My Science School

Volcanoes erupt when molten rock, known as magma, is forced to the Earth's surface by the movement of the Earth's tectonic plates. Sometimes a volcano explodes, sending thick clouds of ash high into the atmosphere. Other volcanic eruptions produce rivers of red-hot lava that flow over the landscape covering everything in their path. Whichever way a volcano erupts, it is one of the natural world's most powerful and destructive forces.

Many of us only notice volcanoes when they are about to explode or disrupt our travel plans, but these spectacular forces of nature can have a significant impact on people living in the local area. While volcanoes can be destructive, they are also responsible for creating rich agricultural soil, minerals like gold and silver, diamonds, hot springs and geothermal energy.

A volcano is like a chimney that allows hot liquid rock, called magma, to flow from a layer within the Earth and erupt onto the surface. The magma can come from as far down as 200 kilometres in the mantle and once it erupts — at a piping hot 700 to 1,200 degrees Celsius — it is called lava.

As magma rises through many kilometres to the Earth's surface, dissolved gases contained within it form expanding bubbles. These bubbles increase the pressure of the magma and, if this pressure is great enough, the volcano will erupt. The amount, temperature and composition of magma, including the amount of trapped gas contained in it, determines the type of volcano formed. The three most common large types of volcanoes are strato, shield and caldera.

Strato volcanoes are cone-shaped mountains that have been built up from layers of ash and lava. They are generally the tallest type of volcano and are known for their violent explosions. Bubbles of gas build up in the magma — which has a high silica content — and explode creating volcanic ash, consisting of tiny gritty sharp fragments of glassy snap-frozen magma and rock from the sides of the volcano vent.

Examples of strato volcanoes include Agung in Bali, Yasur in Vanuatu, Etna in Italy and Fuji in Japan.

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Modern building technologies mean that homes, offices and other buildings can be designed to withstand the effect of an earthquake. Tall buildings are built with a strong central column from which the structure “hangs”. Conical or triangular designs are able to absorb shocks more easily, while the use of new materials allows buildings to be constructed in earthquake zones at a relatively low cost.

After the massive earthquake near Japan one wonders if it’s possible to build an earthquake-proof building. The answer is yes and no. There are of course, engineering techniques that can be used to create a very sound structure that will endure a modest or even strong quake. However, during a very strong earthquake, even the best engineered building may suffer severe damage. Engineers design buildings to withstand as much sideways motion as possible in order to minimize damage to the structure and give the occupants time to get out safely.

Buildings are basically designed to support a vertical load in order to support the walls, roof and all the stuff inside to keep them standing. Earthquakes present a lateral, or sideways, load to the building structure that is a bit more complicated to account for. One way to make a simple structure more resistant to these lateral forces is to tie the walls, floor, roof, and foundations into a rigid box that holds together when shaken by a quake.

The most dangerous building construction, from an earthquake point of view, is unreinforced brick or concrete block.  Generally, this type of construction has walls that are made of bricks stacked on top of each other and held together with mortar.  The roof is laid across the top.  The weight of the roof is carried straight down through the wall to the foundation.  When this type of construction is subject to a lateral force from an earthquake the walls tip over or crumble and the roof falls in like a house of cards.

Construction techniques can have a huge impact on the death toll from earthquakes. An 8.8-magnitude earthquake in Chile in 2010 killed more than 700 people. On January 12, 2010, a less powerful earthquake, measuring 7.0, killed more than 200,000 in Haiti.

The difference in those death tolls comes from building construction and technology. In Haiti, the buildings were constructed quickly and cheaply. Chile, a richer and more industrialized nation, adheres to more stringent building codes.

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Perhaps the world's best known fault line is the San Andreas Fault. Situated in California, USA, it is an area of the world where earthquakes and tremors occur frequently. The citizens of San Francisco know that a very powerful quake (often referred to as “The Big One”) could occur at any time.

Viewed from space, the San Andreas Fault looks like a long, narrow valley that marks where the North America plate meets the Pacific plate. This narrow break between the two plates is called a fault. But viewed up close, there are actually many fractures and faults that mark the zone where the two plates slide past one each other. Sometimes the boundary is a zone of several smaller faults, one or more of which may break during an earthquake. Sometimes it is a single fault. 

On the ground, one can find the San Andreas Fault by looking for landforms it created. For example, sharp cliffs called scarps form when the two sides of the fault slide past each other during earthquakes. "The dominant motion along the fault is primarily horizontal, but some areas also have vertical motion," noted Shimon Wdowinski, a geophysicist at the University of Miami's Rosentiel School of Marine & Atmospheric Sciences who has studied the San Andreas Fault. And stream channels with sharp jogs — the channels are offset across the fault line — can be visited in the central California's Carrizo Plain National Monument.

On the west side of the fault sits most of California's population, riding the Pacific Plate northwest while the rest of North America inches south. The Pacific Plate is moving to the northwest at 3 inches (8 centimeters) each year, and the North American Plate is heading south at about 1 inch (2.3 cm) per year.

The San Andreas Fault was born about 30 million years ago in California, when the Pacific Plate and the North America plate first met. Before then, another oceanic plate, the Farallon plate, was disappearing beneath North America at a subduction zone, another type of plate boundary. The new configuration meant the two plates slid past one another instead of crashing into each other, a boundary called a strike-slip fault.

Researchers have measured identical rocks offset by 150 miles (241 kilometers) across either side of the fault. For example, the volcanic rocks in Pinnacles National Park south of Monterey match volcanic rocks in Los Angeles County (called the Neenach volcanics). Geologists think the total amount of displacement along the fault is at least 350 miles (563 km) since it formed.

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The shock wave of a powerful earthquake can easily destroy buildings and other structures, but there are some side-effects of the quake itself. Underground gas pipes may rupture, leading to serious fires and explosions. The health of survivors is but at risk by damaged sewerage systems allowing disease to spread. In mountainous areas, landslides or avalanches can be triggered, and an undersea earthquake can generate a huge wave called a tsunami.

An earthquake is a sudden shaking movement of the surface of the earth. It is known as a quake, tremblor or tremor. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to toss people around and destroy whole cities. The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.

So far, there have been sixty-two earthquakes in India. The first recorded earthquake in India was on 6th June 1505 it occurred in Saldang, Karnali zone. And the most recent one happened in India as on 31st January 2018 and occurred in Kashmir, Pakistan, Afghanistan, and Tajikistan.

An earthquake is measured on Richter’s scale. A seismometer detects the vibrations caused by an earthquake. It plots these vibrations on a seismograph. The strength, or magnitude, of an earthquake, is measured using the Richter scale. Quakes measuring around 7 or 8 on the Richter scale can be devastating.

Most earthquake-related deaths are caused by the collapse of structures and the construction practices play a tremendous role in the death toll of an earthquake. In southern Italy in 1909 more than 100,000 people perished in an earthquake that struck the region. Almost half of the people living in the region of Messina were killed due to the easily collapsible structures that dominated the villages of the region. A larger earthquake that struck San Francisco three years earlier had killed fewer people (about 700) because building construction practices were different type (predominantly wood). Survival rates in the San Francisco earthquake was about 98%, that in the Messina earthquake was between 33% and 45%) (Zebrowski, 1997). Building practices can make all the difference in earthquakes, even a moderate rupture beneath a city with structures unprepared for shaking can produce tens of thousands of casualties.

Although probably the most important, direct shaking effects are not the only hazard associated with earthquakes, other effects such as landslides, liquefaction, and tsunamis have also played important part in destruction produced by earthquakes.

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Deep beneath the Earth's surface, the Earthquake place where the earthquake actually occurs is called the focus. This is where the greatest amount of rock movement is to he found. The ground directly above the focus is known as the epicentre. This is where the most damage occurs.

An earthquake's hypocenter is the position where the strain energy stored in the rock is first released, marking the point where the fault begins to rupture. This occurs directly beneath the epicenter, at a distance known as the focal or hypocentral depth.

The focal depth can be calculated from measurements based on seismic wave phenomena. As with all wave phenomena in physics, there is uncertainty in such measurements that grows with the wavelength so the focal depth of the source of these long-wavelength (low frequency) waves is difficult to determine exactly. Very strong earthquakes radiate a large fraction of their released energy in seismic waves with very long wavelengths and therefore a stronger earthquake involves the release of energy from a larger mass of rock.

Computing the hypocenters of foreshocks, main shock, and aftershocks of earthquakes allows the three-dimensional plotting of the fault along which movement is occurring. The expanding wave front from the earthquake's rupture propagates at a speed of several kilometers per second; this seismic wave is what is measured at various surface points in order to geometrically determine an initial guess as to the hypocenter. The wave reaches each station based upon how far away it was from the hypocenter. A number of things need to be taken into account, most importantly variations in the waves speed based upon the materials that it is passing through. With adjustments for velocity changes, the initial estimate of the hypocenter is made, then a series of linear equations is set up, one for each station. The equations express the difference between the observed arrival times and those calculated from the initial estimated hypocenter. These equations are solved by the method of least squares which minimizes the sum of the squares of the differences between the observed and calculated arrival times, and a new estimated hypocenter is computed. The system iterates until the location is pinpointed within the margin of error for the velocity computations.

A deep-focus earthquake in seismology (also called a plutonic earthquake) is an earthquake with a hypocenter depth exceeding 300 km. They occur almost exclusively at convergent boundaries in association with subducted oceanic lithosphere. They occur along a dipping tabular zone beneath the subduction zone known as the Wadati–Benioff zone.

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The size, or the magnitude, of an earthquake is recorded using an instrument called a seismometer. Using very heavy weights that remain still while the room it is in is shaking, the machine records the amount of movement on a rotating drum of paper. This type of record is measured on the Richter scale. The physical and visible effects of a quake are measured using the Vertical Modified Mercalli scale (see below).

Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram. The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.

The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but that’s not something scientists can simply measure with a measuring tape since faults are many kilometers deep beneath the earth’s surface. So how do they measure an earthquake? They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was (figure 5). A short wiggly line that doesn’t wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.

The size of the earthquake is called its magnitude. There is one magnitude for each earthquake. Scientists also talk about theintensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.

The Modified Mercalli scale:

1 Only detected by instruments. Doors begin to swing.

2 Some people inside high buildings may feel a tremor.

3 Rapid vibrations possibly felt indoors.

4 Stationary cars rock; windows shake; people indoors feel something.

5 Effects felt outdoors; small objects fall over; some buildings shake.

6 Trees begin to shake; crockery broken; everyone in the area feels it.

7 People alarmed; chimneys begin to crack; windows break.

8 Cars crash; buildings and trees damaged.

9 Many people panic; cracks in the ground; buildings fall down.

I0 Buildings destroyed; underground services disrupted; rivers affected.

II Bridges collapse; landslides happen; railways affected.

12 Widespread devastation; landscape changed.

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Earthquakes can happen anywhere, but they occur most frequently above the boundaries of the Earth's tectonic plates. The most powerful earthquakes occur where the plates are moving deep below the surface. These boundaries are known as transform faults or fault lines.

Earthquakes can strike any location at any time, but history shows they occur in the same general patterns year after year, principally in three large zones of the earth:

The world's greatest earthquake belt, the circum-Pacific seismic belt, is found along the rim of the Pacific Ocean, where about 81 percent of our planet's largest earthquakes occur. It has earned the nickname "Ring of Fire". Why do so many earthquakes originate in this region? The belt exists along boundaries of tectonic plates, where plates of mostly oceanic crust are sinking (or subducting) beneath another plate. Earthquakes in these subduction zones are caused by slip between plates and rupture within plates. Earthquakes in the curcum-Pacific seismic belt include the M9.5 Great Chilean Earthquake [Valdivia Earthquake] (1960) and the M9.2 Great Alaska Earthquake (1964).

The Alpide earthquake belt extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. This belt accounts for about 17 percent of the world's largest earthquakes, including some of the most destructive, such as the 2005 M7.6 shock in Pakistan that killed over 80,000 and the 2004 M9.1 Indonesia earthquake, which generated a tsunami that killed over 230,000 people. 

The third prominent belt follows the submerged mid-Atlantic Ridge. The ridge marks where two tectonic plates are spreading apart (a divergent plate boundary). Most of the mid-Atlantic Ridge is deep underwater and far from human development, but Iceland, which sits directly over the mid-Atlantic Ridge, has experienced earthquakes as large as at least M6.9.

The remaining shocks are scattered in various areas of the world. Earthquakes in these prominent seismic zones are taken for granted, but damaging shocks can occur outside these zones. Examples in the United States include New Madrid, Missouri (1811-1812) and Charleston, South Carolina (1886). However, many years usually elapse between such shocks.

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Earthquakes are one of the most destructive forces on Earth. They happen quite frequently, though most of them are relatively minor. Powerful quakes, depending on where they happen, cause severe damage, toppling buildings and sometimes killing many thousands of people. They happen when tension created by the movement of the Earth's tectonic plates is released, causing the rocks to shift and break suddenly. The incredible amount of force required to break the rocks is what makes earthquakes so devastating.

If your heart beats rapidly during an earthquake, it still doesn’t compete with high-frequency waves generated by the quake. These waves shake the ground faster than your ticker’s thrumming and cause the most damage to smaller structures, such as house­­s.

Researchers now have a new explanation for the source of these poorly understood high-frequency seismic waves. The longer a fault heals between earthquakes, the faster the waves once the fault finally breaks again, according to a new study detailed in the Oct. 31 issue of the journal Nature.

"We can think of a fault as just as crack or a cut in the ground. When they heal, it may not be all that different than how a cut in your skin heals. There are physical and chemical changes that occur right on the surface," said Gregory McLaskey, lead study author and a postdoctoral researcher at the U.S. Geological Survey in Menlo Park, Calif.

Though the next quake may not be bigger in terms of magnitude, it could be much more intense, with more rapid shaking, he said.

"It doesn't just affect the strength of it, it affects the way the ground will shake when it ruptures. The more the fault has healed, the more rapid vibrations and jolts will be produced when the earthquake does come," McLaskey told OurAmazingPlanet.

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The whole of the Earth's crust is subject to continental drift, including the ocean floor. Most of the tectonic plates are both continental (part of the land) and oceanic (part of the ocean floor). Evidence of movement on the sea bed is found in different magnetic alignments in the rock and volcanic activity on the ocean floor.

Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources. The structure of the oceans, starting with the continents, begins usually with a continental shelf, continues to the continental slope – which is a steep descent into the ocean, until reaching the abyssal plain – a topographic plain, the beginning of the seabed, and its main area. The border between the continental slope and the abyssal plain usually has a more gradual descent, and is called the continental rise, which is caused by sediment cascading down the continental slope.

The mid-ocean ridge, as its name implies, is a mountainous rise through the middle of all the oceans, between the continents. Typically a rift runs along the edge of this ridge. Along tectonic plate edges there are typically oceanic trenches – deep valleys, created by the mantle circulation movement from the mid-ocean mountain ridge to the oceanic trench.

Hotspot volcanic island ridges are created by volcanic activity, erupting periodically, as the tectonic plates pass over a hotspot. In areas with volcanic activity and in the oceanic trenches there are hydrothermal vents – releasing high pressure and extremely hot water and chemicals into the typically freezing water around it.

Deep ocean water is divided into layers or zones, each with typical features of salinity, pressure, temperature and marine life, according to their depth. Lying along the top of the abyssal plain is the abyssal zone, whose lower boundary lies at about 6,000 m (20,000 ft). The hadal zone – which includes the oceanic trenches, lies between 6,000–11,000 metres (20,000–36,000 ft) and is the deepest oceanic zone.

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Fossilized remains found in different parts of the world are good evidence that the continents were once joined together. Remains of the same animal have been found in both South America and Africa, which means it must have lived at a time when the continents were part of the same land mass. Plant fossils of the same type and age have been found all over the world, and geologists have identified parts of the same mountain range in different continents.

Alfred Wegener collected diverse pieces of evidence to support his theory, including geological “fit” and fossil evidence. It is important to know that the following specific fossil evidence was not brought up by Wegener to support his theory. Wegener himself did not collect the fossils but he called attention to the idea of using these scientific doc   uments stating there were fossils of species present in separate continents in order to support his claim.

Geological “fit” evidence is the matching of large-scale geological features on different continents. It has been noted that the coastlines of South America and West Africa seem to match up, however more particularly the terrains of separate continents conform as well. Examples include: the Appalachian Mountains of eastern North America linked with the Scottish Highlands, the familiar rock strata of the Karroo system of South Africa matched correctly with the Santa Catarina system in Brazil, and the Brazil and Ghana mountain ranges agreeing over the Atlantic Ocean.

Another important piece of evidence in the Continental Drift theory is the fossil relevance. There are various examples of fossils found on separate continents and in no other regions. This indicates that these continents had to be once joined together because the extensive oceans between these land masses act as a type of barrier for fossil transfer. Four fossil examples include: the Mesosaurus, Cynognathus, Lystrosaurus, and Glossopteris.

The Mesosaurus is known to have been a type of reptile, similar to the modern crocodile, which propelled itself through water with its long hind legs and limber tail. It lived during the early Permian period (286 to 258 million years ago) and its remains are found solely in South Africa and Eastern South America. Now if the continents were in still their present positions, there is no possibility that the Mesosaurus would have the capability to swim across such a large body of ocean as the Atlantic because it was a coastal animal.

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Continental drift is still happening, and the continents will continue to move in the future. They are unlikely to return to the shape of Pangaea, but a map of the world 150 million years from now could look significantly different from today's.

Many times in Earth's past the continents have been dispersed across the globe, kept apart by spreading oceans. But eventually oceans begin to close, and far-flung lands are drawn inexorably together. They fuse in crunching collisions, welding themselves into single vast terrains: supercontinents.

Continents are short-lived unions. Stirred by hot currents below, these great continental collages are destined to break up and once again go their separate ways. It's the planet's version of a family Christmas. Except rather than return every year, Earth's Continent boom-and-bust cycles last 500 million years. Lost worlds litter our planet's past – the ancestral supercontinents of Ur, Kenorland, Nuna, Rhodinia, and Pannotia.

Earth's most recent grand union was 250 million years ago, when a continental mashup brought Pangea together. The giant landmass survived a mere 50 million years. It was undone by splits that tugged its American margins free from its African centre, broke apart the antipodean lands and then cleaved an Atlantic rift northward to release the conjoined bulk of Europe and Asia.

Neighbouring landmasses set off on different trajectories. India, originally snug with Madagascar, sped northwards to plough into Asia, thrusting ancient seafloor up into Himalayan peaks. The divorce of Australia and Antarctica left one to drift off into drier desert latitudes while the other languished in polar isolation. As these vast crustal rafts drifted across the globe, so landscapes and life adjusted. Each continent has been fashioned by that escape from Pangea.

But the continents are starting to come together again. North Africa is advancing into Mediterranean Europe, and over the next few tens of millions of years its shores will crumple into a chain of snowy peaks. Australia – the fastest-moving continent – is already beginning to sweep up New Guinea and the Indonesian archipelago en route to a messy pile-up with Asia. Pangea is slowly reassembling. Give the planet a couple of hundred millions years and we'll have another supercontinent. Geologists even have a name for it: Pangea Ultima.

A glance at a modern map of the world makes it easy to see that all the continents were once joined together. Perhaps the clearest example is the east coast of South America and the west coast of Africa. Their shapes suggest that they would fit closely if brought together.

In the beginning, more than 4.6-billion years ago, the world was a ball of burning gas, spinning through space. At first, super-heated gases were able to escape into outer space, but as the Earth cooled, they were held by gravity to form the early atmosphere.

Clouds began to develop as water vapour collected in the air … And then it began to pour with rain, causing the early oceans to rise up.It took hundreds of millions of years for the first land masses to emerge.

About 250-million years ago, long, long after the Earth had formed, all the continents of the time had joined together to form a super-continent called Pangaea.

This super-continent broke up about 200-million years ago to form two giant continents, Gondwana and Laurasia. Gondwana comprised what is now Africa, South America, Australia, Antarctica and India. The Indian sub-continent lay off the east coast of Africa, before it broke off and moved north rapidly.

It collided with Asia, creating one of the world’s greatest mountain ranges, which extends for more than 2,500 kilometres – the Himalayas. By now, our world had started to look like something we would recognise.

The amazing process of plate tectonics, in which the Earth’s land masses move slowly across the Earth’s crust, is still continuing. Far in the future, some scientists have predicted that the present continents will converge again, to form a new supercontinent.

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There are a number of theories about the causes of continental drift. One puts forward the idea that hot rocks rise through ocean ridges, cool down and then drag the plates downwards. Another theory suggests that the heat from inside the Earth creates movement in the mantle. The resulting currents then shift the plates around. The third idea is the simplest. At the ocean ridges, the plates are higher than elsewhere, resulting in the force of gravity pulling the plates downwards.

The Earth is in a constant state of change. Earth’s crust, called the lithosphere, consists of 15 to 20 moving tectonic plates. The plates can be thought of like pieces of a cracked shell that rest on the hot, molten rock of Earth’s mantle and fit snugly against one another. The heat from radioactive processes within the planet’s interior causes the plates to move, sometimes toward and sometimes away from each other. This movement is called plate motion, or tectonic shift.

Our planet looks very different from the way it did 250 million years ago, when there was only one continent, called Pangaea, and one ocean, called Panthalassa. As Earth’s mantle heated and cooled over many millennia, the outer crust broke up and commenced the plate motion that continues today.

The huge continent eventually broke apart, creating new and ever-changing land masses and oceans. Have you ever noticed how the east coast of South America looks like it would fit neatly into the west coast of Africa? That’s because it did, millions of years before tectonic shift separated the two great continents.

Earth’s land masses move toward and away from each other at an average rate of about 0.6 inch a year. That’s about the rate that human toenails grow! Some regions, such as coastal California, move quite fast in geological terms — almost two inches a year — relative to the more stable interior of the continental United States. At the “seams” where tectonic plates come in contact, the crustal rocks may grind violently against each other, causing earthquakes and volcano eruptions. The relatively fast movement of the tectonic plates under California explains the frequent earthquakes that occur there.

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The earth's crust is divided into enormous slabs of rock called tectonic plates. There are about 15 major plates, covering both the land masses and the ocean floor. They fit together like a huge jigsaw puzzle and, due to continental drift; their boundaries are either colliding with or pulling away from each other.

A tectonic plate (also called lithospheric plate) is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. Plate size can vary greatly, from a few hundred to thousands of kilometers across; the Pacific and Antarctic Plates are among the largest. Plate thickness also varies greatly, ranging from less than 15 km for young oceanic lithosphere to about 200 km or more for ancient continental lithosphere (for example, the interior parts of North and South America).

How do these massive slabs of solid rock float despite their tremendous weight? The answer lies in the composition of the rocks. Continental crust is composed of granitic rocks which are made up of relatively lightweight minerals such as quartz and feldspar. By contrast, oceanic crust is composed of basaltic rocks, which are much denser and heavier. The variations in plate thickness are nature's way of partly compensating for the imbalance in the weight and density of the two types of crust. Because continental rocks are much lighter, the crust under the continents is much thicker (as much as 100 km) whereas the crust under the oceans is generally only about 5 km thick. Like icebergs, only the tips of which are visible above water, continents have deep "roots" to support their elevations.

Most of the boundaries between individual plates cannot be seen, because they are hidden beneath the oceans. Yet oceanic plate boundaries can be mapped accurately from outer space by measurements from GEOSAT satellites. Earthquake and volcanic activity is concentrated near these boundaries. Tectonic plates probably developed very early in the Earth's 4.6-billion-year history, and they have been drifting about on the surface ever since-like slow-moving bumper cars repeatedly clustering together and then separating.

Like many features on the Earth's surface, plates change over time. Those composed partly or entirely of oceanic lithosphere can sink under another plate, usually a lighter, mostly continental plate, and eventually disappear completely. This process is happening now off the coast of Oregon and Washington. The small Juan de Fuca Plate, a remnant of the formerly much larger oceanic Farallon Plate, will someday be entirely consumed as it continues to sink beneath the North American Plate.

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It may not be apparent to us, but the major land masses of the Earth, the seven continents, are not in fixed positions. They are constantly shifted around by forces deep within the Earth. Around 250 million years ago, the land on Earth was made up of one huge continent known today as Pangaea. Over time, this broke up into the continents we know today. This continual movement of the land is known as continental drift.

Wegener thought all the continents were once joined together in an "Urkontinent" before breaking up and drifting to their current positions. But geologists soundly denounced Wegener's theory of continental drift after he published the details in a 1915 book called "The Origin of Continents and Oceans." Part of the opposition was because Wegener didn't have a good model to explain how the continents moved apart. 

Though most of Wegener's observations about fossils and rocks were correct, he was outlandishly wrong on a couple of key points. For instance, Wegener thought the continents might have plowed through the ocean crust like icebreakers smashing through ice. 

"There's an irony that the key objection to continent drift was that there is no mechanism, and plate tectonics was accepted without a mechanism," to move the continents, said Henry Frankel, an emeritus professor at the University of Missouri-Kansas City and author of the four volume "The Continental Drift Controversy".

Although Wegener's "continental drift" theory was discarded, it did introduce the idea of moving continents to geoscience. And decades later, scientists would confirm some of Wegener's ideas, such as the past existence of a supercontinent joining all the world's landmasses as one. Pangaea was a supercontinent that formed roughly 200 to 250 million years ago, according to the U.S. Geological Survey (USGS) and was responsible for the fossil and rock clues that led Wegener to his theory.

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