An Introduction to Plate Tectonics

Good Afternoon!

Here is one that we made earlier.

We’ve touched on plate tectonics in several articles so it would be useful to spend a brief moment summarise plate tectonics and to use earthquake data to show the plate boundaries.

Fig 1: Plot by the author of earthquake depths earthquakes of 4.5 M or more from 1975 to 2020. Purple dots denote depths of 0 – 33km, blue denotes 33 -70km, green denotes 70 – 150 km, yellow denotes 150 – 300 km, orange denotes 300 to 500 km and red denotes > 500 km. Deeper quakes are plotted over shallower ones so that they can be seen. © copyright remains with the author; all rights reserved, 2021.

Plate tectonics is the relative movement of the plates to each other. The Earth’s crust is made up of many rigid blocks called plates. There are 7 major plates: the North American Plate, the Eurasian Plate, the Pacific Plate, the South American Plate, the African Plate, the Indo-Australian Plate and the Antarctic Plate; many smaller plates, such as the Juan de Fuca Plate, the Cocos Plate, the Nazca Plate, the Caribbean Plate; and, several micro-plates in the collision zones between the larger plates.  


The theory of plate tectonics has evolved from early mappers noting the similarities between the shapes of the continents to the current theories set out below.  F B Taylor and Alfred Wegener developed theories (1910 and 1912, resp.) of continental drift, which was followed by the discovery of subduction zones from seismic studies by Wadati and Benioff in the 1920s and 1930s, Holmes theory (1928) that convection currents in the mantle caused by heat from radio-active decay stretched the oceanic crust caused the continents to split (the old oceanic crust cooled and sank back into the mantle to be melted and recycled), Hess’ theory (1950s) that the formation of ocean basins pushed continents apart and oceanic crust is formed at mid ocean ridges,  confirmation in the 1960s of sea-floor spreading from post WWII studies of magnetic lineations on the sea floor, and the discovery of transform faults also in the 1960s.  

The importance of magnetic lineations in the rocks of the sea bed is that when rocks are formed at the mid ocean ridges they are magnetised in the then direction of the earth’s magnetic field; the polarity of the earth’s magnetic field changes over a cycle and these changes are recorded in rocks, giving an indication of when the rocks were formed; the lineations are replicated each side of the spreading ridge.

Later GPS studies have confirmed the plate motions and led to the discovery of some microplates.

Composition of the Plates

The plates are made up of relatively rigid lithosphere with oceanic and / or continental crust.

Oceanic crust is around 7 km thick, principally made up of basalt, whereas continental crust is up to 30 km thick made up of granites and andesite.  Oceanic crust is formed at sea-floor spreading centres (such as the Mid Atlantic Ridge).  Continental crust is created by volcanic activity and the accretion of terranes (terranes are smaller segments of crust). 

The lithosphere is the rocky upper mantle up to 100 km thick which rests on the weaker layer of the upper mantled called the asthenosphere.  In the asthenosphere, heat rising from the Earth’s core, causes solid state convection; rock in the asthenosphere circulates round – heated rock rising and cooling rock sinking.  The plates ride on the moving asthenosphere.  As the plates move, they generate earthquakes.

Types of Plate Boundaries

Fig 2 Schematic by the author of plate separation at a Spreading Ridge (A), ocean-to- continent collision(B) and continent- to-continent collision (C), showing the asthenospheric convection currents at the Spreading Ridge (large red arrows) and at a Backarc Basin (smaller red arrows) by the author after various sources.  White arrows denote lithospheric motion. © copyright remains with the author; all rights reserved, 2021.

There are three distinct types of plate boundary which are clearly delineated by seismic activity at the plate margins:

  • Divergent margins where plates move apart from each other.  Asthenosphere rises to fill the gap, cooling to form new lithosphere.  Ocean basins and ridges are formed in this process. Decompression melting of the mantle leads to the formation of magma and volcanoes.  An example of a divergent margin is the Mid Atlantic Ridge.
  • Convergent margins where the plates move towards each other and one plate descends beneath the other or overrides the other.  In ocean-to-continent or ocean-to-ocean collision, denser the oceanic crust subducts beneath the lighter continental or oceanic crust.  The descending slab causes the formation of a trench and accretionary wedge (sediment scraped off adheres to the overriding plate). As the temperature and pressure increases on the descending plate, water is released which causes hydration melting of the mantle and surface volcanism.  In continent-to-continent collision, there is no marked subduction: the plate edges are compressed, folded and uplifted.  An example of ocean-to-continent collision is the plate boundary between the Pacific Plate and the North American Plate at the Aleutian Arc. 
  • Transform margins where the plates slide past each other with no vertical motion.  An example of this is the San Andreas Fault which marks the western boundary of the North American Plate.

There are other boundaries that are less well defined.

If we plot the earthquakes, we can see the plate boundaries.  We plotted earthquakes downloaded from USGS’s earthquake search with magnitude ≥ 4.5M for the period 1975 to 2020 for coordinates 90°S, 180°W to 90°N, 180°E.

From out plots, most of the plate boundaries are clearly visible (some of the microplate boundaries are hidden due to the scale of the plots): the mid-ocean ridges, ocean-to-continent collision zones marked by deep subduction zones and the shallower continent-to-continent collision zones.   We can also see that the subduction zones bordering the Pacific Ocean and subduction zones of Indonesia are responsible for most of the earthquakes with magnitude of 8.0M or more.

Fig 3: Earthquake plots by the author of earthquakes with magnitude of 4.5M or more; green dots denote earthquakes with magnitude of 4.5M to 6M, yellow stars denoted earthquakes with magnitude 6M to 7M, yellow stars denoted earthquakes with magnitude 7M to 8M and yellow stars denoted earthquakes with magnitude ≥ 8M.  In the left-hand plot, earthquake belts delineate the plate boundaries. The right-hand plot is the geodensity plot of the same earthquake data set. © copyright remains with the author; all rights reserved, 2021.

 What makes the plates move?

The drivers for plate motion are the convection currents in the asthenosphere drag the lithosphere (basal drag) and the pull of the cooler lithosphere sinking into the asthenosphere at subduction zones (slab suction, gravity or slab pull).  Slab pull is considered to be the largest driver of plate motions.  However, as you will see from the plots below, neither the North American Plate nor the Eurasian Plates have active subductions zones so another mechanism is needed. Super plumes of deeper mantle material may drive larger convection currents.  Surge tectonics developed later suggests that the mantle flows in channels beneath the lithosphere which provide basal friction. 

Reykjanes Peninsula – update

The Reykjanes Peninsula straddles the Mid-Atlantic Ridge. Here, the plate boundary consists is a transform margin, made up of a transform fault and localised rifting. At the time of writing, a large earthquake swarm is occurring. We have now added plots of the current rifting / dike formation event to our previous post (“Seismic Activity on the Reykjanes Peninsula“) for earthquake data downloaded on 1 March 2021. IMO is in the process of updating week, 2021 .

The Armchair Volcanologist

© copyright remains with the author; all rights reserved, 2021.

Sources and Further Reading

Plots are the author’s own work.

Earthquake raw data downloaded from USGS earthquake search:

“Global Tectonics – Third Edition”, Philip Kearey, Keith A. Klepeis & Frederick J. Vine, Wiley-Blackwell, 2009.