Main points about plate tectonics

It became clear as geophysical data accumulated prior to the late 1960s that Earth has a strong, solid outer layer (called the lithosphere) that is on a deeper layer in the upper mantle, called the asthenosphere. The lithosphere has two layers: the crust (continental crust or oceanic crust) above the lithospheric upper mantle. The asthenosphere is hot enough to be weak and soft (but still mostly solid), and flows slowly — a bit like silly putty.

The lithosphere is divided into a number of large pieces, called plates. If the total lithosphere is like the shell of an egg, a plate is like a piece of the egg's broken shell.

Most, if not all, edges of lithospheric plates can be defined using the locations of concentrated bands of earthquakes and active/recent volcanoes. (Note that not all earthquakes or active/recent volcanoes occur along plate boundaries, but many do.) Some web resources for these data include the following:

• USGS Earthquake Hazards Program website: https://earthquake.usgs.gov/earthquakes/

• Map of active/recent volcanoes and magnitude ≥7 earthquakes 1900 to 2013: https://earthquake.usgs.gov/static/lfs/learn/worldseis.pdf (12.3 MB PDF)

• This Dynamic Planet -- World Map of Volcanoes, Earthquakes, Impact Craters, and Plate Tectonics: https://pubs.usgs.gov/imap/2800/

• Smithsonian's interactive map of Eruptions, Earthquakes, and Emissions: http://volcano.si.edu/E3/

Lithospheric plates are in motion relative to one another, and that motion accounts for the stresses and magmatism along and near the plates' boundaries with one another. The motion of lithospheric plates also causes the motion of the continents, which are embedded in the plates. So plate tectonics accounts for the following observed phenomena, among others:
• the "fit" of west Africa and east South America
• the distribution of climate-sensitive sedimentary deposits such as coal, glacial deposits, evaporates and so on
• the distribution of land-restricted fossil forms on different continents
• the distribution of distinct rock types and geologic features

These observations had formed the basis for Alfred Wegener's continental drift hypothesis from the early 20th century, which was ultimately unsuccessful because there seemed to be no viable physical mechanism to explain the motion of continents.

Plate Boundaries

Three general types of plate boundaries can be defined based on the direction adjacent plates move relative to each other across their shared boundary:

• divergent boundaries along which adjacent plates move apart from one another

• convergent boundaries along which adjacent plates move toward one another

• transform boundaries along which an adjacent plates moves parallel to the two plates' shared boundary, as observed from one of the plates

Plate boundaries also occur where the plate motion is oblique to the plate boundaries (for example, combining components of convergent and transform motion) and in the form of diffuse deforming boundary zones.

Mid-ocean ridges are combinations of divergent and transform boundaries, with axial rifts alternating with transform faults along the length of the mid-ocean ridge.

How Do We Detect Plate Motion?

We detect plate motion in several ways, including the following:

• directly observing motion of one plate relative to another across a plate boundary

• interpreting motion relative to hotspots in the mantle below the plate

• through Very Long Baseline Interferometry (https://en.wikipedia.org/wiki/Very-long-baseline_interferometry and http://www.cpi.com/projects/vlbi.html)

• through Satellite Laser Rangefinding (http://cddis.nasa.gov/docs/2009/HTS_0910.pdf)

• through the interpretation of GPS data (http://xenon.colorado.edu/spotlight/index.php?action=kb&page=3 )

Why Do Plates Move?

Gravity provides the force responsible for plate motion. Some other important factors include density variations in the lithosphere and asthenosphere, the strength of the lithosphere, the rheology of the relevant layers (e.g., viscous, elastic, visco-elastic, firmo-viscous, etc.)

Some differences in viewpoint exist on the mechanisms that move the plates, but the primary suspects include some combination of the following:

• Slab pull in which the negative buoyancy of a subducting slab pulls the rest of the plate along as the slab sinks into the mantle. Compared with other mechanisms, this is probably the most energetic or strongest mechanism. Most plates that are attached to subducting slabs tend to move in the direction of the subduction zone, as viewed in a reference frame that is tied to the relatively sluggish lower mantle.

• Ridge "push" — a terrible name, because the ridges are not being pushed apart by anything. Rather, the plate is moving down an inclined surface at the base of the lithosphere, perpendicular to the axis of the mid-ocean ridge. Magma then fills-in the gap and cools onto the trailing edge of the plate.

With increasing distance from the axis of the mid-ocean ridge,

(1) the age of the lithosphere increases,

(2) the depth to the sea floor increases,

(3) the thickness of the lithospheric upper mantle increases due to cooling and accretion from below, and so

(4) the thickness of the full lithosphere increases.

Gravity acts on the mass of the lithosphere, causing motion away from the ridge. "Ridge push" might really be more like "ridge slide."

Most plates that have a substantial ridge boundary tend to move perpendicular to the ridge axis, as viewed in a no-net-rotation (NNR) reference frame. (Think of an NNR reference frame as one that is tied to the relatively sluggish lower mantle below the more rapidly moving lithospheric plates.)

• Trench pull caused by counterflow of the asthenosphere

• Maybe some mantle convection apart from plate-induced motion; however, plates are not like packages moving along on top of a conveyor belt (on top of a convection cell).

For the most part, plates move themselves under the influence of gravity.

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The predecessor to plate tectonics: continental drift

Alfred Wegener (1880-1930) collected and synthesized evidence that the continents on Earth have moved throughout Earth's history. His model was called continental drift, and was a significant precursor to plate tectonics. Continental drift is simply the idea that continents are in motion relative to each other. Wegener originated the idea that several continents that are now dispersed across the globe had once been together to form a continuous landmass.

Continental drift was an interesting hypothesis, and the data collected in connection with continental drift were valuable. Ultimately, continental drift failed to convince many geoscientists because it lacked an adequate mechanism for moving continents. This was largely due to our then-primitive and very limited understanding of the ocean floors and the mechanical properties of Earth's crust and upper mantle.

In the mid-1960s, continental drift was subsumed into the newer conceptualization known as plate tectonics, which has proven to be a better explanation of the observed data. Knowledge of the seafloor and the distribution of earthquakes worldwide, that had been accumulated at an accelerated rate during and after World War II, was essential in the development of plate tectonics.

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