The Earth’s outer layer is made up of several large, moving pieces called plates. These plates ride atop the mantle, which is the mostly solid but viscous layer between Earth’s crust and core. The crustal plates “float” on the mantle due to their lower density compared to the ultra-hot material beneath them.
What are crustal plates?
The Earth’s crust is composed of about a dozen major and several minor plates of solid rock material. These plates cover the entire planet much like a spherical jigsaw puzzle. The plates range in size from small microplates, such as the Juan de Fuca and Gorda plates off the northwestern coast of North America, to very large plates like the Pacific and North American plates.
The Pacific plate, for example, extends from the west coast of North America to the Mariana Trench in the western Pacific Ocean south of Japan. It covers an area of about 103,300,000 square kilometers. Meanwhile, the Caribbean plate is about 3,300,000 square kilometers in area.
The crustal plates consist of the crust and the rigid uppermost portion of the mantle. The crust varies in thickness between about 5-10 km on the ocean floor to 30-50 km on continents. Beneath the crust, the upper mantle extends down to a depth of about 100 km below the surface.
What is the mantle?
The mantle is the mostly solid, rocky part of the Earth under the crust and above the core. It has a thickness of about 2,900 km, making up a whopping 84% of the Earth’s total volume. The mantle is divided into sections:
- Upper mantle – from the base of the crust down to 660 km depth
- Transition zone – a 250-400 km thick area from 660-1000 km depth
- Lower mantle – from the bottom of the transition zone to 2,891 km depth
The mantle is composed of very hot rock rich in iron, magnesium, aluminum, silicon and oxygen. While mostly solid, the high temperatures make the mantle rocks ductile, allowing them to slowly deform and flow over geologic timescales in a process called convection.
How do the plates move?
The crustal plates are constantly in motion, shifting at rates of up to 10 cm per year as they ride atop the convecting mantle. This is similar to the motion of droplets of food coloring added to a pot of boiling water. Driven by convection currents in the mantle, the plates experience lateral motions relative to each other in three different ways:
- Divergent boundaries – plates move apart from each other
- Convergent boundaries – plates move towards each other
- Transform boundaries – plates slide past each other
At divergent boundaries like mid-ocean ridges, magma rises from the mantle to fill the gap between separating plates, creating new crust and gradually pushing the plates further apart over time. Subduction zones are convergent boundaries where two plates collide, with one plate sinking beneath the other into the mantle in the process.
The sliding motion along transform faults like the San Andreas fault in California occurs as the plates on either side move laterally past each other. The buildup of friction across the boundary leads to occasional earthquakes as the plates suddenly jerk past each other.
What forces drive plate motion?
The exact forces driving plate tectonics are still not completely understood, but likely involve:
- Slab pull – The sinking of cold, dense oceanic lithosphere at subduction zones provides an important downward pull on the rest of the subducting plate.
- Ridge push – Upwelling magma at mid-ocean ridges elevates and expands the ocean crust, pushing the plates horizontally.
- Mantle convection – Circulation of hot, viscous mantle rock generates drag forces on the plates above.
In addition, forces like slab suction at the leading edges of plates and the shear traction of the asthenosphere help move the plates across Earth’s surface.
On what layer do the plates float?
The crustal plates ride atop the ultra-dense molten rock of the asthenosphere. The asthenosphere is the uppermost layer of the mantle, starting at depths of about 100-150 km below the surface. It exhibits plastic, fluid-like behavior at high temperatures and pressures, allowing the solid overlying plates to slowly move across its surface.
The asthenosphere has relatively low viscosity and shear strength compared to the lithosphere, allowing the two layers to become decoupled. It is thought that convection currents in the plastic asthenosphere help mobilize the plates.
Properties of the asthenosphere
- Partially molten rock containing about 50% melt
- Much lower viscosity than lithosphere above – around $10^{18}-10^{19}$ Pa*s
- Ductile and deformable like silly putty
- Low shear strength facilitates plate motion
- Density of 3.2 to 3.5 g/cm3
The rigid tectonic plates above ride over this weak, flowing layer similar to pieces of wood floating on honey. The low friction between the two layers allows for the relative motion of plates driven by convection currents deeper in the mantle.
Asthenosphere vs Lithosphere
Asthenosphere | Lithosphere |
---|---|
100-250 km deep | 0-100 km deep |
Hot, weakened rock | Cold, rigid rock |
Ductile, can flow | Brittle and rigid |
Allows plate motion | Makes up the plates |
The lithosphere refers to the rigid outer part of the Earth composed of the crust and upper mantle. It ranges from about 50-250 km thick and makes up the moving tectonic plates. Below this is the weaker, mobile asthenosphere on which the lithospheric plates glide.
What causes plates to move?
The exact mechanisms driving plate motions have been long debated, but likely involve a combination of forces:
- Slab pull – The sinking of dense oceanic plates at subduction zones provides a downward pull.
- Ridge push – Upwelling magma at spreading ridges pushes plates apart.
- Mantle drag – Viscous coupling between sinking slabs and the mantle exerts a pull.
- Mantle plumes – Upwelling plumes may locally reorganize plate motions.
However, the most significant driver is mantle convection – the slow churning motion of Earth’s mantle. Hot material rises at mid-ocean ridges, gradually cooling and sinking at subduction zones which pulls the plates along.
The convection is driven by radioactive decay of elements in the mantle producing heat, as well as primordial heat from the Earth’s initial formation. As the hot material rises and cooler material sinks, this creates circulation cells that drag tectonic plates along.
Forces Driving Plate Motion
Force | Description |
---|---|
Slab pull | Downward suction as oceanic plates sink into the mantle at subduction zones |
Ridge push | Upwelling magma at mid-ocean ridges pushes plates horizontally |
Mantle drag | Frictional coupling between sinking oceanic slabs and the mantle |
Mantle plumes | Upward return flow of buoyant mantle material |
Mantle convection | Circulation of material in the mantle drives plate motions |
How do plates move on the mantle?
The tectonic plates are able to move across the underlying mantle due to a process called decoupling. The cold, brittle plates become decoupled from the hot, ductile asthenosphere below. This allows them to move relative to the mantle.
Several factors allow decoupling between the lithosphere and asthenosphere:
- Low viscosity of the asthenosphere – Acts like a lubricant
- Weak asthenosphere rocks deform easily
- Presence of melts/volatiles reduces strength
- Hot temperatures keep the asthenosphere ductile
This is similar to pieces of debris floating on slowly churning lava. The Komatiites lava from ancient mantle plumes had viscosities low enough to allow ~10 cm/year plate motions.
In contrast, the lithosphere is cold and rigid. Its rocks fracture instead of deforming plastically, allowing it to move cohesively as plates. The decoupling across the lithosphere-asthenosphere boundary facilitates plate tectonics.
Plate Motion Across the Mantle
Plate motion is primarily horizontal, driven by basal shear traction against the asthenosphere. But what keeps plates from simply sinking into the mantle?
- Plates have lower density than mantle rocks
- Viscous resistance of the asthenosphere
- Buoyancy from basal topography
The upper mantle has an average density of about 3.3 g/cm3, while the iron- and magnesium-rich oceanic plates are about 3.0-3.1 g/cm3. The plates are compositionally buoyant, like pieces of wood on water.
Drag forces induced by mantle flow help stabilize sideways motion. And the convection itself generates topographic slopes and ridges that provide structural buoyancy.
Together, these factors explain how the entire buoyant lithosphere floats on the mantle while still experiencing shear tractional forces from below.
How deep do tectonic plates go?
The lithospheric plates that make up the Earth’s outer shell extend from the surface down to depths of about 100-250 km. This includes both the crust and the rigid uppermost mantle that breaks apart into relatively small plates.
The thickness of the plates varies depending on composition and temperature:
- Oceanic plates – 80-100 km thick
- Continental plates – 150-250 km thick
Oceanic lithosphere tends to be thinner than continental lithosphere due to its higher density and temperatures. The base of the lithosphere is marked by an increase in plasticity and viscosity.
Below the plates, the weak, ductile asthenosphere allows them to move. Seismic imaging reveals a sharp change in the mechanical properties of the rock across the lithosphere-asthenosphere boundary (LAB).
Plate Thickness By Layer
Layer | Thickness (km) |
---|---|
Oceanic Crust | 5-10 |
Continental Crust | 30-50 |
Oceanic Upper Mantle | 70-90 |
Continental Upper Mantle | 100-200 |
The oceanic crust is thin but dense, while the continental crust is thick and less dense. Below both, the uppermost mantle extends down to the LAB where the asthenosphere begins.
The LAB depth ranges from 80-200 km and is deeper beneath older plates due to greater cooling and thickening over time. The lithospheric plates “float” on the weak asthenosphere, so their total thickness determines how deeply they extend into the mantle.
Do plates sink into the mantle?
The tectonic plates sit atop the mantle and do not generally sink into it due to their buoyancy and the viscos resistance of the asthenosphere. However, plates do descend into the mantle at subduction zones.
At convergent plate boundaries, two plates collide and one dips downward into the mantle. The old, dense oceanic lithosphere sinks into the asthenosphere in a process called subduction. However, subducting slabs eventually get stuck and pile up in the mantle:
- Slabs sink 5-10 cm/year until reaching 660-km boundary
- Transition zone acts as a barrier
- Slabs stagnate and pile up at ~1000 km depth
The sinking velocity decreases with depth as the slab pulls away from the trailing plate. Below the transition zone at 660 km, slabs tend to flatten out and stagnate due to increasing viscosity and density.
Over millions of years, these accumulated slabs may instigate mantle plumes and thermal pulses contributing to surface tectonics. Otherwise, plates generally float horizontally along on top of the mantle.
Conclusion
The Earth’s outer crust is made of rigid tectonic plates that ride along on top of the soft, ductile asthenosphere. The upper mantle below the lithosphere acts like a lubricating layer that facilitates plate motions driven by mantle convection.
Decoupling between the cold, strong plates and the hot, weak asthenosphere prevents the sinking of plates into the mantle under normal conditions. Only at subduction zones do slabs of dense oceanic lithosphere plunge down into the mantle, eventually piling up due to increasing density and viscosity with depth.
So while the crustal plates are embedded in and coupled to the mantle, they float atop the asthenosphere which allows for the great continental drifts we see slowly reshape the Earth over geological time.