Plate tectonics is the theory that explains the structure of the Earth’s crust and associated movements and phenomena that result from the interaction of the stiff rigid lithospheric plates which move slowly over the mantle. Click here to read more about the fundamentals of plate tectonics.
Historical Context
In 1915, Alfred Wegener, a German scientist, hypothesized that the continents moved around the Earth. He based this observation on a few lines of evidence: 1) the edges of the contents seemed to fit together, much like the pieces of a puzzle; 2) where the continents seemed to fit together, the ancient climate at similar bands of latitude were similar; 3) fossil species that are distributed on different continents today occurred in the same areas when the continents were fit back together; and 4) evidence of glaciers (scrapes on solid rock, called striations, masses of boulders and pebbles left over from the glaciers, called glacial till), that occur on different continents today occurred in similar areas when the continents were put back together. Later, geologists came to realize that similar or the same rock formations can be found on opposite sides of the ocean. For example, rocks that are found on the east coast of North America in the Appalachian Mountains are also found on the west coast of Europe and in Greenland. The first major hypothesis as to how the plates became situated in their current orientation was proposed by Wegener in 1912. He stated that the continents had drifted apart through time, which was later referred to as continental drift. His ideas were largely ignored due to his lack of mechanism to cause these large scale movements. Read more about his history here and watch an awesome song by the Amoeba People here.
After Wegener’s idea of continental drift, in 1929 Arthur Holmes suggested the hypothesis that thermal convection, or heat circulating in the upper part of the Earth’s crust, is the driving mechanism behind the plate’s movements. Thermal convection occurs when a substance is heated. When the substance is hot enough, its density decreases and it rises. When the substance then cools, it condenses and sinks again. Think about a lava lamp: when the ‘lava’ becomes hot enough, it rises to the top of the lamp. When that ‘lava’ at the top then cools, it sinks back down to the bottom of the lamp.
It was not until the scientific community developed the tools to examine both Wegener and Holmes’ ideas 30 years from when they were proposed did we begin to support them. Read more about Wegener and Holmes here.
It wasn’t until the 1950’s until scientists would ultimately find concrete evidence for the hypothesis of moving continents through time. During an oceanic expedition, a research vessel was tugging a magnetometer behind it, looking for signs of submarines and sea mines that might be in the Pacific Ocean. A magnetometer measures the magnetic intensity of the surrounding, and the instrument began to pick up bizarre signals from the seafloor. The signal first appeared as intensity variation. If Earth’s magnetic intensity is subtracted from this, it shifts to a series of alternating positive and negative values, or stripes!
These magnetic ‘stripes’ are preserved on the seafloor, in the sediments. The magnetometer was picking up on switches in the Earth’s poles (read more about magnetic reversals here)! When scientists finally mapped all of these seafloor stripes, they found that they were very symmetrical on either side of a spreading center, called a mid ocean ridge, where new seafloor extruded from the Earth as magma is formed. The magnetic stripes were then ‘rewound’, and lo and behold, they showed that the continents did align at one point in geologic history!
Today, Wegener’s theory is understood as plate tectonics. Here is how it works in a nutshell then we will go into more detail below: The Earth’s surface comprises a series of crustal plates, that are continually in motion. There are convection currents within the mantle There are several mechanisms that drive plate motions: (1) slab pull, where a plate is being subducted (sucked under) another and it ‘pulls’ along the rest of the plate; (2) ridge push, where new material is formed at mid-ocean ridges forming a higher topography and gravity ‘pushes’ on that material; and (3) mantle drag, where convection currents within the mantle and under these crustal plates that aid in moving these plates in different directions. The source of the convection currents is radioactive heat from deep within the Earth’s mantle.
Types of Crust
Continental crust is made up of granitic, a type of igneous rock. This rock tends to be rather old, and considered to be felsic. Felsic rocks contain high amounts of silica. Silica is one of the most common elements found on Earth, and when it combines with oxygen, it makes silica dioxide. Silica dioxide is another name for a mineral that is highly sought after by rock and mineral collectors: quartz! In other words, the crust that makes up our continents contains a lot of quartz, among other minerals in smaller quantities. Because quartz is made of a silica ion and two oxygens, it tends to be very light compared to other minerals, especially the minerals that make up the crust upon which the oceans are contained. This is why continental crust ‘floats’ above oceanic crust: it is much lighter and less dense!
As stated above, oceanic crust is made up of basalt, another type of igneous rock. This rock tends to be younger and mafic. Mafic rocks tend to be made up of heavier iron- and magnesium-rich minerals that are often darker in color than felsic rocks (those that contain lots of quartz). Earth’s crust behaves similar to rubber ducks and toys floating in water. Objects can float differently based on their weight/density relative to the material they’re floating in. If a floating object is barely less dense that what it’s floating in, it will barely rise out of the surface. On the other hand, if the floating object is much less dense, it will float higher. This is based on buoyancy which leads to isostasy – stable balance between floating and sinking of material based on density. Because oceanic crust is heavier, it sinks below or lower into the Earth’s crust than felsic crust.
Plate Boundaries & Generation of New Crust
Interesting, and scary, geologic phenomena do occur where two tectonics come together or slide past one another. There are also places on the Earth where new oceanic (mafic) and continental (felsic) crust is being created. Here, we’ll talk about the three three main types of plate boundaries: (1) Convergent; (2) Divergent and; (3) Transform, and also discuss where new continental and oceanic crust is formed.
Convergent Plate Boundaries
‘Convergent’ simply means coming together. Thus at converging boundaries, two (or more) tectonic plates that are coming together or colliding. These plates, depending on what type of crust (oceanic or continental) they are made of, can collide in a variety of ways. If two plates made of continental crust are colliding they will crumple to form mountains. One modern example is the Himalayan mountains, caused by the northward-moving Indian plate colliding with the Eurasian plate.
When a denser oceanic plate crashes into a lighter and less dense continental plate, the more dense oceanic plate will subduct, or sink underneath, the less dense plate. The oceanic plate will then be sucked into the Earth’s mantle, where it will eventually melt. One major example of subducting oceanic plates beneath a continental plate is in the Pacific Ocean. This area is more commonly known as ‘The Ring of Fire’, where the Pacific Plate is subducting underneath the Australian and North American plates. Boundaries such as this are called subduction zones and are areas with deep earthquakes, volcanic activity, and in the case of the Pacific Ocean, tsunamis caused by strong earthquake activity.
And finally, when two oceanic plates come together, whichever crust is more dense will be subducted. This often relates to the ages of the oceanic crust, with older crust being subducted because it has cooled over its lifespan and become more dense than newer crust which is warmer and less dense.
Divergent Plate Boundaries
Divergent means to spread apart. At divergent plate boundaries, new crust is being generated which pushes two plates away from one another. These spreading centers commonly occur on the seafloor, and are areas where new oceanic (heavy, dense, mafic crust) is being brought to the surface of the Earth. A good way to think about spreading centers are as long zippers that occur on the seafloor. These areas are not explosive like many volcanoes on the Earth’s surface, but instead lava oozes out of the Earth’s crust. Divergent boundaries are often associated with many minor faults, or breaks, in the Earth’s crust. One of the major spreading centers today occurs in the middle of the Atlantic Ocean, called the Mid Atlantic Ridge. Today the ridge produces about 2.5 centimeters (0.98 inches) of new crust per year. This means that the Atlantic Ocean is becoming about 1 inch wider per year!
As new plate material is produced, plate material is subducted and reworked into the mantle meaning the Earth is not ‘growing’ or ‘expanding’ but maintaining the total amount of crust at its surface. Also, remember these motions are often slow processes that take immense amounts of time.
Transform Plate Boundaries
In plate tectonic terms, ‘transform’ means sliding past one another. At transform plate boundaries, two plates slide against one another, thus causing shallow earthquakes. At these boundaries, the plates typically do not simply slide past one another through time. Instead, they may stick together through friction until enough energy is built up, then they move. It is this action that causes intense earthquakes.
Today, the most famous example in the United States of a transform fault is the San Andreas Fault system, where you can easily identify the movement between the two plates by matching up corresponding rock on either side of the boundary. Another famous example is the Alpine Fault that cuts New Zealand in half.
Commonly, transform faults are confused with strike-slip faults. A strike-slip fault is a simple offset between rock layers, within the same tectonic plate. Transform faults are offsets, but between two tectonic plate boundaries.
To learn more about Plate Tectonics, visit these sites and free online textbooks: