introduction to plate tectonics

Wegener proposed that at one time, all the present-day continents actually were combined into a "super-continent" which he called Pangaea (or Pangea). These spots are often at the head of a track of volcanic activity that has "burned through" the lithosphere. How many major pieces are there? Live Science is part of Future US Inc, an international media group and leading digital publisher. Many spectacular volcanoes are found along subduction zones, such as the "Ring of Fire (opens in new tab)" that surrounds the Pacific Ocean. We often think of hot spots as stationary features (relative to the lithospheric plates) and they provide a reference frame that we use to estimate the absolute velocity of the plates with respect to the mantle beneath the lithosphere. Wegener didn't have an explanation for how continents could move around the planet, but researchers do now: Plate tectonics. When two continental plates push together, the result is an upward movement of the accumulating land. <<86A4D498951746479445F5E90B1DD6CA>]>> V)gB0iW8#8w8_QQj@&A)/g>'K t;\ $FZUn(4T%)0C&Zi8bxEB;PAom?W= The thermal expansion of the material beneath the ridge maintains the high elevation and produces a gravitational force that helps slide the two plates apart (this force is called ridge push). This process happens incredibly slowly. The complex shapes of the tectonic plates, the geometric constraints of plates moving on the confined surface of a sphere and changes in the forces acting on the plates all lead to changes in plate motion over time. [4shwz\s`A9C>2O$Tt^~~ggS}q~:J B(cwuhFY)RlDDJ@_& kJCD7v@gpu!;9x2'FaD0I(3!B(*4hoN^%MYi^Cn(|N9gKtn7UeefV&%M2kXdYm7hff& /uM?oK q-w0)bmDVr~,i\vLR endstream endobj 133 0 obj<> endobj 134 0 obj<>stream Her work has appeared in Yale Climate Connections, The Farmers' Almanac, and otherpublications. What they found is that at irregular intervals, Earth's magnetic field reverses polarity. endstream endobj 123 0 obj<> endobj 124 0 obj<> endobj 125 0 obj<>/Font<>/ProcSet[/PDF/Text]/ExtGState<>>> endobj 126 0 obj<> endobj 127 0 obj<> endobj 128 0 obj[/ICCBased 135 0 R] endobj 129 0 obj<> endobj 130 0 obj<> endobj 131 0 obj<> endobj 132 0 obj<>stream Plate tectonics is the theory that Earth's outer shell is divided into large slabs of solid rock, called plates, that glide over Earth's mantle, the rocky inner layer above Earths core. 0000001362 00000 n Geologists refer to the places where segments meet and divide as plate boundaries. Plate tectonics is the means through which mountains are formed. 0000020667 00000 n They were able to date the age of lava flows using radioactive dating techniques (which we discussed earlier) and identify the orientation and strength of the magnetic field during the past.

Plates are large pieces of the upper few hundred kilometers of Earth that move more or less as a single unit. It is 100 km (60 miles) thick, according to the Encyclopedia Britannica (opens in new tab). This motion is resisted by viscous forces on the base of the plates, by the strength of the plate itself, which resists bending into the mantle at subduction zones, and by the frictional and viscous forces acting between adjacent plates. The red dots are shallow earthquakes, the green are intermediate depth, and the blue and purple are deep. As the name suggests, divergent boundaries are tectonic boundaries where plates "diverge" or are tugged apart. 0000002901 00000 n This fracture likely formed when the ocean crust was formed, but now it is being turned into a new plate boundary. This movement under Earth's crust should be familiar. Stay up to date on the latest science news by signing up for our Essentials newsletter. By now you already know that Earth has layers, similar to the layers of an onion. However, according to a 2012 article in Nature (opens in new tab), earthquakes over the last few decades are evidence that the Indo-Australian plate has cracked over the last 10 million years, creating a separate Indian Plate and Australian Plate (opens in new tab) which will increase the number of major plates to eight. Earthquakes deeper than 500 km are restricted to only a few regions surrounding the Pacific and part of the Indian oceans. Faulting along transform margins is strike-slip. Individual measurements in any region can be different from the values plotted, but the regional averages are well represented in the diagram. When you purchase through links on our site, we may earn an affiliate commission. There was a problem. A convection current is the movement of heat energy through liquids or gases. These continents are still on the move today. The north magnetic pole and the south magnetic pole exchange places (the geographic poles do not change, the magnetic field changes). The ridges are regions of earthquake activity. Some volcanic regions such as the Hawaiian Islands are isolated. California's San Andreas Fault (opens in new tab), where the North American and Pacific tectonic plates grind past each other with a mostly horizontal motion, is one famous example of a transform boundary. Related: The Alps are still growing faster than they're eroding (opens in new tab). We measure heat flow at different points on Earth's surface by measuring the change in temperature with depth (a higher temperature gradient indicates higher heat flow, assuming that the measurements were made in the same material). Explain III: Mountain Building and Earthquakes, Governor's Committee on People with Disabilities. When two oceanic plates converge, one will be thrust under the other, and the same volcanic processes will occur. With a little study you can also deduce that the growth rates vary. 0000003700 00000 n The forces are produced by mantle convection and gravity. When two oceanic plates converge, a deep trench forms, such as the Mariana Trench (opens in new tab) in the North Pacific Ocean which is believed to be the deepest point on Earth. startxref n3kGz=[==B0FX'+tG,}/Hh8mW2p[AiAN#8$X?AKHI{!7. It measures 39,768,522 square miles (103,000,000 square kilometers) in size, and lies hidden beneath the ocean. %PDF-1.4 % The precise thickness of a plate varies from place to place, but away from plate margins, plates are usually on the order of 100-200 km thick. That current is what causes the movement of the plates in these cases. Therefore, it is important for geophysicists to be able to both use observations to determine plate motions, and conversely to use to plate motions to make predictions about how those motions are connected to other observations. Click the NASA logo below to visit its Science News site and learn a little bit about this theory and how NASA satellites are involved. What you may not know is that the lithosphere is broken into many pieces. The major plates that you need to know about are the following: You can see the plates again here on the map. The map above shows the distribution of earthquakes with magnitudes greater than 5.0 that occurred between 1965 and 1995. ", Signs of cancer found in mysterious 'pregnant' Egyptian mummy, 560 million-year-old tentacled creature may be the animal kingdom's first known predator, Monkeypox: Symptoms, pictures, treatments & vaccines, Ice age children frolicked in 'giant sloth puddles' 11,000 years ago, footprints reveal, Strange, never-before-seen diamond crystal structure found inside 'Diablo canyon' meteorite, Rare and bizarre tentacle-trailing sea creature caught on video, expedition scientists 'mind is blown', The ultimate action-packed science and technology magazine bursting with exciting information about the universe, Engaging articles, amazing illustrations & exclusive interviews, Issues delivered straight to your door or device, Delve deeper into plate tectonics with this. As the system evolved, part of coastal California has been "captured" by the Pacific plate and as a result the Los Angeles region (which is on the Pacific plate) is moving towards the San Francisco region (which is on the North American plate) at about four centimeters per year. Earths solid outer layer, which includes the crust and the uppermost mantle, is called the lithosphere. endstream endobj 135 0 obj<>stream g>lNEdy j7xgY/=86(M^Sb/|[&f?q s^&yjH Zf >On;:"uMyOO0~tQJV\: , =*Y~` X

In the 1950's and 1960's scientists used the magnetic field-information stored in rocks to investigate the behavior of the geomagnetic field. The motion of tectonic plates are primarily driven by gravitational buoyancy forces associated with sinking of cold material into the mantle at subduction zones and rising of hot material at mid-ocean spreading centers. All Rights Reserved. and changes in plate boundaries (e.g., How long until the Juan de Fuca ridge reaches the Cascadia subduction zone?). 0000000676 00000 n They're thought to wrap around the Earth like seams on a baseball. Explore III: Which Movement Is Different? But converging plates don't always collide upward. Since the construction of the first good maps of the continents, people have puzzled over the close match between the coastlines of South America and Africa. In the 1950's and 1960's scientists worked out a way of estimating the age of the ocean floor using characteristics of Earth's magnetic field.

(o ( This map shows the seafloor and deformation below it at a fracture in the Wharton Basin in the Indian Ocean. We are unsure where they originate (upper mantle/lower mantle/ core-mantle boundary?) You certainly noticed that the sliding plates and the plates coming together and building up the land have not yet been discussed.

The magma is lighter than the surrounding rock and floats upward. In the last century we have gathered much evidence to support the idea that Earth's surface is broken up into "lithospheric" plates that slowly move over the top of the mantle. 0000001145 00000 n While the Earth is estimated to be 4.54 billion years old (opens in new tab), oceanic crust is constantly recycled at subduction zones. x- [ 0}y)7ta>jT7@t`q2&6ZL?_yxg)zLU*uSkSeO4?c. R -25 S>Vd`rn~Y&+`;A4 A9 =-tl`;~p Gp| [`L` "AYA+Cb(R, *T2B- The map below shows the major plates. Underwater mountains and volcanoes can rise along this seam, in some cases forming islands. The color of the symbol identifies the hypocentral depth and continent coastlines are shown in blue. One theory is that convection within the Earth's mantle pushes the plates, in much the same way that air heated by your body rises upward and is deflected sideways when it reaches the ceiling. The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Thingvellir Valley in Iceland is only place where the Mid-Atlantic Ridge is above sea level. One of the earliest big supercontinents, called Rodinia, assembled about 1 billion years ago. Finally, we will learn how to use a model of present-day plate motion to answer questions related to relative plate motions (e.g., How long until San Diego is at the same latitude as Sacramento?) "Plate tectonics unified all these descriptions and said that you should be able to describe all geologic features as though driven by the relative motion of these tectonic plates.". Most scientists think modern plate tectonics took over from earlier planetary development about 3 billion years ago (opens in new tab), based on ancient magmas and minerals preserved in rocks from that period. The result of collisions are dramatic mountain ranges and plateaus such as the Himalayas and Tibetan Plateau which have formed as a result of the collision of India and southern Eurasia. The paper curls upward because the force coming from each side leaves nowhere else for the paper to go. Visitors can walk down the rift between the North American plate and the Eurasian plate. It's similar to laying a piece of paper flat and pushing each end of the piece of paper toward the center. Plate tectonics explains the movement of Earth's surface. Another theory is that gravity is pulling the older, colder, and thus heavier ocean floor with more force than the newer, lighter seafloor. A more recent supercontinent called Pangaea (opens in new tab) formed about 300 million years ago. Additionally, the rocks composing the continental crust are very old, some formed as much as 3.8 billion years ago. The original digital data are courtesy of researchers at the Scripps Institute of Oceanography). The Baird Mountains in Alaskas Kobuk Valley National Park formed when two tectonic plates along a convergent boundary collided, causing solid rock to buckle and fold. 140 0 obj<>stream If you study the map of ocean age, you will notice that there are places where the regions of different age ocean are juxtaposed and segments of the ridge are offset. From the deepest ocean trench to the tallest mountain, plate tectonics explains the features and movement of Earth's surface in the present and the past. The land masses continued to move apart, riding on separate plates, until they reached the positions they currently occupy. The surface of the Earth is divided into rigid blocks, called tectonic plates that are bounded by narrow regions of high deformation called plate boundaries. As the plates are pulled apart, magma is extracted from the mantle to fill in the void. nQt}MA0alSx k&^>0|>_',G! RD,*b-LJWEL5!n19L According to World Atlas (opens in new tab), seven major plates exist: the North American, Pacific, Eurasian, African, Indo-Australian, South American and Antarctic tectonic plates. The spreading is a result of forces pulling the oceanic lithosphere on either side of the ridge in opposite directions. When two pieces of crust move apart from each other, magma escapes and hardens to form new crust. %%EOF The places where the destruction takes place are called convergent plate boundaries, or subduction zones. Okay, let's check your understanding of plates and some of their movement so far. This is the result of the magma escaping through Earth's crust, becoming lava, and finally hardening to form new ocean basins, or ocean floors. Now if the ocean floor is growing in some places, but the overall surface are of the planet is constant, somewhere ocean floor must be destroyed. 1@}0a@_|E0La How many major pieces are there? 0000020394 00000 n Visit our corporate site (opens in new tab). Geophysical Research Letters (2020); Some of the major modern-day plates under our feet. The large number of hot spots in the Atlantic ocean are suspected to have played a role in the breakup of Pangaea. The oceanic material is forced down into the mantle because it is more dense than the continent. HTn@}Wvg8!A"zY>,MK*Iw9GZARN The list of Earths minor plates includes the Arabian Plate, Caribbean Plate, Cocos Plate, Nazca Plate, Philippine Plate, Scotia Plate, and more. Tectonic plates move at rates of a few to tens of centimeters per year. We know that the oceans have been around much longer than 200 Ma so the implication is that the entire sea floor is recycled in a few hundred million years. It is a convection current. The first to seriously investigate the connection was Alfred Wegener, a German meteorologist who proposed the continental drift hypothesis (between 1919-1929) to explain the observed shape of the coastlines and several other geologic observations regarding the observation of fossils and rocks on opposite sides of the ocean. The water mixes with the mantle and reduces the mantle rocks melting point, and magma forms. At about 1300C typical mantle material begins to melt, and softens dramatically. The "geomagnetic" field is generated by motions of the iron in the outer core. What they found was the the intensity of the field alternated between high and low values as if the ocean floor was also recording the polarity reversals of Earth's magnetic field. The average continental heat flow is about 57 milliwatts per square meters (mW/m^2), the oceanic heat flow is about 100 mW/m^2. wG xR^[ochg`>b$*~ :Eb~,m,-,Y*6X[F=3Y~d tizf6~`{v.Ng#{}}jc1X6fm;'_9 r:8q:O:8uJqnv=MmR 4

xref However, when a mountains mass becomes too large to resist gravity, it will cease to grow (opens in new tab). At the time not much was known about the oceanic and continental crust and they were assumed to be similar in age and structure (that's the simplest idea). and it may turn out they the are not perfectly stationary but are slowing drifting (but moving slower than the plates). Related: Plate tectonics are 3.6 billion years old, oldest minerals on Earth revea (opens in new tab)l, Nicholas van der Elst, a seismologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, considers plate tectonics to be the unifying theory of geology., "Before plate tectonics, people had to come up with explanations of the geologic features in their region that were unique to that particular region," said Van der Elst. The rocks in the oceanic crust are formed and spend a great deal of time in an environment rich in water. As the mash-up continues, those mountains grow higher and higher. However, it is clear that the most active deformation of the plates occurs along their boundaries, where they interact with other plates. These types of collisions can also lead to underwater volcanoes. Ninety percent of all earthquakes occur along plate margins and by far the greater amount of energy released as seismic waves comes from subduction zones. Notice the pointed shape of Rupal Peak in the far western portion of the Himalaya Mountains.

The offsets occur on transform faults, which form the second type of plate boundary. In a way, compasses are instruments for measuring the direction of Earth's magnetic field. It then descends into the Earth's mantle, the layer underneath the crust, melts in the mantle's hot magma, and is spewed out in a volcanic eruption. Whether that new divide counts as a boundary or not, the Pacific Plate is still the largest of all tectonic plates. Exactly what drives plate tectonics is not known. 0000000016 00000 n Because Earth is spherical, its tectonic or lithospheric plates are fractured into dozens of curved sections. Geologists have discovered the Swiss Alps are being lifted faster than they are being lowered through erosionand are thus growing every year, according to a 2020 study in the journal Earth-Science Reviews (opens in new tab). The Sea Floor Spread (divergent boundary), The Continental Slide (convergent boundary), The Continental Crush (collisional boundary), Slippin' and a Slidin' (transform boundary), WGBH | PBS Online | Search | Feedback | Shop Its breakup is linked to a global glaciation called Snowball Earth. As the rock continues to cool, its temperature decreases below what we call the "blocking temperature" and the magnetically induced alignment of iron is is frozen into the rock. The result of this work (and other geologic investigations) is shown in the map above. The North American and Eurasian plates are moving away from each other, resulting in a large underwater mountain range called the Mid-Atlantic Ridge. We call that part of the mantle asthenosphere, to indicate that it is a weak zone, that "decouples" the plate from the overlying mantle (actually, there is undoubtedly some "drag" forces that act between the two, but the lithosphere can move independently of the deeper mantle. As a result of that work, the variety of structures in the ocean bottom were identified and mapped, including a 40,000 km long ridge system that encircles the planet (the ridge system is identified in the map above by the light shaded regions near away from the continents). 0000001686 00000 n It lubricates the undersides of Earth's tectonic plates, allowing the lithosphere to move around. Second, we will review some of the ways in which we can observe plate motions, such as using magnetic anomalies on the seafloor or GPS measurements. The alternating patterns were symmetric about bathymetric ridges that were recently mapped and known encircled the globe. The oldest ocean rocks are found in the northwestern Pacific Ocean and the eastern Mediterranean Sea. Near the equator, the magnetic field is nearly horizontal, near the poles it becomes more vertical. Hot material near the Earth's core rises, and colder mantle rock sinks. For example, the east Pacific region is spreading faster than the central Atlantic. Tiffany Means is a meteorologist turned science writer based in the Blue Ridge mountains of North Carolina. To understand the line of argument, we must review some basic ideas about magnetism and rocks. The ocean ridge system shows up as an interconnected ribbon of red and red-range indicating that the ridges are the youngest part of the oceans. Why do you think they are moving? Variations in the heat-flow measurements were averaged to provide a mathematical representation of Earth's heat flow. As the lava cools the iron they contain is preferentially oriented by the magnetic field of Earth, like mini-compasses. Plate tectonics is an ongoing process, so long in the future these plates could be as unrecognizable as Earth's surface was a billion years ago.

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