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Earth’s core is the very hot, very dense center of our planet. The ball-shaped core lies beneath the cool, brittle crust and the mostly-solid mantle. Click through the gallery to learn more about the core and the rest of Earth’s interior.Variations in rotation, conductivity, and heat impact the magnetic field of a geodynamo. Mars, for instance, has a totally solid core and a weak magnetic field. Venus has a liquid core, but rotates too slowly to churn significant convection currents. It, too, has a weak magnetic field. Jupiter, on the other hand, has a liquid core that is constantly swirling due to the planet’s rapid rotation.Earth is the “Goldilocks” geodynamo. It rotates steadily, at a brisk 1,675 kilometers per hour (1,040 miles per hour) at the Equator. Coriolis forces, an artifact of Earth’s rotation, cause convection currents to be spiral. The liquid iron in the outer core is an excellent electrical conductor, and creates the electrical currents that drive the magnetic field. The energy supply that drives convection in the outer core is provided as droplets of liquid iron freeze onto the solid inner core. Solidification releases heat energy. This heat, in turn, makes the remaining liquid iron more buoyant. Warmer liquids spiral upward, while cooler solids spiral downward under intense pressure: convection. Earth’s Magnetic Field: Earth’s magnetic field is crucial to life on our planet. It protects the planet from the charged particles of the solar wind. Without the shield of the magnetic field, the solar wind would strip Earth’s atmosphere of the ozone layer that protects life from harmful ultraviolet radiation. Although Earth’s magnetic field is generally stable, it fluctuates constantly. As the liquid outer core moves, for instance, it can change the location of the magnetic North and South Poles. The magnetic North Pole moves up to 64 kilometers (40 miles) every year. Fluctuations in the core can cause Earth’s magnetic field to change even more dramatically. Geomagnetic pole reversals, for instance, happen about every 200,000 to 300,000 years. Geomagnetic pole reversals are just what they sound like: a change in the planet’s magnetic poles, so that the magnetic North and South Poles are reversed. These “pole flips” are not catastrophic—scientists have noted no real changes in plant or animal life, glacial activity, or volcanic eruptions during previous geomagnetic pole reversals.

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In geology, the crust is the outermost solid shell of a rocky planet, dwarf planet, or natural satellite. It is usually distinguished from the underlying mantle by its chemical makeup; however, in the case of icy satellites, it may be distinguished based on its phase (solid crust vs. liquid mantle).The crusts of Earth, Moon, Mercury, Venus, Mars, Io, and other planetary bodies formed via igneous processes, and were later modified by erosion, impact cratering, volcanism, and sedimentation. Most terrestrial planets have fairly uniform crusts. Earth, however, has two distinct types: continental crust and oceanic crust. These two types have different chemical compositions and physical properties, and were formed by different geological processes. Primary crust / primordial crustThis is a planet’s “original” crust. It forms from solidification of a magma ocean. Toward the end of planetary accretion, the terrestrial planets likely had surfaces that were magma oceans. As these cooled, they solidified into crust. This crust was likely destroyed by large impacts and re-formed many times as the Era of Heavy Bombardment drew to a close.The nature of primary crust is still debated: its chemical, mineralogic, and physical properties are unknown, as are the igneous mechanisms that formed them. This is because it is difficult to study: none of Earth’s primary crust has survived to today.[4] Earth’s high rates of erosion and crustal recycling from plate tectonics has destroyed all rocks older than about 4 billion years, including whatever primary crust Earth once had.However, geologists can glean information about primary crust by studying it on other terrestrial planets. Mercury’s highlands might represent primary crust, though this is debated. The anorthosite highlands of the Moon are primary crust, formed as plagioclase crystallized out of the Moon’s initial magma ocean and floated to the top; however, it is unlikely that Earth followed a similar pattern, as the Moon was a water-less system and Earth had water. The Martian meteorite ALH84001 might represent primary crust of Mars; however, again, this is debated. Like Earth, Venus lacks primary crust, as the entire planet has been repeatedly resurfaced and modified.Secondary crust: Secondary crust is formed by partial melting of silicate materials in the mantle, and so is usually basaltic in composition.[This is the most common type of crust in the Solar System. Most of the surfaces of Mercury, Venus, Earth, and Mars comprise secondary crust, as do the lunar maria. On Earth, we see secondary crust forming primarily at mid-ocean spreading centers, where the adiabatic rise of mantle causes partial melting. Tertiary crust: Tertiary crust is more chemically-modified than either primary or secondary. It can form in several ways: Igneous processes: partial-melting of secondary crust, coupled with differentiation or dehydration. Erosion and sedimentation: sediments derived from primary, secondary, or tertiary crust

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Abraham Gottlob Werner, (born Sept. 25, 1750, Wehrau, Saxony—died June 30, 1817, Freiberg), German geologist who founded the Neptunist school, which proclaimed the aqueous origin of all rocks, in opposition to the Plutonists, or Vulcanists, who argued that granite and many other rocks were of igneous origin. Werner rejected uniformitarianism (belief that geological evolution has been a uniform and continuous process). A member of an old iron-mining family, Werner worked with his father for five years in the ironworks at Wehrau and Lorzendorf. In 1775 he was appointed inspector and teacher in the Freiburg School of Mining. During his 40-year tenure, the school grew from a local academy into a world-renowned centre of scientific learning. Werner was a brilliant lecturer and a man of great charm, and his genius attracted students who, inspired by him, became the foremost geologists of Europe. A distinguishing feature of Werner’s teaching was the care with which he taught the study of rocks and minerals and the orderly succession of geological formations, a subject that he called geognosy. Influenced by the works of Johann Gottlob Lehmann and Georg Christian Füchsel, Werner demonstrated that the rocks of the Earth are deposited in a definite order. Although he had never travelled, he assumed that the sequence of the rocks he observed in Saxony was the same for the rest of the world. He believed that the Earth was once completely covered by the oceans and that, with time, all the minerals were precipitated out of the water into distinct layers, a theory known as Neptunism. Because this theory did not allow for a molten core, he proposed that volcanoes were recent phenomena caused by the spontaneous combustion of underground coal beds. He asserted that basalt and similar rocks were accumulations of the ancient ocean, whereas other geologists recognized them as igneous minerals. It was primarily disagreement on this point that formed one of the great geological controversies. Werner wrote only 26 scientific works, most of them short contributions to journals. His aversion to writing grew, and finally he adopted the practice of storing his mail unopened. Elected a foreign member of the Académie des Sciences in 1812, he learned of the honour much later, when he happened to read about it in a journal. In spite of his failure to produce extensive geological writings, Werner’s theories were faithfully adopted and widely spread by his loyal students. Even though many of them eventually discarded his Neptunist theories, they would not publicly renounce them while Werner still lived.

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An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth’s lithosphere that creates seismic waves. 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 is the frequency, type, and size of earthquakes experienced over a period of time. The word tremor is also used for non-earthquake seismic rumbling.At the Earth’s surface, earthquakes manifest themselves by shaking and displacing or disrupting the ground. When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides and occasionally, volcanic activity. In its most general sense, the word earthquake is used to describe any seismic event—whether natural or caused by humans—that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests. An earthquake’s point of initial rupture is called its focus or hypocenter. The epicenter is the point at ground level directly above the hypocenter.

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The methods of measuring the rates of geological processes vary considerably. However, this book is not a methodological guide and, therefore, only the most important facts about these methods are mentioned. Expressing the rate of a process represents a considerable problem. Is it at all possible to find a unit which would be suitable for all geological processes, ranging from crustal movements to the motions of groundwaters? Efforts to introduce such a unit have and partly are still being made in the world literature. Fischer (1969)suggested 1 Bubnoff (B) which he defined as the rate of 1 m per Ma (i.e. 1 mm per ka or 1 ~tm [micrometre] per a). The unit was named after the well-known German geologist who advocated the measuring the absolute rates of geological processes. Other authors, such as Ericson (1969) suggested introducing megabubnoffs, microbubnoffs, etc. However, Bubnoffs did not become particulartly popular and we do not intend to propagate them either. Some authors, like Berg and Gaugi (1971), were against the introduction of the units in no uncertain terms and were sorry for Professor Bubnoff that his name had been misused for such dubious purposes. In the present state of affairs, an attempt to introduce a unit for the rate of all geological processes would encounter grave difficulties. We, therefore, consider it best to express the rate of various processes in terms of the units established in the literature: the rate of sedimentation is most frequently given in cm per 1 000 a (smaller units), or in m per Ma (larger units); the rate of erosion in centimetres per 100 a; the rate of weathering also in centimetres per 100 a; the rate of crustal movements in millimetres per year; the rate of sea level changes also in millimetres per year; the rate of crystallization in centimetres per second; the rate of motion of groundwater in metres per year; the rate of slow mass movements in centimetres per year. In many cases, the rate of processes can be expressed in alternative ways, for example in terms of area or volume ( e.g., in measuring the rate of sedimentation or the rate of lava production).

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Earth’s lithosphere is divided into several rigid tectonic plates that migrate across the surface over many millions of years. About 71% of Earth’s surface is covered with water, mostly by oceans. The remaining 29% is land consisting of continents and islands that together contain many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earth’s polar regions are covered in ice, including the Antarctic ice sheet and the sea ice of the Arctic ice pack. Earth’s interior remains active with a solid iron inner core, a liquid outer core that generates the Earth’s magnetic field and a convecting mantle that drives plate tectonics.Within the first billion years of Earth’s history, life appeared in the oceans and began to affect the Earth’s atmosphere and surface, leading to the proliferation of anaerobic and, later, aerobic organisms. Some geological evidence indicates that life may have arisen as early as 4.1 billion years ago. Since then, the combination of Earth’s distance from the Sun, physical properties and geological history have allowed life to evolve and thrive. Early edition, published online before print. In the history of life on Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that ever lived on Earth are extinct. Estimates of the number of species on Earth today vary widely; most species have not been described. Over 7.7 billion humans live on Earth and depend on its biosphere and natural resources for their survival. Humans have developed diverse societies and cultures; politically, the world has around 200 sovereign states.

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The rock cycle is a basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and change as they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle. When rocks are pushed deep under the Earth’s surface, they may melt into magma. If the conditions no longer exist for the magma to stay in its liquid state, it cools and solidifies into an igneous rock. A rock that cools within the Earth is called intrusive or plutonic and cools very slowly, producing a coarse-grained texture such as the rock granite. As a result of volcanic activity, magma (which is called lava when it reaches Earth’s surface) may cool very rapidly while being on the Earth’s surface exposed to the atmosphere and are called extrusive or volcanic rocks. These rocks are fine-grained and sometimes cool so rapidly that no crystals can form and result in a natural glass, such as obsidian, however the most common fine-grained rock would be known as basalt. Any of the three main types of rocks (igneous, sedimentary, and metamorphic rocks) can melt into magma and cool into igneous rocks.

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Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός “pertaining to building”) is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth’s lithosphere, since tectonic processes began on Earth between 3.3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.The lithosphere, which is the rigid outermost shell of a planet (the crust and upper mantle), is broken into tectonic plates. The Earth’s lithosphere is composed of seven or eight major plates (depending on how they are defined) and many minor plates.Where the 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). The relative movement of the plates typically ranges from zero to 100 mm annually.Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction, or one plate moving under another, carries the lower one down into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading. In this way, the total surface of the lithosphere remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories, since disproven, proposed gradual shrinking (contraction) or gradual expansion of the globe.Tectonic plates are able to move because the Earth’s lithosphere has greater mechanical strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection; that is, the slow creeping motion of Earth’s solid mantle. Plate movement is thought to be driven by a combination of the motion of the seafloor away from spreading ridges due to variations in topography (the ridge is a topographic high) and density changes in the crust (density increases as newly formed crust cools and moves away from the ridge). At subduction zones the relatively cold, dense crust is “pulled” or sinks down into the mantle over the downward convecting limb of a mantle cell.Another explanation lies in the different forces generated by tidal forces of the Sun and Moon. The relative importance of each of these factors and their relationship to each other is unclear, and still the subject of much debate.

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The geologic time scale (GTS) is a system of chronological dating that relates geological strata (stratigraphy) to time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that have occurred during Earth’s history. The table of geologic time spans, presented here, agree with the nomenclature, dates and standard color codes set forth by the International Commission on Stratigraphy (ICS). The primary defined divisions of time are eons, in sequence the Hadean, the Archean, the Proterozoic and the Phanerozoic. The first three of these can be referred to collectively as the Precambrian supereon. Eons are divided into eras, which are in turn divided into periods, epochs and ages.The following four timelines show the geologic time scale. The first shows the entire time from the formation of the Earth to the present, but this gives little space for the most recent eon. Therefore, the second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, and the most recent period is expanded in the fourth timeline.

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The internal structure of the Earth is layered in spherical shells: an outer silicate solid crust, a highly viscous asthenosphere and mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. Scientific understanding of the internal structure of the Earth is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanoes or volcanic activity, analysis of the seismic waves that pass through the Earth, measurements of the gravitational and magnetic fields of the Earth, and experiments with crystalline solids at pressures and temperatures characteristic of the Earth’s deep interior.

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