Category Science

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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Most of the earth is made of various solid rocks. The 2000km- (1240-mile-) thick outer core is the only part of the Earth that exists in an entirely liquid form. Iron, nickel and other materials are liquefied by the extremely high temperatures. Molten rock is found in parts of the mantle, some of which comes to the surface as lava.

The Earth’s interior is composed of four layers, three solid and one liquid—not magma but molten metal, nearly as hot as the surface of the sun.

The deepest layer is a solid iron ball, about 1,500 miles (2,400 kilometers) in diameter. Although this inner core is white hot, the pressure is so high the iron cannotmelt.

The iron isn’t pure—scientists believe it contains sulfur and nickel, plus smaller amounts of other elements. Estimates of its temperature vary, but it is probably somewhere between 9,000 and 13,000 degrees Fahrenheit (5,000 and 7,000 degrees Celsius).

Above the inner core is the outer core, a shell of liquid iron. This layer is cooler but still very hot, perhaps 7,200 to 9,000 degrees Fahrenheit (4,000 to 5,000 degrees Celsius). It too is composed mostly of iron, plus substantial amounts of sulfur and nickel. It creates the Earth’s magnetic field and is about 1,400 miles (2,300 kilometers) thick.

The next layer is the mantle. Many people think of this as lava, but it’s actually rock. The rock is so hot, however, that it flows under pressure, like road tar. This creates very slow-moving currents as hot rock rises from the depths and cooler rock descends.

The mantle is about 1,800 miles (2,900 kilometers) thick and appears to be divided into two layers: the upper mantle and the lower mantle. The boundary between the two lies about 465 miles (750 kilometers) beneath the Earth’s surface.

The crust is the outermost layer of the Earth. It is the familiar landscape on which we live: rocks, soil, and seabed. It ranges from about five miles (eight kilometers) thick beneath the oceans to an average of 25 miles (40 kilometers) thick beneath the continents.

Currents within the mantle have broken the crust into blocks, called plates, which slowly move around, colliding to build mountains or rifting apart to form new seafloor.

Continents are composed of relatively light blocks that float high on the mantle, like gigantic, slow-moving icebergs. Seafloor is made of a denser rock called basalt, which presses deeper into the mantle, producing basins that can fill with water.

Except in the crust, the interior of the Earth cannot be studied by drilling holes to take samples. Instead, scientists map the interior by watching how seismic waves from earthquakes are bent, reflected, sped up, or delayed by the various layers.

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     The magnetic field of the Earth at any given time is preserved in the magnetic minerals within rocks that solidified during that period. Geologists are thus able to study the magnetic field of rocks thousands of years old, such as those used to build the pyramids at Giza, Egypt.

     A magnetic field is the area around a material in which its magnetic forces can be detected. Those forces stem from the activity of tiny, negatively charged particles called electrons, which are within all atoms. A material’s magnetism is determined by the way its electrons move around the outside of its atoms’ nuclei — particularly those electrons that aren’t paired with other electrons in certain ways. If a large number of unpaired electrons rotate in the same direction (imagine a large number of tops spinning on a table or other flat surface), then an object’s magnetic field can be strong. If all of the unpaired electrons spin in random directions, the object’s magnetic field is either very weak or missing.

     Some materials, such as lodestones, create a persistent magnetic field. Others with unpaired electrons, such as iron, can become magnetized when they’re placed within a magnetic field and their atoms rotate and align.

     Scientists don’t know how some types of rocks, including lodestones, become so strongly magnetized. But new lab tests show how some other rocks can become naturally magnetized.

     Charles Aubourg is a geologist at the University of Pau and the Adour Countries in France. He and his colleagues heated samples of a type of sedimentary rock to as much as 130 degrees Celsius (about 266 degrees Fahrenheit). Sedimentary rock is made from material eroded from other rocks. The eroded materials transform into stone when exposed to high pressure deep within Earth for a lengthy period of time, sometimes millions of years.

     Aubourg’s team got its rock samples from northern France, but similar rocks can be found worldwide. Each sample contained large amounts of clay and silt (both of which are made of tiny particles eroded from other rocks). But importantly, the rocks also contained a small amount of an iron-bearing mineral called pyrite.

     First, the team used a strong magnetic field to erase any magnetism naturally trapped in the sample. Then the researchers heated the rock inside a strong magnetic field according to a specific recipe: 25 days at 50 degrees Celsius, then 25 days at 70 degrees, 25 days at 80 degrees, 10 days at 120 degrees, and a final 10 days at 130 degrees. This temperature range is the same as that of rocks located between 2 kilometers and 4 kilometers deep in Earth’s crust, explains Aubourg.

     The rocks’ magnetic field rose during each stage of heating. It increased most quickly during the earliest days of each step. The growing magnetism of the samples suggests that the heat triggered reactions that caused some of the pyrite to chemically transform into magnetic minerals.

     Analyses conducted after the heating suggest that the magnetic minerals were very tiny grains of magnetite. These grains were so small, less than 20 nanometers across, that it would take more than 1,000 of them side by side to stretch across the width of a single human hair. The researchers reported their results online August 10 in the scientific journal Geochemistry, Geophysics, Geosystems.

     Because the grains of magnetite were so small, looking for one “would be like trying to find a needle in a haystack,” says Douglas Elmore. He is a sedimentary geologist at the University of Oklahoma in Norman. Nevertheless, he notes, the evidence is convincing that the heating experiments created small grains of magnetite, not other types of magnetic minerals.

     Studies that investigated rocks in their natural environment have hinted that rocks buried in shallow layers of Earth’s crust and heated there naturally can become magnetized, says Elmore. The new lab tests provide even stronger evidence that such magnetization occurs naturally, he adds.

     Studying the magnetic field trapped in ancient rocks helps scientists better understand Earth’s history, including how the planet’s magnetic field has changed through time.

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    The earth is rather like an enormous magnet. Otherwise known as the magnetosphere, the Earth’s magnetic field stretches out into space, helping to protect the Earth from the Sun’s radiation. The magnetic poles are close to the geographic North and South Poles.

    A magnetic field is a vector field that describes the magnetic influence of electric charges in relative motion and magnetized materials. The effects of magnetic fields are commonly seen in permanent magnets, which pull on magnetic materials (such as iron) and attract or repel other magnets. Magnetic fields surround and are created by magnetized material and by moving electric charges (electric currents) such as those used in electromagnets. They exert forces on nearby moving electrical charges and torques on nearby magnets. In addition, a magnetic field that varies with location exerts a force on magnetic materials. Both the strength and direction of a magnetic field vary with location. As such, it is described mathematically as a vector field.

    In electromagnetics, the term “magnetic field” is used for two distinct but closely related fields denoted by the symbols B and H. In the International System of Units, H, magnetic field strength, is measured in the SI base units of ampere per meter. B, magnetic flux density, is measured in tesla (in SI base units: kilogram per second2 per ampere), which is equivalent to newton per meter per ampere. H and B differ in how they account for magnetization. In a vacuum, B and H are the same aside from units; but in a magnetized material, B/{\displaystyle \mu _{0}}  and H differ by the magnetization M of the material at that point in the material.

    Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. Magnetic fields and electric fields are interrelated, and are both components of the electromagnetic force, one of the four fundamental forces of nature.

    Magnetic fields are widely used throughout modern technology, particularly in electrical engineering and electromechanics. Rotating magnetic fields are used in both electric motors and generators. The interaction of magnetic fields in electric devices such as transformers is studied in the discipline of magnetic circuits. Magnetic forces give information about the charge carriers in a material through the Hall Effect. The Earth produces its own magnetic field, which shields the Earth’s ozone layer from the solar wind and is important in navigation using a compass.

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The molten iron that partly makes tip the Earth’s core continually flows around. As this happens, it generates powerful electric currents that create the Earth’s magnetic field. This is similar to the way magnetic currents are generated by an electric motor.

Earth’s magnetic field, also known as the geomagnetic field, is the magnetic field that extends from the Earth’s interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic field is generated by electric currents due to the motion of convection currents of molten iron in the Earth’s outer core: these convection currents are caused by heat escaping from the core, a natural process called a geodynamo. The magnitude of the Earth’s magnetic field at its surface ranges from 25 to 65 microteslas (0.25 to 0.65 gauss). As an approximation, it is represented by a field of a magnetic dipole currently tilted at an angle of about 11 degrees with respect to Earth’s rotational axis, as if there were an enormous bar magnet placed at that angle through the center of the Earth. The North geomagnetic pole, which was in 2015 located on Ellesmere Island, Nunavut, Canada, in the northern hemisphere, is actually the south pole of the Earth’s magnetic field, and conversely.

While the North and South magnetic poles are usually located near the geographic poles, they slowly and continuously move over geological time scales, but sufficiently slowly for ordinary compasses to remain useful for navigation. However, at irregular intervals averaging several hundred thousand years, the Earth’s field reverses and the North and South Magnetic Poles respectively, abruptly switch places. These reversals of the geomagnetic poles leave a record in rocks that are of value to paleomagnetists in calculating geomagnetic fields in the past. Such information in turn is helpful in studying the motions of continents and ocean floors in the process of plate tectonics.

The magnetosphere is the region above the ionosphere that is defined by the extent of the Earth’s magnetic field in space. It extends several tens of thousands of kilometers into space, protecting the Earth from the charged particles of the solar wind and cosmic rays that would otherwise strip away the upper atmosphere, including the ozone layer that protects the Earth from harmful ultraviolet radiation.

The Earth’s magnetic field serves to deflect most of the solar wind, whose charged particles would otherwise strip away the ozone layer that protects the Earth from harmful ultraviolet radiation. One stripping mechanism is for gas to be caught in bubbles of magnetic field, which are ripped off by solar winds. Calculations of the loss of carbon dioxide from the atmosphere of Mars, resulting from scavenging of ions by the solar wind, indicate that the dissipation of the magnetic field of Mars caused a near total loss of its atmosphere.

The study of the past magnetic field of the Earth is known as paleomagnetism. The polarity of the Earth’s magnetic field is recorded in igneous rocks, and reversals of the field are thus detectable as “stripes” centered on mid-ocean ridges where the sea floor is spreading, while the stability of the geomagnetic poles between reversals has allowed paleomagnetists to track the past motion of continents. Reversals also provide the basis for magnetostratigraphy, a way of dating rocks and sediments. The field also magnetizes the crust, and magnetic anomalies can be used to search for deposits of metal ores.

Humans have used compasses for direction finding since the 11th century A.D. and for navigation since the 12th century. Although the magnetic declination does shift with time, this wandering is slow enough that a simple compass can remain useful for navigation. Using magnetoreception various other organisms, ranging from some types of bacteria to pigeons, use the Earth’s magnetic field for orientation and navigation.

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