entrainment around the slab is discussed in Box 1. Morgan and Smith, 1992). In addi- tion, the subducting slab tends to drag the asthenosphere in comparison with larger than the decrease in thermal thin sheets of asthenosphere into the the underlying cooler mantle (Dq ⁄q = buoyancy due to cooling associated deeper mantle at its top and bottom ) ) aDT, with a 3.3 · 10 5 °C 1). Any with the latent heat of partial melting boundaries (Fig. 1b,c). All of these partial melt extraction from upwelling (cf. Phipps Morgan et al., 1995b). Dep- effects are clearly visible in our numer-
Load more

Recommended publications
  • Ocean Basin Bathymetry & Plate Tectonics
    13 September 2018 MAR 110 HW- 3: - OP & PT 1 Homework #3 Ocean Basin Bathymetry & Plate Tectonics 3-1. THE OCEAN BASIN The world’s oceans cover 72% of the Earth’s surface. The bathymetry (depth distribution) of the interconnected ocean basins has been sculpted by the process known as plate tectonics. For example, the bathymetric profile (or cross-section) of the North Atlantic Ocean basin in Figure 3- 1 has many features of a typical ocean basins which is bordered by a continental margin at the ocean’s edge. Starting at the coast, there is a slight deepening of the sea floor as we cross the continental shelf. At the shelf break, the sea floor plunges more steeply down the continental slope; which transitions into the less steep continental rise; which itself transitions into the relatively flat abyssal plain. The continental shelf is the seaward edge of the continent - extending from the beach to the shelf break, with typical depths ranging from 130 m to 200 m. The seafloor of the continental shelf is gently sloping with undulating surfaces - sometimes interrupted by hills and valleys (see Figure 3- 2). Sediments - derived from the weathering of the continental mountain rocks - are delivered by rivers to the continental shelf and beyond. Over wide continental shelves, the sea floor slopes are 1° to 2°, which is virtually flat. Over narrower continental shelves, the sea floor slopes are somewhat steeper. The continental slope connects the continental shelf to the deep ocean with typical depths of 2 to 3 km. While the bottom slope of a typical continental slope region appears steep in the 13 September 2018 MAR 110 HW- 3: - OP & PT 2 vertically-exaggerated valleys pictured (see Figure 3-2), they are typically quite gentle with modest angles of only 4° to 6°.
    [Show full text]
  • Asthenosphere–Lithospheric Mantle Interaction in an Extensional Regime
    Chemical Geology 233 (2006) 309–327 www.elsevier.com/locate/chemgeo Asthenosphere–lithospheric mantle interaction in an extensional regime: Implication from the geochemistry of Cenozoic basalts from Taihang Mountains, North China Craton ⁎ Yan-Jie Tang , Hong-Fu Zhang, Ji-Feng Ying State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, PR China Received 25 July 2005; received in revised form 27 March 2006; accepted 30 March 2006 Abstract Compositions of Cenozoic basalts from the Fansi (26.3–24.3 Ma), Xiyang–Pingding (7.9–7.3 Ma) and Zuoquan (∼5.6 Ma) volcanic fields in the Taihang Mountains provide insight into the nature of their mantle sources and evidence for asthenosphere– lithospheric mantle interaction beneath the North China Craton. These basalts are mainly alkaline (SiO2 =44–50 wt.%, Na2O+ K2O=3.9–6.0 wt.%) and have OIB-like characteristics, as shown in trace element distribution patterns, incompatible elemental (Ba/Nb=6–22, La/Nb=0.5–1.0, Ce/Pb=15–30, Nb/U=29–50) and isotopic ratios (87Sr/86Sr=0.7038–0.7054, 143Nd/ 144 Nd=0.5124–0.5129). Based on TiO2 contents, the Fansi lavas can be classified into two groups: high-Ti and low-Ti. The Fansi high-Ti and Xiyang–Pingding basalts were dominantly derived from an asthenospheric source, while the Zuoquan and Fansi low-Ti basalts show isotopic imprints (higher 87Sr/86Sr and lower 143Nd/144Nd ratios) compatible with some contributions of sub- continental lithospheric mantle. The variation in geochemical compositions of these basalts resulted from the low degree partial melting of asthenosphere and the interaction of asthenosphere-derived magma with old heterogeneous lithospheric mantle in an extensional regime, possibly related to the far effect of the India–Eurasia collision.
    [Show full text]
  • Planet Earth in Cross Section by Michael Osborn Fayetteville-Manlius HS
    Planet Earth in Cross Section By Michael Osborn Fayetteville-Manlius HS Objectives Devise a model of the layers of the Earth to scale. Background Planet Earth is organized into layers of varying thickness. This solid, rocky planet becomes denser as one travels into its interior. Gravity has caused the planet to differentiate, meaning that denser material have been pulled towards Earth’s center. Relatively less dense material migrates to the surface. What follows is a brief description1 of each layer beginning at the center of the Earth and working out towards the atmosphere. Inner Core – The solid innermost sphere of the Earth, about 1271 kilometers in radius. Examination of meteorites has led geologists to infer that the inner core is composed of iron and nickel. Outer Core - A layer surrounding the inner core that is about 2270 kilometers thick and which has the properties of a liquid. Mantle – A solid, 2885-kilometer thick layer of ultra-mafic rock located below the crust. This is the thickest layer of the earth. Asthenosphere – A partially melted layer of ultra-mafic rock in the mantle situated below the lithosphere. Tectonic plates slide along this layer. Lithosphere – The solid outer portion of the Earth that is capable of movement. The lithosphere is a rock layer composed of the crust (felsic continental crust and mafic ocean crust) and the portion of the mafic upper mantle situated above the asthenosphere. Hydrosphere – Refers to the water portion at or near Earth’s surface. The hydrosphere is primarily composed of oceans, but also includes, lakes, streams and groundwater.
    [Show full text]
  • Label and Describe the Earth Diagram
    Name_________________________class______ Label and Describe the Earth Diagram Read the definitions then use the information to color code, label and describe IN YOUR OWN WORDS each section of the diagram below. Definitions: crust – (green) the rigid, rocky outer surface of the Earth, composed mostly of basalt and granite. The crust is the thinnest of all layers. It is thicker on continents & thinner under the oceans. inner core – (gray) the solid iron-nickel center of the Earth that is very hot and under great pressure. mantle – (orange) a rocky layer located under the crust - it is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium. Convection (heat) currents carry heat from the hot inner mantle to the cooler outer mantle. outer core – (red) the molten iron-nickel layer that surrounds the inner core. ______________________________________________ ______________________________________________ ______________________________________________ _______________________________________ _______________________________________ _______________________________________ ___________________________________ ___________________________________ ___________________________________ ___ __________________________________ __________________________________ __________________________________ __________________________ Name_________________________________cLass________ Label the OUTER LAYERS of the Earth This is a cross section of only the upper layers of the Earth’s surface. Read the definitions below and use the information to locate label and describe IN TWO WORDS the outer layers of the Earth. One has been done for you. continental crust – thick, top Continental Crust – (green) the thick parts of the Earth's crust, not located under the ocean; makes up the comtinents. Oceanic Crust – (brown) thinner more dense parts of the Earth's crust located under the oceans. Ocean – (blue) large bodies of water sitting atop oceanic crust. Lithosphere– (outline in black) made of BOTH the crust plus the rigid upper part of the upper mantle.
    [Show full text]
  • The Ocean Basin & Plate Tectonics
    Sept. 2010 MAR 110 HW 3: 1 Homework 3: The Ocean Basin & Plate Tectonics 3-1. THE OCEAN BASIN The world ocean basin is an extensive suite of connected depressions (or basins) that are filled with salty water; covering 72% of the Earth’s surface. The ocean basin bathymetric profile in Figure 3-1 highlights features of a typical ocean basin, which is bordered by continental margins at the ocean’s edge. Starting at the coast, the sea floor the slopes slightly across the continental shelf to the shelf break where it plunges down the steeper continental slope to continental rise and the abyssal plain, which is flat because accumulated sediments. The continental shelf is the flooded edge of the continent -extending from the beach to the shelf break, with typical depths ranging from 130 m to 200 m. The sea floor slopes over wide shelves are 1° to 2° - virtually flat- and somewhat steeper over narrower shelves. Continental shelves are generally gently undulating surfaces, sometimes interrupted by hills and valleys (see Figure 3-2). Sept. 2010 MAR 110 HW 3: 2 The continental slope connects the continental shelf to the deep ocean, with typical depths of 2 to 3 km. While appearing steep in these vertically exaggerated pictures, the bottom slopes of a typical continental slope region are modest angles of 4° to 6°. Continental slope regions adjacent to deep ocean trenches tend to descend somewhat more steeply than normal. Sediments derived from the weathering of the continental material are delivered by rivers and continental shelf flow to the upper continental slope region just beyond the continental shelf break.
    [Show full text]
  • Marine Science and Oceanography
    Marine Science and Oceanography Marine geology- study of the ocean floor Physical oceanography- study of waves, currents, and tides Marine biology– study of nature and distribution of marine organisms Chemical Oceanography- study of the dissolved chemicals in seawater Marine engineering- design and construction of structures used in or on the ocean. Marine Science, or Oceanography, integrates different sciences. 1 2 1 The Sea Floor: Key Ideas * The seafloor has two distinct regions: continental margins and deep-ocean basins * The continental margin is the relatively shallow ocean floor near shore. It shares the structure and composition of the adjacent continent. * The deep-ocean floor differs from the continental margin in tectonic origin, history and composition. * New technology has allowed scientists to accurately map even the deepest ocean trenches. 3 Bathymetry: The Study of Ocean Floor Contours Satellite altimetry measures the sea surface height from orbit. Satellites can bounce 1,000 pulses of radar energy off the ocean surface every second. With the use of satellite altimetry, sea surface levels can be measured more accurately, showing sea surface distortion. 4 2 5 The Physiography of the Ocean Floor Physiography and bathymetry (submarine landscape) allow the sea floor to be subdivided into three distinct provinces: (1) continental margins, (2) deep ocean basins and (3) mid-oceanic ridges. 6 3 The Topography of Ocean Floors The classifications of ocean floor: Continental Margins – the submerged outer edge of a continent Ocean Basin – the deep seafloor beyond the continental margin Ocean Ridge System - extends throughout the ocean basins A typical cross section of the Atlantic ocean basin.
    [Show full text]
  • Origin and Evolution of Asthenospheric Layers
    Two mechanisms of formation of asthenospheric layers L. Czechowski and M. Grad Institute of Geophysics, Faculty of Physics, University of Warsaw Ul. Pasteura 5, 02-093 Warszawa, Poland Phone: +48 22 5532003 E-mail: [email protected] Corresponding author: Leszek Czechowski: [email protected] The theory of plate tectonics describes some basic global tectonic processes as a result of motion of lithospheric plates. The boundary between lithosphere and asthenosphere (LAB) is defined by a difference in response to stress. Position of LAB is determined by: (i) the ratio (melting temperature)/( temperature) and (ii) an invariant of the stress tensor. We consider the role of these both factors for origin and decay of asthenosphere. We find that the asthenosphere of shear stress origin could be a transient, time-dependent feature. Key words: asthenosphere, evolution, LAB, origin of asthenosphere 1. Introduction The plate tectonics theory describes some of the basic tectonic processes on the Earth as motion of lithospheric plates. The plates are moved by large-scale thermal convection in the mantle. The lithosphere is mechanically resistant. Its thickness varies from ~50 to ~250 km. The lithosphere is underlain by the asthenosphere. The boundary between the lithosphere and the asthenosphere (LAB) is defined by a difference in response to stress: the asthenosphere deforms viscously. The bulk of asthenosphere is not melted (1), but at the deformation rate typical for the mantle convection (about 10-14 s-1) it behaves as a fluid with the viscosity η of about 5 1019 kg m-1 s-1. The mantle below has higher effective viscosity (e.g.
    [Show full text]
  • Plate Boundaries - Where the Action Is! Modeling Activity
    Plate Boundaries - Where the action is! Modeling Activity The Earth’s outer shell, called the lithosphere, is broken up into tectonic plates. The lithosphere is rock that is rigid and brittle - it is composed of the crust and the uppermost mantle. The rigid plates move around on top of a hotter more mobile layer of the mantle known as the asthenosphere. Where the plates meet is where most of the world’s earthquakes and volcanic activity occurs. Let’s explore what happens at different plate boundaries. What you need: • Graham crackers • Waxed paper • Peanut butter or frosting and a knife to spread • Small bowl of water PUSH Set up: • Spread peanut butter or frosting on the waxed paper. Cover an area almost twice the size of a whole graham cracker. This is the asthenosphere – soft and mobile. • Break a graham cracker in half, now you have two tectonic plates. The PULL plates are rigid and brittle. When you broke the graham cracker... you caused an earthquake! What you’ll do: Place two graham cracker halves (tectonic plates) on your asthenosphere. Using one finger on each plate, very slowly and gently move your plates around on the asthenosphere. This is how tectonic plates on the Earth’s asthe- nosphere move. Now let’s check out the different types of boundaries between plates. Divergent Plate Boundary These are boundaries where two plates move away from each other. These boundaries are often found on ocean floors. Put the two graham cracker halves (tectonic plates) touching each other in the middle of your asthenosphere.
    [Show full text]
  • Uplift of Earth's Crust
    Standards—7.3.4: Explain how heat flow and movement of material within Earth causes earthquakes and vol- canic eruptions and creates mountains and ocean basins. 7.3.7: Give examples of some changes in Earth’s surface that are abrupt, such as earthquakes and volcanic eruptions, and some changes that happen very slowly, such as uplift and wearing down of mountains and the action of glaciers. Also covers: 7.2.7 (Detailed standards begin on page IN8.) Uplift of Earth’s Crust Building Mountains One popular vacation that people enjoy is a trip to the mountains. Mountains tower over the surrounding land, often providing spectacular views from their summits or from sur- I Describe how Earth’s mountains rounding areas. The highest mountain peak in the world is form and erode. Mount Everest in the Himalaya in Tibet. Its elevation is more I Compare types of mountains. than 8,800 m above sea level. In the United States, the highest I Identify the forces that shape mountains reach an elevation of more than 6,000 m. There are Earth’s mountains. four main types of mountains—fault-block, folded, upwarped, and volcanic. Each type forms in a different way and can pro- The forces inside Earth that cause duce mountains that vary greatly in size. Earth’s plates to move around also are responsible for forming Earth’s Age of a Mountain As you can see in Figure 11, mountains mountains. can be rugged with high, snowcapped peaks, or they can be rounded and forested with gentle valleys and babbling streams.
    [Show full text]
  • What Are the Layers of the Earth? What Are the Characteristics of Each Layer?
    What are the layers of the Earth? What are the characteristics of each layer? The Composition of Earth • The Earth is divided into several layers with different composition and physical properties. • Composition: Earth’s layers are made of different mixtures of elements. • Physical Properties: Temperature, density, and viscosity (ability to flow). The Composition of Earth • The Earth can be divided into three layers by the composition of each layer. • These three layers are the CRUST, MANTLE, and CORE. The Composition of Earth • The lightest materials make up the outermost layer. The more dense material make up the inner layer. The Crust • The outermost layer • Ranging from 5-100 km thick • Thinnest layer of Earth – less than 1% of the Earth’s mass • Least Dense • Made of two layers. • The coolest layer. The Crust • Top layer is primarily granite. Bottom layer is primarily basalt. • Two types of crust: Continental and Oceanic. • Both layers found under continents. • Only basalt layer under the oceans. Continental vs. Oceanic Crust • Makes up Earth’s • Makes up the continents. ocean floor. • Thickest layer of • Not as thick as the Earth’s crust. continental crust. • Least Dense. • More dense than • Composition continental crust. similar to Granite. • Composition similar to Basalt. Mantle • The layer of Earth beneath the crust is called the mantle. • Approximately 67% of Earth’s mass is found in this layer. • The top boundary, the MOHO, is made up of solid rock. In the center of the mantle, the rock is viscous -- it flows like syrup. Mantle • Scientists have been able to examine the molten rock from active ocean volcanoes to gather information about this layer.
    [Show full text]
  • Layers of the Earth
    Notes: Layers of the Earth Crust Outer layer; covers the whole earth; varies in thickness from 5 to 60 Km. Together with the upper mantle, is part of a zone called the lithosphere. There are 2 kinds of crust: continental crust and oceanic crust. Continental Crust Exists under continents Average thickness is 30-50 Km (thickest under mountains), although it can be as thin as 10 Km in places Chemical composition: rocks rich in calcium and aluminum silicates **Note: a silicate contains the molecule, SiO2 Common rock types: granite and rhyolite Rocks are less dense, lighter in color than oceanic crust Oceanic Crust Exists under oceans Average thickness is 7 Km Chemical composition: rocks rich in iron and magnesium silicates Common rock types: basalt, obsidian, gabbro Rocks are more dense, darker in color than continental crust Mantle (Chemical Composition: iron & magnesium silicates) Lies underneath the crust 2900 Km thick The lithosphere is a zone made of the upper mantle and entire crust. It is made of cool, hard rock. Most (but not the very upper part) of the mantle is plastic rock: is both solid and molten at the same time. This zone is called the asthenosphere. Underneath the asthenosphere is the mesosphere, which is solid. The asthenosphere has convection currents, where matter rises to the top, cools, then comes back down again, in a continuous cycle. Core Center of the Earth; ~ 3500 Km thick Outer Core Made of molten iron and nickel 2270 Km thick Inner Core Made of solid iron and nickel. Is solid because of the extreme pressures it is under.
    [Show full text]
  • Crust and Mantle Vs. Lithosphere and Asthenosphere
    Crust and Mantle vs. Lithosphere and Asthenosphere Why do we use two names to describe the same layer of the Earth? Well, this confusion results from the different ways scientists study the Earth. Lithosphere, asthenosphere, and mesosphere (we usually don't discuss this last layer) represent changes in the mechanical properties of the Earth. Crust and mantle refer to changes in the chemical composition of the Earth. Lithosphere and Asthenosphere The lithosphere (litho:rock; sphere:layer) is the strong, upper 100 km of the Earth. The lithosphere is the tectonic plate we talk about in plate tectonics. The asthenosphere (a:without; stheno:strength) is the weak and easily deformed layer of the Earth that acts as a “lubricant” for the tectonic plates to slide over. The asthenosphere extends from 100 km depth to 660 km beneath the Earth's surface. Beneath the asthenosphere is the mesosphere, another strong layer. Crust and Mantle The crust is a chemically distinct layer at the surface of the Earth. Crustal material contains lighter elements like Si, O, Al, Ca, K, Na, etc... Feldspars (Anorthite, Albite, Orthoclase) are comon minerals in the crust (CaAL2Si2O8, NaALSi3O8 , KALSi3O8). The crust may be divided into 2 types: oceanic and continental. Oceanic crust is usually 5-10 km thick and continental crust is 33 km thick on average. Beneath the crust is the mantle. The mantle is made up of Si and O, like the crust, but it contains more Fe and Mg. Thus, Olivine (Fe2SiO4-Mg2SiO4) and pyroxene (MgSiO3-FeSiO3) are abundant in the mantle. The mantle extends to the core-mantle interface at approximately 2900 km depth.
    [Show full text]