Plate tectonics is a scientific theory describing the large-scale motion of the plates making up the Earth's lithosphere. The theory builds on the concept of continental drift and was only widely accepted after seafloor spreading was validated in the late 1950s and early 1960s. The rigid outermost shell of Earth is broken into tectonic plates. Where plates meet, their relative motion determines the type of boundary (convergent, divergent, or transform). Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries or faults associated with them. 

Continental Drift vs. Plate Tectonics

What is Plate Tectonics Theory?

To understand how rocks formed in the ocean can end up on mountain tops we have to understand why continents and oceans seem to have mobility. This is where plate tectonics come in. Plate tectonics is an extremely efficient theory that explains why the Earth’s crust moves and behaves the way it does. Plate tectonics is known as a great unifying theory because it brought together many separate observations and it is referred to as a paradigm shift because before plate tectonics geologists knew mountains, volcanoes and earthquakes occurred in some places but not others without really understanding why. Plate tectonics connected the dots. It helped us understand why these features and processes occur where they do.

What is Continental Drift Hypothesis?

Continental drift is a hypothesis that the Earth's continents have moved over geologic time relative to each other, thus appearing to have "drifted" across the ocean bed. The idea of continental drift has since been subsumed by the theory of plate tectonics, which explains that the continents move by riding on plates of the Earth's lithosphere (crust and the upper most part of the mantle).

Who Discovered Continental Drift?

Continental drift hypothesis is widely attributed to Alfred Wegner (1912) even though he wasn’t the only one to come up with this idea independently. Speculation that continents might have 'drifted' was first put forward by Abraham Ortelius in 1596. However, Wegner was the first to come up with a lot of evidence so we will focus on him. Wegner was a meteorologist who did much of his work in Greenland and specialized in snowfall and ice. As he worked with glaciers he noticed that glaciers can actually move (slowly but move nonetheless). In fact, while these big masses don’t appear to be mobile they can actually move a significant amount over time. With this in mind he thought it could be possible that the continents were moving in the same way since it appeared as though the continents fit together like a puzzle piece.

What Evidence Supported Continental Drift?

Wegner further supported his hypothesis using geologic data, he studied the geology across different continents. He found that there were in fact geologic belts that existed across South America and Africa. There were fossil assemblages (like a fresh water crocidillian that couldn’t swim across the ocean) that matched across continents and there was also strong evidence of ice movement across continents all supporting the idea that South America and Africa were once joined. Similar observations were made that connected the other continents as well leading to his hypothesis that all of Earth’s land mass was once joined as one giant land mass. He named it Pangea which means “All Earth.”

Why Was Continental Drift Not Accepted?

Wegner had a lot of evidence but the major flaw in the continental drift hypothesis was the mechanism he proposed. He suggested that the continents moved by flowing through the crust in the same way that an ice breaker ship would force its way through ice. He suggested that tidal forces of the moon and sun were pulling the continents which geophysicists outright did not accept – basically saying it was not physically possible. Geologists pointed to the rock strength contradiction – if the ocean crust was so weak that it could be plowed through then why don’t we see buckled and folded oceanic crust – instead we see buckled and folded continental crust forming mountains which wouldn’t make sense if it was so strong that it could plow through oceanic crust. Anytime you have a drastic new idea you need good evidence and Wegner did have good evidence but it wasn’t complete. Not every scientist thought he was completely off base. Many southern hemisphere geologists thought his observations fit very well but the mechanism part of his theory needed to be reworked.

When Did Plate Tectonics Theory Take Hold?

It wasn’t until World War II when new data became available, largely due to the search for enemy submarines, that the idea of drifting continents resurfaced. Data detailing things like the ocean bottom topography, oceanic crust age, and magnetism of the seafloor helped Harry Hess and Robert Dietz propose a mechanism that could explain why the continentals appear to have moved. They suggested that oceanic crust was spreading apart at under water mountain belts, carrying the continents apart through a process they called seafloor spreading.

This all renewed interest in Wegner’s idea of continental drift which morphed into the theory of plate tectonics that proposes the Earth’s crust is broken up into a dozen or so rigid lithospheric slabs or tectonic plates that move relative to each other; moving away each other, toward each other or past each other. Tectonic activity (sometimes just called tectonics) are processes that deform the lithosphere – things like volcanism and earthquakes. Plate Tectonic Theory explains why we have tectonic activity where we do and it overwhelmingly tends to be at the boundaries of tectonic plates.

Layers of the Earth

What are Earth’s Compositional Layers?

Earth’s layers based on chemical composition are the crust, mantle, and core. The crust is the outermost layer and the thinnest layer. It is composed of either continental or oceanic material. Continental crust has the average composition similar to a granite and averages a thickness of 20 to 25 miles. Oceanic crust has an average composition similar to basalt which is a common volcanic rock. Oceanic crust is thinner than continental crust averaging about 4 miles but oceanic crust is more dense than continental crust that is why oceanic crusts sits at the bottom of ocean basins and continental crust sits higher, above sea-level.

The mantle is primarily composed of the green mineral olivine and it extends from the base of the crust down to 1800 miles. That is a thickness roughly equal to the distance between Denver CO and New York City! The mantle is the thickest layer and makes up most of Earth’s volume.

Lastly, the core is the inner most layer. It includes the outer core – which is liquid and the inner core which is solid. The outer core is about 1,400 mi thick, and the inner core has a radius of almost 760 miles. They are not chemically distinct from each other as both the inner and outer core is mainly composed of nickel and iron, but they are chemically distinct from the mantle.

What are Earth’s Mechanical Layers?

The mechanical layers of the Earth are differentiated by their strength or rigidity. These layers are not exactly the same as the compositional layers of the Earth, such as the crust, mantle, and core, though some of the boundaries align.

The uppermost part of the mantle is relatively strong and solidly attached to the overlying crust. The crust and the uppermost part of the mantle together form the lithosphere. That uppermost part of the mantle that is part of the lithosphere is called the lithospheric mantle. The lithosphere is rigid and behaves brittlely meaning that when stress is applied to the lithosphere it will fracture or crack. This is where tectonic plates “live.” Tectonic plates are giant slabs of broken up lithosphere that move relative to each other. The Earth is made up of 7 main plates and a several smaller ones. Every plate is made up of both continental crust and oceanic crust with a few exceptions that are composed entirely of oceanic crustal material.

The lithosphere can move around because it sits on top of a softer, weaker zone called the asthenosphere. The asthenosphere is about 62 miles thick and it is hotter and weaker than the lithospheric mantle above it. The asthenosphere responds plastically to stress (flows under pressure over long periods of time). It also behaves like a fluid in that it can carry convection currents but it is not liquid!

The rest of the mantle beneath the asthenosphere is called the mesosphere and it is much more rigid than the upper mantle but not brittle like the crust (similar to stiff plastic). Under the mesosphere is the outer core which is made of liquid iron and nickel. This is the only layer of the that is a true liquid. The core-mantle boundary is the only boundary of Earth’s layers that is both mechanical and compositional. Flow of the liquid outer core is responsible for Earth’s magnetic field. The inner core has the same composition as the outer core but it is solid.

Evidence for Seafloor Spreading

How was Seafloor Spreading Discovered?

The continental drift hypothesis did not gain much traction because it did not provide a viable mechanism to explain how the continents could move around the planet until the development of the bathometer during World War 2. A bathometer is an instrument that measures the depth of the oceans and it was an essential instrument for merchant ships and for submarines to navigate the oceans. One captain of a merchant ship, Harry Hess, who was a geology professor at Princeton in his civilian life, decided he would run his bathometer continuously (he was not suppose to do that). But the more he did this the more he could map the seafloor and he found that the ocean floor was not a flat abyss. This wasn’t a novel idea by any means as this had been hinted at before but now there were detailed measurements to prove it.

How Do Guyots Support Seafloor Spreading?

As more detailed bathometric maps were made they found that many ocean basins had long linear mountain ranges. Hess also observed flat topped mountains that he named guyots – named after a friend but some say the name comes from the idea of a guillotine because these mountains appear to have been beheaded. You can think of them that way and what beheaded them was wave action. Near the underwater ridges the guyots were much closer to the surface of the water. As you moved further from the sea ridge, the guyots got deeper and deeper. How then could wave action flatten the guyots further away from the ridge if they were below the wave action? It would reason that the deeper guyots are too far from down to be affected by wave action.

Harry Hess thought perhaps they used to be at the sea surface at a time when they were closer to the sea ridge and the guyots have been moving away from the sea ridge with time. Another interesting observation was that the ridges along the ocean floor formed in a pattern that mimicked the shapes of the continental coastlines. This lead Hess and colleagues to the idea of seafloor spreading. Maybe instead of continents plowing through the seafloor like the continental drift hypothesis suggests, they are actually being spread apart at these sea ridges. And the spreading of the ridges is what is pushing the continents along and that is the mechanism by which the continents have drifted around the Earth. At the time even Harry Hess was himself skeptical of this idea without more evidence. The guyots were just not enough but it looked promising.

How Does Radiometric Dating Support Seafloor Spreading?

A significant piece of evidence came at the end of the 1950’s with the discovery of radiometric dating which is a technique used to date materials using the relative proportions of radioactive isotopes. Now we had the ability to measure the absolute ages of rocks in the seafloor. It so happens that the seafloor is made out of basalt which is an mafic igneous rock that works very well for this dating technique. As technology became available, cores were drilled into the ocean floor and samples were taken and tested. The results showed a very interesting pattern. The youngest rocks were closer to the ridge, the oldest seafloor was away from the ridge. While this observation supported the idea of seafloor spreading it still wasn’t enough to say for sure that seafloor spreading was really happening.

How Does Paleomagnetism Support Seafloor Spreading?

It wasn’t until the 1960’s that more evidence emerged that started to solidify the idea of seafloor spreading as a principal explanation for what we now call plate tectonics but at the time referred to as continental drift. This big piece of evidence came with the publication by Frederick Vine and Drummond Matthews that described magnetic anomalies over oceanic ridges. When magnetic surveys were conducted of the seafloor they discovered a remarkably regular set of magnetic bands or stripes of alternating polarity across the seafloor. Bear in mind, there is strong evidence that magnetic poles have reversed many times throughout Earth’s history and igneous rocks (like those found in ocean crust) have ferromagnetic minerals that will align themselves with the current magnetic field as they cool and solidify and thus preserving the magnetic field at that time.

The stripes were parallel to the ridges and the stripes themselves were symmetrical across the ridge. It was a mirror image of the stripes on the either side of the ridge. How do you get a pattern like that? Well seafloor spreading could explain that very well. Imagine you have new basaltic magma rising at the center of an ocean ridge creating new sea floor and recording the current magnetic field as it cools. But as seafloor spreading continues, new material is brought to the surface and this crust that just formed gets ripped in half and pushed aside as new crust is made. When the magnetic field flips, this too is preserved in the new oceanic crust. And this goes on and on through geologic time and with varying rates in magma rising and varying frequencies in magnetic pole reversals. And you end up with these striped, parallel, and symmetrical magnetism across the ocean floor. This was one of the strongest pieces of evidence to support seafloor spreading. More observations that support seafloor spreading has emerged since then but these were the foundational pieces that solidified seafloor spreading as a viable mechanism driving plate tectonics.

The Reason Plates Move

The process of plate tectonics creates and destroys lithosphere and it helps circulate material between the asthenosphere and the lithosphere. Some asthenosphere becomes lithosphere at mid-ocean spreading centers and then takes a slow trip across the ocean floor before going back down into the asthenosphere at a subduction zone. This process is also an way to transport heat to the surface. We know that seafloor spreading is the mechanism that facilitates the process of plate tectonics but why does the seafloor spread in the first place? There are 3 ideas on the cause of seafloor spreading.

What is Mantle Convection?

The mantle convection (conveyor belt) idea suggests plates are passively riding along a convecting mantle. Plates are are very rigid and they sit atop the asthenosphere which is plastic and mobile and has the ability to flow. Flow like a very slow-moving fluid (think silly putty). If you were to bring material from the asthenosphere to the Earth’s surface and hit it with a hammer, it would crack and break (deform brittlely). However, under the intense heat and pressure found at the depths of the asthenosphere, the asthenosphere behaves like a fluid. And when you heat a fluid from below convention currents form. This is what happens at the base of the asthenosphere. Heating from deeper in the mantle leads to the formation of convection cells where you get rising asthenosphere. As the asthenosphere rises, it comes up to the base of the lithosphere and then spreads out (at spreading centers). When the rising asthenosphere gets to the lithosphere and spreads out it can carry the lithosphere with it like a conveyor belt. When the asthenosphere spreads apart it provides an opening for magma to rise and fill in the void that would otherwise form. This is where you get basaltic eruptions, and these are your spreading ridges or mid ocean ridges. Eventually it dives back into to the asthenosphere (at subduction zones) to make large circulating convection cells.

What is Ridge Push?

Ridge push or gravity sliding is another idea to explain what drives plate tectonics. The mid-ocean ridge is naturally higher than the ocean floor away from the ridge because lithosphere near the ridge is thinner and hotter and the asthenosphere is bulging up at that point. The idea with ridge push is that gravity pushes downward at these ridges which will exert a force that causes the plate to slide away from the topographically high ridge and push the plate outward.

What is Slab Pull?

Slab pull or trench suction is the third idea to explain why plate move. At subduction zones you have two plates coming toward each other and one gets overpowered and subducts beneath the other. As plates are buried deeper and deeper, they experience higher and higher temperature and pressure. At a certain point some of the minerals will alter their crystal forms and become denser mineral versions. Eclogite, for example, forms when you get basaltic material deep enough and it’s a dense mineral – it is like heavy weights forming at the leading edge of the subducting plate. The subducting plate becomes denser and heavier and gravity pulls that plate downward into the asthenosphere where that lithosphere will eventually melt. Slab pull is a significant force and subducting plates tend to move faster than plates at other boundaries. There is some debate among geologists as to which of these forces is most important or the controlling driving force. Although slab pull seems to have a stronger influence, they are all in play and they all seem to contribute to the system as a whole.

Divergent Boundaries

What is a Divergent Boundary?

A divergent boundary is a type of plate boundary where two plates move apart or diverge relative to one another. When a divergent boundary occurs between two oceanic plates a mid-ocean ridge. This is where new oceanic lithosphere forms. Mid-ocean ridges are also called spreading centers because of the way the plates spread apart. A narrow trough, or rift runs along the axis of most mid-ocean ridges. The rift forms because large blocks of crust slip down as spreading occurs. That movement causes faulting which results in frequent small to moderate-sized earthquakes.

As the plates move apart, an opening is created which allows the asthenosphere to rise toward the surface to fill that space. As the plates pull apart, the load on the asthenosphere decreases which decreases the pressure in the asthenosphere and leads to decompression melting. The magma rises along narrow conduit, accumulates in magma chambers beneath the rift and eventually becomes part of the oceanic lithosphere. Much of the magma solidifies at depth but some erupts on the seafloor forming submarine lava flows. These eruptions create new ocean crust that is incorporated into the oceanic plates as they move apart. Mid-ocean ridges are elevated above the surrounding seafloor because they consist of hotter, less dense materials and the underlying asthenosphere is thinner and bulging right beneath the ridges. The elevation of the seafloor decreases away from the ridge because the rock cools and contracts.

What is Continental Rifting?

Most divergent boundaries are beneath oceans but a divergent boundary can form within a continent. When you have divergence within a continent it is called continental rifting and it creates a continental rift. If continental rifting progresses it can lead to seafloor spreading and the formation of a new ocean basin. The initial stage of continental rifting commonly includes broad uplift of the surface as mantle-derived magma ascends into crust. That magma can melt parts of the continental crust which produces additional magma. Increased heating causes increased expansion which translates to further uplift. But the uplift is short lived because the crust is stretched and pulled at a divergent boundary so it will begin to thin.

The next stage of continental rifting is progressive stretching that causes large crustal blocks to drop down along faults, forming a continental rift. The down dropped blocks may form basins that can trap sediment and water. Continued rifting causes decompression melting and the resulting magma from the mantle may solidify beneath the surface or may erupt from volcanoes and long fissures on the surface. The entire continental crust thins as it is pulled apart and elevation of the central rift decreases over time.

If rifting continues, the continent splits into two pieces and a narrow ocean basin forms as seafloor spreading takes place. As the edges of the continents move away from the heat associated with active spreading , the thinned continental crust cools and drops in elevation, eventually dropping below sea level. At this point the continental margin ceases to be a plate boundary. A continental edge that lacks tectonic activity is called a passive margin. With continuing seafloor spreading, the ocean basins become progressively wider, eventually becoming a broad ocean like the modern-day Atlantic Ocean.

Convergent Boundaries

Convergent boundaries form when two plates move toward each other. There are 3 types of convergent boundaries: ocean-ocean, ocean-continent, and continent-continent convergence.

What Happens at an Ocean-Ocean Convergent Boundary?

When two oceanic plates converge, we call that ocean-ocean convergence. When two oceanic plates come together it is hard to tell which one will lose and get pushed down but for sure one will always lose. The losing plate will bend and slide beneath the other plate along an inclined zone called a subduction zone and the whole process is called subduction.

An oceanic trench forms as the subducting plate bends down. Sediment and slices of oceanic crust collect in the trench forming a wedge called an accretionary prism which continues to grow over time because material is continually added as subduction progresses. Trenches tend to be very deep oceanic features much of which has to do with the bending of that subducting plate. One well known example of this feature is the Mariana’s Trench in the western Pacific.

As the plate subducts, its temperature increases, releasing water from minerals in the oceanic crust. This water causes melting in the overlying asthenosphere. This is because the presence of water significantly lowers the melting temperature of rocks. The resulting magma from all this melting is buoyant and rises into the overlying plate.

At the start of subduction, some magma will erupt under the ocean but as subduction progresses magma will erupt from volcanoes that rise above the sea. With continued activity, you end up with multiple volcanoes along the plate boundary that form a curving belt of islands called an island arc. Magma that cools and solidifies at depth adds to the volume of the crust. Over time, the crust gets thicker and volcanic islands can actually join to become more continuous strips of land.

What Happens at an Ocean-Continent Convergent Boundary?

When an oceanic plate and a continental plate converge it’s called ocean-continent convergence. Along this boundary, the denser oceanic plate subducts beneath the more buoyant continental plate. An oceanic trench marks the plate boundary and receives sediment from the adjacent continent. This sediment and material scraped off the oceanic plate form an accretionary prism. Volcanoes form on the surface of the overriding continental plate in the same wat the volcanoes form in an ocean-ocean convergent boundary. These volcanoes erupt, often violently producing large amounts o volcanic ash, lava, and mudflows.

Magma forms by melting of the asthenosphere above the subduction zone. It can solidify at depth, rise into the overlying continental crust before solidifying, or reach the surface and cause a volcanic eruption. Compression associated with the convergent boundary deforms and thickens the overriding continental crust. Uplift and volcanism can produce high mountain ranges like the Andes.

What Happens at an Continent-Continent Convergent Boundary?

When two continental plates converge it’s called continent-continent collision or continental collision. This type of boundary creates big mountain ranges. Continental collisions typically start as ocean-continent convergence. As the oceanic part of the plate continues to subduct, the two continents become closer to each other. Magmatic activity occurs in the overriding plate above the subduction zone. The edge of the approaching continent has no such activity because it is not a plate boundary, yet.

When the converging continent arrives at the subduction zone, it may partially slide under the other continent or it may just clog up the subduction zone as the two continents collide. Because the two continents are thick and have the same density, neither can be easily subducted beneath the other let alone into the asthenosphere. Along the boundary, faults slice up the continental crust, stacking one slice on top of another. These slices are distinct from the accretionary prism that formed along the convergent boundary prior to the actual continental collision. Continental collisions form enormous mountain belts and high plateaus like the Himalaya and Tibetan Plateau.

Transform Boundaries

At transform boundaries, plates slip past each other horizontally along transform faults. Transform boundaries exist along oceanic and continental plates. In the oceans, transform faults are commonly associated with mid-ocean ridges. Transform faults combine with seafloor spreading to form a zigzag pattern on the seafloor. The zigzag pattern is basically what happens when you try to draw a straight line on a curved surface.

Spreading centers are these straight lines but they are breaking a curved surface. So what happens is that transform boundaries form to help accommodate some divergent motion. The spreading direction is parallel to the transform faults and perpendicular to the spreading segments. If we use the mid-ocean ridge in the south Atlantic Ocean as an example you’ll see that spreading occurs along north-south oriented ridges but the spreading is perpendicular to the ridges orientation the spreading is in the east-west direction.

Why do the ocean floors have zig-zags? 

The offset you see that are oriented east-west (in the south Atlantic Ocean) are transform faults along which the two diverging plates simply slide past one another. These transform faults link the different spreading segments. The zigzag pattern of mid-ocean ridges reflects the alternation of spreading segments with transform faults. The overall shape of the ridge mimics the edges of Africa and South America and so was largely inherited from the shape of the original rift that split the two continents apart.

Continuing outward from most transform faults is an oceanic fracture zone, which is a step in the elevation of the seafloor. A fracture zone is a former transform fault that now has no relative motion across it. It no longer separates two plater and instead is within a single plate. Opposite sides of the fracture zone have different elevations because they formed by seafloor spreading at different times in the past, so they have had different amounts of time to cool and subside after forming at the spreading center. Younger parts of the plate are warmer and higher than older parts.

Transform faults or transform boundaries also link different types of plate boundaries. If you look at most plates, the different sides of the plates will move differently relative to the surrounding plates. Some transform boundaries occur beside or within a continent, sliding one large crustal block past another, the San Andreas fault in California is an example of this.

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