If I said ‘plate tectonics’ you would probably reminisce on a science class from fifth or sixth grade where you learned about Pangea and Earth’s super continents. However, the theory of plate tectonics is more than the Earth processes that slowly push continents together or apart. It is a magnificent display of physical and chemical processes that unify the geosciences. From mountain building to volcanic activity and continental rearrangement, these are the processes which have shaped Earth for the past ~4 billion years.
To understand plate tectonics, we must first start with the interior of the Earth. As you may know the Earth is layered like an onion, only each layer is thousands of kilometers thick, and are made up of either liquid or solid Earth elements. Earth’s layers include the lithosphere, asthenosphere, lower mantle, and the outer and inner core. The lithosphere includes the Earth’s crust from the surface to about 100 km below the surface. From 100km to approximately 700km is the asthenosphere, a region of the upper mantle which is thought to be fluid-like, and the area which produces the energy required to move the Earth’s crust. Beneath the asthenosphere, the lower mantle extends to about 2800 km, until it transitions to the liquid outer core which extends to a depth of 5100 km. At the Earth’s center is a solid iron and nickel core. The mantle, however, is the dominant component of the earth, making up approximately 82.5% by volume, while the core comprises 16.1%, and the crust only accounts for 1.4% of the Earth’s total volume.
The Earth’s composition coupled with it’s rotation on axis is also what creates our protective magnetic field. So not only does remnant heat from Earth’s creation 4.6 billion years ago protect us from a solar death, it also powers surficial processes that have helped to create a life supporting atmosphere and shape the continents and oceans. If we could somehow send a temperature probe to the center of the Earth, it would return some pretty HOT readings. Generally speaking, the geothermal gradient is 15° to 30° Celsius per kilometer for the first 100 km of depth beneath the surface of the crust. Once below the lithosphere, temperatures increase even more rapidly to around 5000°C at the core. As you would expect, pressure also increases dramatically with depth. On the surface of the Earth we recognize standard temperature and pressure as 1 atm (atmosphere), 14.7 psi (pounds per square inch), or 29.92 inHg (inches of mercury) at 15° C/59° F. This is a very different story as you move through the interior of the Earth, with pressures reaching as high as 3 million atm or ~44 million psi!
I like to think of the asthenosphere as a ‘tectonic engine’ that drives the motion of continental and oceanic crustal plates. Earlier I mentioned that the asthenosphere is fluid-like, but in reality it is more like a malleable plastic that is so hot it produces convection beneath the surface. Convection is the movement within a fluid produced by the rising of warmer less dense material and the sinking of cooler, more dense material. So, if we think of the asthenosphere as a fluid that the crust ‘floats’ on, then it becomes much easier to visualize how the Earth’s plates move. As heat creates these convective currents beneath the crust, the crust begins to move in different directions establishing geologic boundaries. Geologic boundaries represent the interaction between two different tectonic plates, either a continental-continental, continental-oceanic, or oceanic-oceanic plate. The type of boundary created depends on the direction of movement in respect to another plate and includes convergent, divergent, and transform boundaries.
Convergent plate boundaries are formed where tectonic plates collide, and result either mountain building (orogenies), volcanic activity, or both. During collision, one plate will typically slide beneath the other in a process known as subduction. This usually occurs when an oceanic plate is thrust beneath a continental plate. However, in rare cases, an oceanic plate can be thrust on top of a continental plate in what’s known as obduction. Divergent plate boundaries are created when two plates move away from each other in a process known as rifting. Rifting occurs as magma from the mantle rises and melts overlying crustal material. As the crust thins it stretches, allowing magma to push the plate apart as it extrudes onto the surface. If two adjacent plates are neither colliding, or diverging, then they are sliding parallel to each other, but in opposite directions, creating a transform boundary.
All of this movement results in the cyclical opening and closing of ocean basins and crustal formation and rearrangement every ~250 million years, known as the Wilson Cycle. Interestingly, it wasn’t until 1967 that the theory of plate tectonics and continental drift was accepted by the scientific community, and only after all of the major and most of the minor crustal plates were mapped out. Today, GPS tracks the relative movement of Earth’s tectonic plates, which ranges anywhere from a few millimeters to tens of centimeters per year.
Claiming that the theory of plate tectonics is the unifying theory for the geosciences is a grand statement, however it is absolutely true. Plate tectonics is the backbone to almost all geologic processes, and that makes it fundamental to understanding our Earth. Active tectonics explain earthquakes and the associated normal, reverse, and transform fault planes. The subduction of crust is responsible for creating active volcanic island arcs, hot spots, and volcanoes on mainlands, while colliding plates lead to rising mainlands and mountain building events.
Without active plate tectonics there would be no rock cycle which buries/exhumes the crust beneath our feet, earthquakes, ocean basins, mountains, or the volcanic activity which was partly responsible for creating the atmosphere we have today. Earth would be a very different place, one that likely would not foster the evolution of life nor hold any real value with it’s place in the solar system.
It is important to understand that our Earth has been constantly changing since the very beginning. With every era, Earth’s physical and chemical conditions change, moving forward with time regardless of anything else but the natural and physical laws of The Universe.
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Fichter, L.S., 2000, The Wilson Cycle: http://csmres.jmu.edu/geollab/Fichter/Wilson/wilsonsimp.html (accessed February 2017)
Mulroy, P., Mr. Mulroy’s Earth Science: http://peter-mulroy.squarespace.com/earths-interior/ (accessed February 2017)
http://www.geosci.usyd.edu.au/users/prey/Teaching/Geol-3101/Rifting02/rift.html (accessed February 2017)
Photo Credit: 05 Dec 2012, Tolbachik Volcano erupting, Kamchatka, Russia, Image by © Sergey Gorshkov/Minden Pictures/Corbis