Skip to content

Asthenosphere And Lithosphere Compare Contrast Essays

Both the lithosphere and asthenosphere are part of Earth and are made of similar material. Lithosphere is made up of Earth's outermost layer, the crust, and the uppermost portion of the mantle. In comparison, the asthenosphere is the upper portion of Earth's mantle (which is also the middle layer of Earth). The lithosphere lies over the asthenosphere. In fact, if any material from the asthenosphere were to solidify, it would become part of the lithosphere.

...

Both the lithosphere and asthenosphere are part of Earth and are made of similar material. Lithosphere is made up of Earth's outermost layer, the crust, and the uppermost portion of the mantle. In comparison, the asthenosphere is the upper portion of Earth's mantle (which is also the middle layer of Earth). The lithosphere lies over the asthenosphere. In fact, if any material from the asthenosphere were to solidify, it would become part of the lithosphere.

Being closer to the Earth's core, the asthenosphere is a higher temperature as compared to the lithosphere and hence its rocks are plastic and can flow. In comparison, the lithosphere's rocks are more rigid. The asthenosphere is more dense and viscous in comparison to the lithosphere. The lithosphere is comprised of a large number of fragments, each of which is known as the tectonic plate. These tectonic plates are in constant motion and are floating over the plastic material underneath.

Hope this helps. 

Viscous coupling at the lithosphere-asthenosphere boundary

Authors

  • Tobias Höink,

    1. Department of Earth Science, Rice University, Houston, Texas, USA
    Search for more papers by this author
  • A. Mark Jellinek,

    1. Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia, Canada
    Search for more papers by this author
  • Adrian Lenardic

    1. Department of Earth Science, Rice University, Houston, Texas, USA
    Search for more papers by this author

Abstract

Tectonic plate motions reflect dynamical contributions from subduction processes (i.e., classical “slab-pull” forces) and lateral pressure gradients within the asthenosphere (“asthenosphere-drive” forces), which are distinct from gravity forces exerted by elevated mid-ocean ridges (i.e., classical “ridge-push” forces). Here we use scaling analysis to show that the extent to which asthenosphere-drive contributes to plate motions depends on the lateral dimension of plates and on the relative viscosities and thicknesses of the lithosphere and asthenosphere. Whereas slab-pull forces always govern the motions of plates with a lateral extent greater than the mantle depth, asthenosphere-drive forces can be relatively more important for smaller (shorter wavelength) plates, large relative asthenosphere viscosities or large asthenosphere thicknesses. Published plate velocities, tomographic images and age-binned mean shear wave velocity anomaly data allow us to estimate the relative contributions of slab-pull and asthenosphere-drive forces for the motions of the Atlantic and Pacific plates. Whereas the Pacific plate is driven largely by slab pull, the Atlantic plate is predicted to be strongly driven by basal forces related to viscous coupling to strong asthenospheric flow, consistent with recent observations related to the stress state of North America. In addition, compared to the East Pacific Rise (EPR), the relatively large lateral pressure gradient near the Mid-Atlantic Ridge (MAR) is expected to produce significantly steeper dynamic topography. Thus, the relative importance of this plate-driving force may partly explain why the flanking topography at the EPR is smoother than at the MAR. Our analysis also indicates that this plate-driving force was more significant, and heat loss less efficient, in Earth's hotter past compared with its cooler present state. This type of trend is consistent with thermal history modeling results which require less efficient heat transfer in Earth's past.

1. Introduction

Extensive recent work has investigated how the asthenosphere, a sub-lithospheric zone of low viscosity, governs the viscous resistance to plate motions, and, in turn, the wavelength of mantle convection [Richards et al., 2001; Busse et al., 2006; Lenardic et al., 2006; Höink and Lenardic, 2008, 2010]. Three-dimensional mantle convection simulations [Höink and Lenardic, 2010] show also that buoyancy effects related to lateral temperature variations in the asthenosphere may influence the driving force for plate tectonics under certain conditions. For example, Figure 1a shows a snapshot from a mantle convection simulation in which a cold, high viscosity lithosphere that is subducting at the right side of the box overlies a warm, low-viscosity asthenosphere. Each vertical velocity profile shown can be decomposed into the superposition of a linear velocity gradient related to simple shear imparted to the asthenosphere at the base of the moving lithosphere (i.e., the “Couette flow” component) and a parabolic velocity profile in the asthenosphere associated with the flow driven by lateral temperature variations (i.e., the “Poiseuille flow” component). The downstream evolution of the profiles shows a monotonic reduction in the strength of the Poiseuille component with distance. In particular, over lateral length scales comparable to or less than the mantle depth, flow in the asthenosphere is dominantly of Poiseuille type, which implies the potential for a significant contribution to the observed surface plate velocity. Flow in the asthenosphere can provide a plate driving force acting on the base of the plate, which is distinct from the ridge-push force, in which elevated topography, generated at mid-ocean ridges, is acting on the plate volume. In the limit in which plates are not moving, i.e. the limit of stagnant-lid convection, ridge-push forces cannot exist for the lack of ridges. In contrast, the asthenosphere-drive mechanism is still predicted to exert a force at the base of plate, but at a magnitude too small to overcome the strength of the plate.

Related to the asthenosphere-drive mechanism, but developed independently, Alvarez [2010] has proposed a mechanism termed “continental undertow” in which continental plates are driven by basal tractions, explaining protracted continental collisions which lack a driving mechanism associated with slab pull.

Moreover, Höink and Lenardic [2010] find two distinct plate motion regimes with a transition that is governed by the nature of the asthenosphere flow and the wavelength of the plate-scale convection (cf. distinct breaks in scaling trends in Figures 1b and 1c). In more detail, they identify a power law relationship between the convective cell aspect ratio and the asthenosphere velocity ratio, i.e. the ratio of pressure-driven velocity component and velocity component due to shear from the overriding lithosphere. The empirical relationship is found to be universal for the system explored numerically in that it holds across the transition from short to long wavelength flow regimes. Here, we use theoretical scaling analysis to investigate how the numerically observed trend across the regime transition might emerge as a result of the viscous coupling between the asthenosphere and lithosphere layers. This analysis is able to predict numerical simulation results. We apply our analysis to make predictions for the leading-order force balances driving the motions of the Atlantic and Pacific plates. We also discuss implications for Earth's thermal history.

2. Lithosphere-Asthenosphere Model

Motivated by first-order observations of oceanic plates (progressive lithosphere thickening with distance from the ridge [e.g., Parsons and McKenzie, 1978] and the existence of a low-viscosity asthenosphere [e.g., Gutenberg, 1959; Hager and Richards, 1989]), we consider a simple lithosphere-asthenosphere model, shown in Figure 2. In this model, the thermal lid thickness, marked by an isotherm, TL, increases with distance from the ridge, x, as a result of continuous cooling to the surface. The rate of thermal lid thickening decreases away from the ridge. The relatively cold temperatures of the lithosphere lead to an average high viscosity. Underlying the lithosphere is the asthenosphere, a layer of higher temperature and thus lower average viscosity. Dehydration at the ridge generates a column of dehydrated material, hdry, which advances with the plate, leading to an additional viscosity stratification [Hirth and Kohlstedt, 1996; Lee et al., 2005]. For the following scaling analysis (beginning in section 4) we define lithosphere and asthenosphere in terms of viscosity, and use average viscosities in lithosphere, μL, and asthenosphere, μA. We assume that the low viscosity of the asthenosphere is related to its temperature, the concentration of water and the presence of partial melt. The vertical temperature and viscosity variations in the asthenosphere are very small in comparison to the lithosphere. Consequently the vertical structure and thickness of the asthenosphere is probably governed by the amount of dissolved water with some contribution from partial melt. Both of these effects are particularly sensitive to pressure and thus we take the asthenosphere to have a relatively flat lower boundary. The thickness of the asthenosphere, hA, is taken to be approximately constant, which is strictly appropriate at distances greater than hA from the ridge. For the thickness of the lithosphere we set hL = hdry.

In this work we define the asthenosphere in terms of relatively low viscosity. Earlier work has investigated different plate models (e.g. “plate model” [McKenzie, 1967] and “CHABLIS”, in which a constant heat flow is applied to the bottom lithospheric isotherm [Doin and Fleitout, 1996]) with respect to how well observations of topography and heat flow can be matched. For the purpose of this paper we do not assume a specific model of thermal lithospheric thickening. The only assumption here is that the thermal lithosphere thickens with distance from the ridge.

A constant surface temperature and progressive thermal lid thickening away from the ridge lead to two opposing flows in the asthenosphere. Whereas lateral differences in hydrostatic pressure (Figure 2b) essentially “squeezes” the asthenospheric material towards the ridge, related lateral temperature variations (Figure 2a) cause relatively cold asthenosphere to spread away from the ridge. Where either mechanism governs the dynamics in the asthenosphere and whether these dynamics influence the overlying plate in a significant way is considered in section 5 below.

In our analysis we consider the lithosphere and asthenosphere system to be a viscously coupled two-layer fluid system where an effectively high-viscosity mechanical lithosphere of depth hL(x), average viscosity μL and density ρL overlies a low-viscosity asthenosphere of depth hA, average density ρA and viscosity μA. Whereas lateral variations in temperature lead to only minor variations in viscosity in the asthenosphere (most of the temperature effect on viscosity occurs in the lithosphere), they are gravitationally unstable and can drive flow.

Viscous coupling across the lithosphere-asthenosphere interface implies that the average motions within one layer will affect the mean velocity of the other layer. Viscous coupling has previously been considered mainly in one direction: Large scale plate motions, driven by the buoyancy-regulated slab-pull force [e.g., Davies and Richards, 1992], can shear the asthenosphere. However, for appropriate asthenosphere viscosities and sufficiently large asthenospheric flow velocities viscous coupling at the lithosphere-asthenosphere boundary leads to another plate driving force: asthenosphere-drive. We will discuss the origin of the asthenosphere-drive more fully below, and we will identify the conditions in which this force can enhance plate motions.

3. Asthenosphere-Drive Versus Convective Traction

The concept of convective traction goes back to the 1930's when Holmes [1931] suggested the “conveyor belt” hypothesis, in which continents are passive rafts driven by convective flow within Earth's mantle. After the advent of plate tectonics and numerous studies on the causes and mechanics of plate motion [e.g., Elsasser, 1967; McKenzie, 1969], the idea of convective traction driving plates fell out of favor. Richter [1973] concluded that fluid dynamic models of the lithosphere-asthenosphere system are “incapable of generating flows in the asthenosphere that move overlying lithospheric plates by viscous traction”. He further concluded that a descending slab provides the dominant plate driving force. Noting that viscous coupling at the base of the plate, depending on the relative velocity between plate and asthenosphere, can either be driving or resisting plate motion, Forsyth and Uyeda [1975] and subsequent workers concluded that the asthenosphere is passive, and convective tractions only resist plate motion. Recently, Alvarez [2010] has brought back the potential of tractions driving plate motions. He explained protracted continental collisions on Earth, which lack a driving mechanism associated with the subduction of oceanic plates (i.e., slab pull), by a mechanism named “continental undertow”, in which continents move as a result of horizontal traction of the mantle acting on the edges and base of deep continental roots.

The idea of shear tractions driving oceanic plates has on the other hand not been revitalized. Although the possibility is acknowledged [e.g., Schubert et al., 2001] the majority view remains that the asthenosphere resists motion.

Although we build on the concept of shear tractions at the base of the plate, the idea of asthenosphere-drive is conceptually different from this traditional concept of shear tractions: Asthenosphere-drive results from viscous coupling of channelized flow in the asthenosphere to the base of a plate, which is moving slower than flow in the asthenosphere. They key assumptions here are that the asthenosphere is a channel of low viscosity and that flow is strongly channelized (i.e. horizontal mantle flow does not extend below the low-viscosity region of the asthenosphere). This leads to distinctly different predictions than older convective traction ideas. Our treatment of this problem allows us to make specific predictions, such as under which conditions asthenosphere-drive is an important driving force, and how the velocity ratio of plate to asthenosphere depends on the plate length. We also point out that these predictions can be tested against numerical simulations and, as we show in section 8, can be compared with observations on Earth to determine if they are consistent with the observations.

4. Asthenosphere-Drive Versus Ridge-Push

The asthenosphere-drive mechanism is also distinct from the classical ridge-push picture. Ridge-push forces arise in response to gradients in hydrostatic head resulting from elevated topography at mid-ocean ridges [Forsyth and Uyeda, 1975]. This force is a body force that acts perpendicular to the strike of the ridge to push the lithosphere away from the ridge. Ridge-push forces are of much smaller magnitude than forces related to the subduction of cold, dense slabs [McKenzie, 1969; Conrad and Lithgow-Bertelloni, 2002].

In contrast, the asthenosphere-drive force is a surface force acting on the base of plate. It results from lateral temperature gradients that lead, in turn, to lateral gradients in hydrostatic pressure that drive flow in the asthenosphere. When asthenospheric flow velocities exceed plate velocities, viscous drag imparted at the asthenosphere-lithosphere boundary acts to draw the plate in the direction of this flow. We note that asthenosphere-drive is only a driving force when flow velocities below the plate exceed plate velocities, and we discuss the conditions for this to occur below.

The qualitative difference between asthenosphere-drive and ridge-push leads to a new class of predictions. In particularly, in contrast to ridge-push, which assumes that topographic differences are isostatically balanced at depth, asthenosphere-drive predicts that topographic differences are balanced by dynamic contributions from pressure gradients that exist in the asthenosphere. How these dynamic contributions can explain the observed difference of mid-ocean ridge topography is discussed in section 8.4.

5. Stress Coupling Between Lithosphere and Asthenosphere

The surface layer velocity can be written as

where the “plate component” UP is the contribution to the plate velocity from the slab-pull force and UD is the velocity component resulting from the asthenosphere-drive force. At very high Rayleigh number (Ra > 106) heat transfer is approximately independent of the layer depth [Moore, 2008]. In this case, the sinking velocity of a drip such as the cold plume shown at the right side of Figure 1 is close to that of a discrete thermal [cf. Griffiths, 1986] and is proportional to Ram1/3 [Turner, 1979; O'Neill et al., 2007]. Following standard boundary layer assumptions we assume that the surface velocity scales as the sinking velocity [e.g., Turcotte and Oxburgh, 1967; Jellinek and Manga, 2004, and references therein]. The scaling for the plate component is then

where κ is thermal diffusivity, H is the depth of the convecting system (e.g. the mantle in Earth's terms), Ram is the mantle Rayleigh number.

In the simulations by Höink and Lenardic [2010], lateral flow within the asthenosphere is, in part, a response to lateral temperature variations ΔTlat, arising as a result of a constant surface temperature and increasing lithosphere thickness away from a ridge in proportion to the square root of age (Figure 2c). These lateral temperature variations give rise to density differences and, in turn, to gradients in hydrostatic pressure that drive flow away from the ridge. The strength of this flow depends on the magnitude of the buoyancy force, which can be expressed in terms an effective gravitational acceleration

where g is the acceleration due to gravity, α is the thermal expansion coefficient and ΔTlat is the scale of lateral temperature variations (Figure 2c). We note that in the Earth these lateral temperature variations may be enhanced locally by the spreading of plume material [Morgan and Smith, 1992; Morgan et al., 1995; Gaherty, 2001], and thus our models will give, on average, a lower bound on the magnitude of these buoyancy effects.

The increasing lithosphere thickness with age also depresses and “squeezes” the asthenosphere, resulting in an additional, but opposing, pressure gradient, the so-called “lubrication pressure” [e.g., Joseph, 1980]. If the thermal lithosphere thickens proportionally with square-root of plate age (Figure 2b) until it flattens out between 20 Ma [Stein and Stein, 1994] and 80 Ma [Parsons and Sclater, 1977], it scales with distance from the ridge, L, as dhL/dL, where κ is the thermal diffusion coefficient. Lithosphere thickening produces a lateral pressure gradient dp/dx = gtopo ρA dhL/dL, where gtopo is the effective gravitational acceleration and ρA is the average asthenosphere density, which generates asthenosphere flow according to μAUA/hA2 = dp/dx. The gravitational acceleration from this effect scales as

For reasonable physical and geometric conditions [e.g., Parsons and McKenzie, 1978; Hillier and Watts, 2004] (g = 10 m/s2, α = 5 × 105K−1, ΔTlat ∼ 100 K, μA = 1019Pa s, ρA = 3300 kg/m3, κ = 10−6m2/s, hA = 70 km, UAUP ∼ 1 cm/a) and assuming that hA is approximately constant at distances of order hA away from the ridge, we determine from (3) and (4) that the two gravitational accelerations, or pressure gradients, become comparable in magnitude at length scales (L) larger than 50,000 km (i.e., greater than the Earth's circumference). This demonstrates that the effect of temperature variations is generally more important for the dynamics of the asthenosphere.

Assuming LH an appropriate scale for the spreading rate for the gravity current that originates from lateral temperature variations is

where νA = μA/ρ is the kinematic viscosity of the bottom layer. To characterize the strength of this flow it will be useful to introduce an asthenosphere Rayleigh number as

Whether flow in the asthenosphere can influence plate motions depends critically on the extent of the viscous coupling between them. Continuity of viscous stresses at the asthenosphere-lithosphere interface demands that

Combining (6) and (7) leads to

The relative contributions of drag and plate components to the observed surface velocity are given by the lithosphere velocity ratio

where Γ = L/H is the aspect ratio and only the dependence on mantle Rayleigh number Ram remains to be identified.

6. Characteristic Length Scale

We start by recognizing that the Rayleigh number is foremost defined as a ratio of two time scales, i.e. the ratio of characteristic times for diffusive to advective heat transfer across a scale length. In our model of the lithosphere-asthenosphere system (see Figure 2) the convection cell length L is a natural choice for a characteristic length scale when LH since we are concerned with lateral heat transport. Moreover, such a choice is appropriate in any convecting system in which the wavelength of the flow substantially exceeds the layer depth.

With time scales for diffusion and advection

the mantle Rayleigh number is

from which the velocity ratio in the surface layer (equation (9)) follows

We note that the theory assumes a channel of low viscosity (μAμL)and finite thickness (hA > 0), consistent with many observations and inferences [Gutenberg, 1959; Hager and Richards, 1989; Thoraval and Richards, 1997; Paulson and Richards, 2009]. Further it will break down if μA/μL ≈ 1, because the whole notion of a channelized flow disappears.

Before we apply this approach to a specific example we will take a closer look at equation (12) in the context of lithosphere and asthenosphere. The lithosphere velocity ratio UD/UP tends towards zero in the limits of vanishing asthenosphere viscosity (μA/μL ≪ 1) or very large asthenosphere thickness (hA/hL ≫ 1). This end-member characterizes the classic picture of a top-driven lithosphere. The other end-member is a bottom-driven lithosphere, i.e. a case where the lithosphere is dragged by the underlying asthenosphere, and the lithosphere velocity ratio is much larger than one (UD/UP ≫ 1). This regime can exist for sufficiently small aspect ratios (small L/H), or for a thick asthenosphere (the buoyancy force scales with hA3). The transition between those two regimes, between plate driven lithosphere and asthenosphere-driven lithosphere, should occur around UD/UP ≈ 1 and can be estimated with equation (12).

7. Ratio of Asthenosphere Velocity Components