Plate tectonics II: Earths structure and plate boundaries
Important: This chapter follows mainly: Chap. 1 in Turcotte and Schubert Chap. 2 in Fowler. Earth structure: The main units
Compositional: Crust Mantle Core Rheological: Lithosphere Asthenosphere Mesosphere Earth structure: The main units
Crust versus mantle: The crust is a product of mantle melting. Typical mantle rocks have a higher magnesium to iron ratio, and a smaller portion of silicon and aluminum than the crust. Lithosphere versus asthenosphere: While the lithosphere behaves as a rigid body over geologic time scales, the asthenosphere deforms in ductile fashion. The lithosphere is fragmented into tectonic plates, which move relative to one another. There are two types of lithosphere: oceanic and continental. Upper versus lower mantle: Together the lithosphere and the asthenosphere form the upper mantle. The mesosphere, extending between the 660 boundary and the outer core, corresponds to the lower mantle. The region between 410 and 660 km is referred to as the transition zone. Earth structure: Mantle phase changes
410 km: Above this depth the Mg, Fe, Si and O are primarily within olivine and pyroxene. Below this depth the olivine is no longer stable and is replaced by a higher density polymorph - spinel. The material has a similar overall composition but the minerals have a more compact structure. 660 km: Below this depth the spinel gives way to the minerals Mg-perovskite and Mg-wustite. (In fact, Mg-perovskite is probably the most abundant solid of the earth since it appears to be stable through much of the mantle.) Earth structure: Seismic discontinuities
Moho: The dept at which the P-wave velocity exceeds 8.1 Km/S is referred to as the moho (after the seismologist Mohorovicic). The moho is both a seismic and a compositional boundary, marking the transition between crust and mantle materials. Low Velocity Zone (LVZ): The low velocity is more strongly visible for S-waves than for P-waves. It marks the boundary between the lithosphere and the asthenosphere. Earth structure: Seismic discontinuities
Thickness of the Earth's crust (by the USGS). Since the Moho is at the base of the crust this map also shows depth to Moho. Earth structure: Seismic discontinuities
The LVZ is deeper under shield and platforms, than it is under oceanic basins and continental rifts. Earth structure: Seismic discontinuities
D: There is evidence of a seismic discontinuity about 200 km above the core-mantle boundary (CMB). This is known as the D" discontinuity, and while we don't know much about it, it appears to be ubiquitous, although its position varies from less than 100 km to over 300 km above the CMB. Earth structure: Seismic discontinuities
Tomographic images of the P and S velocity perturbation, averaged vertically over the deepest 1000 km of the mantle, reveal structures with vertical continuity over that depth range. Negative (reddish) anomalies indicate the presence of lower mantle plumes. Figure from Montelli et al., 2006 Earth structure: Seismic discontinuities
Seismic images suggesting that some mantle plumes originate at the D. Figure from Montelli et al., 2004 Earth structure: Seismic discontinuities Earth structure: Seismic discontinuities
It has been suggested, based on tomography (i.e., seismic imaging), that the D is a slab graveyard and/or plume factory Figure from: Earth structure: Core The shadow zones Earth structure: Core Does Earths inner core rotate slower, faster or at the same rate as the rest of the plant? see animation on: Plate boundaries: MOR Lithospheric plates are created at ocean ridges.
The two plates on either side of an ocean ridge move away from each other with near constant velocities of a few tens of millimeters per year. As the two plates diverge, hot mantle rock flows upward to fill the gap. The upwelling mantle rock cools by conductive heat loss to the surface. The cooling rock accretes to the base of the spreading plates, becoming part of them Plate boundaries: MOR As the plates move away from the ocean ridge, they continue to cool and the lithosphere thickens. As the lithosphere cools, it becomes more dense; as a result it sinks downward into the underlying mantle rock. The topographic elevation of the ridge is due to the greater buoyancy of the thinner, hotter lithosphere near the axis of accretion at the ridge crest. Plate boundaries: MOR The elevation of the ocean ridge also provides a body force that causes the plates to move away from the ridge crest. A component of the gravitational body force on the elevated lithosphere drives the lithosphere away from the accretional boundary. This force on the lithosphere is known as ridge push and is a form of gravitational sliding. Plate boundaries: MOR The volcanism at ocean ridges is caused by pressure-release melting. As the two adjacent plates move apart, hot mantle rock ascends to fill the gap. The temperature of the ascending rock is nearly constant, but its pressure decreases. When the temperature of the ascending mantle rock equals the solidus temperature, melting occurs. Plate boundaries: MOR In some localities slices of oceanic crust and underlying mantle have been brought to the surface. These are known as ophiolites; they occur in such locations as Cyprus, Newfoundland, Oman, and New Guinea. Field studies of ophiolites have provided a detailed understanding of the oceanic crust and underlying mantle. Plate boundaries: MOR Layer 1 is composed of sediments that are deposited on the volcanic rocks of layers 2 and 3. The thickness of sediments increases with distance from the ridge crest; a typical thickness is 1 km. Layers 2 and 3 are composed of basaltic rocks of nearly uniform composition. Plate boundaries: MOR Layer 2 of the oceanic crust is composed of extrusive volcanic flows that have interacted with the seawater to form pillow lavas and intrusive flows primarily in the form of sheeted dikes. A typical thickness for layer 2 is 1.5 km. Plate boundaries: MOR Plate boundaries: MOR Layer 3 is made up of gab- bros and related cumulate rocks that crystallized directly from the magma chamber. Gabbros are coarse-grained basalts; the larger grain size is due to slower cooling rates at greater depths. The thickness of layer 3 is typically 4.5 km. Plate boundaries: Subduction
The negative buoyancy of the dense rocks of the descending lithosphere results in a downward body force. Because the lithosphere behaves elastically, it can transmit stresses and acts as a stress guide. The body force acting on the descending plate is transmitted to the surface plate, which is pulled toward the ocean trench. This is one of the important forces driving plate tectonics and continental drift. It is known as slab pull. Deriving forces: Ridge push Slab pull Resisting Forces: Viscous traction Frictional resistance Additional forces: Slab suction Elastic bending Fig from Heki and Mitsui, EPSL, 2013 Plate boundaries: Subduction
Since the gravitational body force on the subducted lithosphere is downward, it would be expected that the subduction dip angle would be 90. In fact, the typical dip angle for a subduction zone is near 45. One explanation is that the subducted slab is supported by the induced flow above the slab. The descending lithosphere induces a corner flow in the mantle wedge above it. The dip of a subducting lithosphere is a direct consequence of the balance between the gravitational torque and the lifting pressure torque, i.e. the slab suction. Plate boundaries: Subduction
In some trench systems a secondary accretionary plate margin lies be- hind the volcanic line. This back-arc spreading is very similar to the seafloor spreading that is occurring at ocean ridges. Plate boundaries: Subduction
A number of explanations have been given for back-arc spreading: Option 1, the descending lithosphere induces a secondary convection cell (panel-a). Option 2, the ocean trench migrates away from an adjacent continent because due to the sinking of the descending lithosphere, and the back-arc spreading is required to fill the gap (panel-b). Plate boundaries: Subduction
Isotherms in a lithosphere descending at an angle of 45 into the mantle As the subducted lithosphere descends into the mantle, frictional heating occurs at its upper boundary. The effect of frictional heating gives rise to the isotherms in the slab. The low temperatures in the descending lithosphere cause it to have a higher density than the surrounding mantle. The higher density results in a body force driving the descending lithosphere downward. Plate boundaries: Subduction
An additional downward body force on the descending slab is provided by the distortion of the olivinespinel phase boundary in the slab. The olivinespinel phase boundary is elevated in the descending lithosphere as compared with its position in the surrounding mantle because the pressure at which the phase change occurs depends on temperature. P T spinel olivine Sketch of the Clapeyron curve, which gives the pressures and temperatures at which two phases of the same material, such as olivine and spinel, are in equilibrium. Plate boundaries: Subduction
The same approach can also be applied to the transition of spinel to perovskite. In this case the slope of the Clapeyron curve is negative and the transition occurs at a deeper depth (higher pressure) in the slab. P T spinel post-spinel Plate boundaries: Subduction
The phase change from spinel to perovskite could act to deter penetration of the descending lithosphere. Shallow subduction earthquakes generally indicate extensional stresses where as the deeper earthquakes indicate compressional stresses. This is also an indication of a resistance to subduction. Fig. from: Wolfgang, Meschede and Blake Plate boundaries: Subduction
Earthquakes terminate at a depth of about 660 km, but termination of seismicity does not imply cessation of subduction. Fig. from: Wolfgang, Meschede and Blake Plate boundaries: Subduction
The fate of the descending plate has important implications regarding mantle convection. Figure from Fukao et al., 2001 Blue = fast anomaly = dense = cold Red = slow anomaly = buoyant = hot Plate boundaries: Subduction
Currently, it seems that the answer to this fundamental question is in the eye of the beholder. (learn more at:) Figure from Zhao et al., 2004 Plate boundaries: Subduction
The remaining of the Farallon plate underneath N. America? Plate boundaries: Subduction