|Gneiss, tuff |
and a little wacke
|Know your schist|
Plate tectonics is a theory in geology that describes the lithosphere[note 1] of the Earth as being broken up into many large and small rigid "plates" floating on the asthenosphere of the mantle. They float because the siliceous material that makes up most of the crust is less dense than the mantle,[note 2] and can move around because the asthenosphere below them is relatively weak.[note 3]. While a "new" theory, dating only from the 1970s, plate tectonics is widely accepted for its explanatory and predictive strengths. The theory of plate tectonics can explain a wide variety of observations in geology, including volcanoes, earthquakes, continental drift, geomagnetism, and seafloor spreading.
Where plates collide, mountain ranges are often thrust up, sometimes due to compression such as with the Himalayas and other times due to uplift and volcanism at subduction zones as is the case with Andes. Where plates move apart, new material from the mantle rises up to fill the gap, creating new crust that makes up mid-ocean ridges, with the Great Rift Valley in east Africa being one of the few cases of this occurring on land. The crust is returned back to the mantle in subduction zones like the Mariana Trench, where one plate sinks under another it is colliding with. Thus the planet's oceanic crust is continually "recycled" to and from the mantle. As a result oceanic crust tends to be relatively young - the oldest ocean crust is 200 to 300 million years old - while some continental crust is as old as 4 billion years. Where plates move laterally relative to one another, major earthquake faults develop. The San Andreas fault in California is one of the most famous of these.
Some volcanoes, like the Hawaiian archipelago, form when a plate slowly moves over a "hot spot" below it in the mantle. Because the hot spot melts through successive sections of the plate, this creates a chain of mountains that record the plate's history of motion. The "spot" in Hawaii is still "hot", resulting in active volcanoes on the newest island, and yet another island, Lōihi, will emerge tens of thousands of years from now. Some other examples of well-known hotspots include Yellowstone and Iceland.
In 1912 the German polymath Alfred Wegener (1880-1930) put forward the hypothesis of continental drift. Mapmakers had noted centuries before that continents across the Atlantic Ocean (especially eastern South America and western Africa) seem to fit together, but Wegener was the first to suggest that the continents moved over time. Although Wegener presented scientific evidence for his hypothesis, he could not explain what forces drove plate motion [note 4] and the mechanism he proposed (continents "plowing" through the ocean floor) was correctly seen as impossible.
Wegener's theory was widely rejected during his lifetime, not least because Wegener insisted on the aforementioned impossible mechanism. Geologists also avoided the evidence that Wegener did have, something now considered to have been a mistake.[note 5] It did not help that geologists saw Wegener, a professor of geophysics but a climatologist by profession, as an outsider trying to overturn all the established knowledge in their field.
During the 1930s, the US Navy conducted oceanic mapping surveys using sonar but largely kept the data secret. One of the researchers who conducted that work, Harry Hammond Hess (also known as Henry Hess), later became a professor of geology at Princeton University, where he continued to study oceanic survey data. Hess was one of several people who described the modern theory of plate tectonics, but the Navy's secrecy likely delayed the description and acceptance of plate tectonics for decades.
During the 1950s and 1960s interest in continental drift was rekindled as geologists discovered several additional lines of evidence. Ocean floor mapping revealed by Marie Tharp and Bruce Heezen revealed a massive mountain range that encircles the world, and scientists studying magnetic anomalies and samples from ocean drilling found that new seafloor was being created along the mid-oceanic ridge. From then on, continental drift became incorporated into the theory of plate tectonics, which is now as fundamental to geology as evolution is to biology. Where Wegener suggested that the continents were blocks of crust plowing through the ocean floor, in plate tectonics the ocean crust acts more like a conveyor belt that drags the older and more stable continents around.
Researchers have since measured the rate of plate movement by using sophisticated positioning equipment placed along major fault lines, as well as by using satellite observations. As you might have guessed, the plates don't move very fast - about the same rate as your finger nails grow.
Other important concepts
The reason why plate tectonics is such an effective theory is because it explained a wide variety of evidence that geologists had been building for years, as well as having predictive power for discoveries made since. Most of our entire current understanding of geological events like earthquakes and volcanoes is based on our understanding of plate tectonics, which is why it might be surprising that the theory was developed only after things like relativity and quantum physics. Still, it took a few discoveries to fully establish the theory, as well as some observations that had been going for centuries.
Perhaps the most important discovery to the theory was that of seafloor spreading. Geologist Harry Hess had been a submarine commander during World War II, and during his missions he discovered hundreds of undersea mountains in the Pacific. Later, a massive ridge was found in the middle of the Atlantic Ocean. Soon, a canyon would be found in the middle of the ridge, which would inspire Hess to revisit the data and in 1960, published his theory of seafloor spreading — and unlike Wegener, he could provide a geological mechanism. Later discoveries would provide more evidence, most notably the discovery that magnetic reversals and the age of the ocean floor are symmetrical along the ridges. By this point, most geologists had accepted the idea of continental drift.
Obviously, this still left many unanswered questions. They knew how crust was created, but is crust destroyed as well, or does the Earth just expand over time or get more compressed? The answer to these questions was discovered by seismic imaging. There are two kinds of seismic waves: s-waves which cause shaking (think the motion of a shaking rope) and p-waves which expand and compress the ground (which are more similar in motion to a slinky). P-waves are faster and can travel between any form of matter, while s-waves only travel through solids. Both of them will travel at different velocities through different densities of matter and can be recorded using instruments such as a seismometer. By collecting a lot of data, a map can be made of the underground.[note 6] This is where the important discovery came in: people were able to find slabs of oceanic crust pushing below the surface in volcanic places such as the Marianas. This led to the discovery of the mechanism in which oceanic crust is destroyed, which is subduction zones, where old oceanic crust is pushed below continental or less dense oceanic crust, and also led to a greater understanding of the formation of volcanoes in this region, as the water in the sinking oceanic crust results in the melting of rocks and by extension volcanism.
Location of volcanoes and earthquakes
One of the more obvious evidences for plate tectonics, and one that was known for years, was that earthquakes and volcanoes typically don't occur randomly; most of them tend to occur in lines. The Ring of Fire around the Pacific Ocean that contains two thirds of the world's volcanoes is the most famous example. Indeed, most of them occur along mid-ocean ridges and subduction zones, and the gaps are filled with faults like the San Andreas where plates slide past each other; these have little volcanism but many earthquakes. There are exceptions to this in hot spots, and while at first that may seem to throw a wrench in the theory, it actually strengthens it. This is because these hot spots leave behind many volcanoes (the Hawaiian–Emperor seamount chain has over 80) that occur laterally in descending order of age. This shows that the plates are moving on top of the hot spots, so the island chains can actually provide a record of the direction and rate of movement of plates. This evidence, in addition to geomagnetic data telling us which direction from the north pole a rock was formed, allows us to reconstruct maps of what the world looked like millions or even billions of years ago, as well as allowing us to predict what it will look like in the future. There are a couple of hot spots that are still poorly understood by geologists, such as Iceland and Paektu, so new research is ongoing to this day.
Pangaea is a "supercontinent" predicted, in a reverse fashion, by plate tectonics. At a point far back in time (ca. 250 million years ago), all of today's landmasses were conglomerated in one large feature. This had the result of allowing land animals to spread over the whole earth, including dinosaurs and mammals, and also produced a giant desert in the center of the continent.
Where supercontinents come from and go to
Supercontinents form in cycles; many before Pangaea are theorised to have existed and many possible future ones have also been predicted.
- As the ocean floor between the continents is subducted and disappears, continental plates move together to form supercontinents.
- They generally break up after a few hundred million years due to changes in mantle convection — the supercontinent acts as a cap and the mantle then overheats and wells up and splits the plate above. The Great Rift Valley in Africa is a current example of this process in action.
- They generally merge together after another couple hundred million years.
Continental drift, and supercontinents such as Pangaea and Gondwana have turned out to be remarkably useful in matching patterns of fossils. The patterns of fossils on currently-separated continents turn out to make lots of sense if those continents were butted up next to each other at the age of the fossils.
Some creationists think that the statement that the Earth divided in the days of Peleg, as described in Genesis 10:25, refers to Pangaea, and that the antedeluvian Earth had a single supercontinent, i.e. Pangaea. This is an uber-literal position: even Answers in Genesis thinks the statement refers to the Tower of Babel. No, Answers in Genesis believes the pre-Flood Earth was the pre-Cambrian Rodinia, and that Pangaea simply formed and broke apart during the Flood.
Likewise, others have similarly claimed either the Flood describes Pangaea's breakup during the Triassic period (meaning Noah was around in those times) or that there were two Floods: one sent by God ten million years ago in the Miocene attempting to destroy Satan and the angels to his side, presumably confusing in the Zanclean flood that filled the Mediterranean basin, and other the Biblical one, that took place much later, in the times described in the Bible, and caused Pangaea's breakup too (the consequences of cramming many millions of years of geologic activity into forty days at best are of course not addressed). It's left to the reader to consider which position is the stupidest one.
In the peculiar Saturn Myth offshoot from Immanuel Velikovsky's planetary billiards scenario, it is claimed that, within human memory, Earth was a satellite of Saturn, orbiting in a tidal lock configuration so that the Pangea supercontinent always faced toward Saturn — which glowed as a brown dwarf star, thanks to the Electric Universe. Megafauna apparently flourish in this gravitational environment. Pangea was broken up when the Sun and/or Jupiter crashed into the Saturnian system and Earth ended up orbiting the Sun instead. This idea is nothing if not imaginative.
Before evidence was gathered for the existence of tectonic plates and the mechanisms by which they move, there were a variety of other theories to explain how continents appeared, moved, changed, and disappeared. Today, a few eccentric people still believe in them.
- Expanding Earth: Robert Mantovani suggested the world was expanding, pushing Africa and South America apart; the idea is still popular with some cranks.
- Contracting Earth: James Dwight Dana suggested the earth cooled and wrinkled, forming mountain ranges.
- Crust displacement: Charles Hapgood believed the earth could sometimes spin really fast, rearranging continents.
- Pole-fleeing force (Polflucht): Alfred Wegener's early theory of continental drift suggested that centrifugal force or some other mechanism would cause continents to move from the poles to the equator.
- Flat earth tectonics: some proponents of the flat earth thing believe there are tectonic plates below the flat earth, these plates collide producing mountain ranges, and hot rock rises from below to produce other mountains.
- the crust and the uppermost portion of the mantle
- The crust forms when part of the mantle melts and rises to the surface, and this partial melting naturally creates rocks that are less dense than the material left behind.
- Despite the high temperatures in Earth's innards, the mantle is solid due to the tremendous pressures that the rocks there have to withstand (see here). Like glacial ice, solid mantle rock can "flow" like a liquid over long time scales, but this "flow" (and plate motion) is at speeds of millimeters to a few centimeters per year.
- Which is still an outstanding question in modern geology!
- However, overall, the delay in the success of the continental drift theory can be considered an object lesson in what happens when a scientist who happens to be right is a jerk about it.
- This is also how we know about the structure of the inside of the Earth.
- US Navy's Secrecy Likely Stalled Ocean Science Progress for Decades by Mindy Weisberger (December 13, 2018) Live Science.
- Marie Tharp’s groundbreaking maps brought the seafloor to the world: Her deep understanding of geology made for gorgeous and insightful views by Betsy Mason (January 13, 2021 at 11:00 am) Science News.
- Great Global Rift is discovered, PBS
- Hess proposes sea-floor spreading, PBS
- Seafloor Spreading Activity, NOAA
- Compare-Contrast-Connect: Seismic Waves and Determining Earth’s Structure, Manoa
- Seismic imaging, Peter Mora and Albert Tarantola, SpringerLink 1990
- Seismic tomography, Tjeerd H. van Andel, Britannica
- Subduction Zone, ScienceDirect
- Plate Tectonics and the Ring of Fire, National Geographic
- What is a hotspot and how do you know it's there?, USGS
- Ancient Earth, Dinosaur Database
- AIG:In the days of Peleg
- The Created World, Answers in Genesis 20 August 2008
- See the Wikipedia article on Continental drift.
- Formation of Mountains and Volcanoes, Flat Earth Society wiki, accessed May 24, 2018