Forces Acting on the Plates (according to C.M.Fowler’s The solid Earth )

Forces Acting on the Plates(according to C.M.Fowler’sThe solid Earth) By: Jacob Hadley

Forces Acting on the Plates The Lithosphere: – Cold upper thermal boundary layer – Forms in models of thermal convection of the mantle – Motion of lithospheric plates relative to each other involves many different forces – Some of these forces “drive” motion, and some “resist” motion

Lithospheric Forces… – If plates are at constant velocity, conservation must be in action – Driving = Resistive

Possible Lithospheric Forces: – FDF Mantle-Drag – FCD Extra Mantle-Drag Beneath Continents – FRP Ridge-Push – FTF Transform-Fault Resistance – FSP Slab-Pull – FSR Slab-Resistance (on the slab as it penetrates the asthenosphere) – FCR Colliding-Resistance (which acts between two plates with equal magnitude and and opposite direction) – FSUSuctional Force (which may be capable of pulling the overriding plate towards the trench)

Driving Forces:(primary driving forces) – FRP Ridge-Push – FSP Slab-Pull

Driving Forces: – Ridge-Push Force: • Acts at mid-ocean ridges near edge of plate • Made of two parts: – The pushing by the upwelling mantle material – The tendency of newly formed plate to slide down the sides of the ridge

Ridge-Push: – e = elevation of ridge axis above cooled plate – ρm= density of mantle at base – ρw= density of sea water – L = plate thickness

Driving Forces: (continued) – Another force is the Slab-Pull Force which is simply the “negative buoyancy” of the subducting plate at a convergent boundary – This occurs because the subductingplate is cool and more dense than the mantle in which it descends – An estimate is made “per unit length of subducting zone,” occurring at depth z and simply caused by density contrast

Slab-Pull Force: – z = depth beneath base of plate – α = coefficient of thermal expansion – T1 = temperature of mantle – d+L = thickness of upper mantle – Ret = {cp= specific heat ; and v = rate at which mantle slab sinks}

Slab-Pull Force: – The total force available is evaluated at FSP(0) where z = 0 – As FSP(z) decreases with depth into the mantle it approaches 0 at z = d ; FSP(d) = 0

Driving Forces: (continued) – This graph plots FSP(0) as a function of subduction velocity v – As v −> ∞ a limit becomes prevalent – This plot of subduction velocity displays the dependence of total force on composition of velocity

Driving Forces: (conclusion) – Total FSP is estimated to be ~1013 N m-1 in magnitude – This is greater than the estimate of FRP at ~1012N m-1 – Both of these driving forces (FSP+FRP) are caused by the density difference between the hot and cold mantle – Hot mantle rises as cold mantle sinks

Resisting Forces:(primary resisting forces) – FDF Mantle-Drag – FCD Extra Mantle-Drag Beneath Continents – FTF Transform-Fault Resistance – FSR Slab-Resistance – FCR Colliding-Resistance – FSUSuctional Force

Resisting Forces:

Resistive Force: – These resistive forces occurs at ridge axis of plates • causing shallow earthquakes – Resistivity is alsoalong bases of plates as mantle-dragforce [FDF] •assuming mantle flow < plate velocity (if opposite occurs it simply becomes a driving force)

Resistive Force: (continued) – Transform-Fault Resistance [FTF] • Produces earthquakes – Slab-Resistance [FSR] • Estimates of forces suggest the resistive force on top of sinking slab to be greater than forces acting on its sides

Resistive Force: (continued) – Resistive forces acting on the base of plate are proportional to the area of plate; also, of the same magnitude of resistive forces on descending slab – These forces are calculated by D.E. for flow in a fluid –This is logically proportional to the product of mantle viscosity [η] and plate velocity [v] – The product of [ηv] isestimated at ~1013 N m-1 in magnitude

Resistive Force: (conclusion) – It is difficult to estimate resistive forces on faults – Transform-Fault [FTF] resistive forces are possibly of same magnitude as FRP at ~1012 or smaller – Again, this is still smaller than the mantle-drag force [FDF]

Lithospheric Forces: (conclusion) – Driving Forces • Slab-Pull [FSP] • Ridge-Push [FRP] – Resistive Forces • Slab-Resistance • Transform-Fault Resistance

Mantle Convection ControllingPlate Tectonics????? – Does the mantle convection drag plates around? OR – Do the acting forces at edges of plates simply drive tectonics? Tough to decide…

Mantle Convection: (continued) – Analysis of driving and resistive forces shows that Slab-Pull is vital in determination of mantle flow – Difficulty lies in the fact that not all ridges are above convection cells • Not all plates have a ridge AND subduction • Antarctic and African plates are surrounded by ridges –Ridge-Push and Continental-Collision control stresses in most tectonics

Mantle Convection: (continued) – Analysis of N. American plate displaysforces driving and deforming the continental section of the plate – Primary driving force of N. American plate is the Ridge-Push force from the Mid-Atlantic Ridge – The resistive forces of the Pacific plate against N. American plate is 25% of driving force, resulting in compression of the plate

Mantle Convection: (conclusion) – Flow of mantle and motion of lithospheric plates is still obscure – The pull of descending plates at boundaries due to temperature decrease is a major role on thermal modeling of mantle flow and mechanical models of forces

Archaean Tectonic Activity: – Force-balancing models are used to inspect probability of tectonic activity during Archaean • Plate velocities can be estimated – Earth was likely much hotter in the past • Temperatures at 1700oC during Archaean as opposed to 1300–1400oC today

Archaean Tectonic Activity: (concluded) – Equating driving/resistive forces allows for estimation of viscosity and plate velocities – Plate velocities must have been high (~50cm yr-1) around four billion years ago – High plate velocities ought to have been necessary during Archaean to maintain high rate of heat loss through oceans

Forces Acting on the Plates (according to C.M.Fowler’s The solid Earth )