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A 2007 eruptive column at Mount Etna producing volcanic ash, pumice and lava bombs Santa Ana Volcano, El Salvador, a close up aerial view of the nested summit calderas and craters, along with the crater lake as seen from a United States Air Force C-130 Hercules flying above El Salvador. Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. One such hazard is that volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature; the melted particles then adhere to the turbine blades and alter their shape, disrupting the operation of the turbine. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's lower atmosphere (or troposphere); however, they also absorb heat radiated up from the Earth, thereby warming the upper atmosphere (or stratosphere). Etymology Plate tectonics Divergent plate boundaries "Hotspots"

Structure of the Earth Structure of the Earth Assumptions[edit] The force exerted by Earth's gravity can be used to calculate its mass, and by estimating the volume of the Earth, its average density can be calculated. Astronomers can also calculate Earth's mass from its orbit and effects on nearby planetary bodies. Structure[edit] Earth's radial density distribution according to the preliminary reference earth model (PREM).[1] Earth's gravity according to the preliminary reference earth model (PREM).[1] Comparison to approximations using constant and linear density for Earth's interior. The layering of Earth has been inferred indirectly using the time of travel of refracted and reflected seismic waves created by earthquakes. Core[edit] The average density of Earth is 5,515 kg/m3. The inner core was discovered in 1936 by Inge Lehmann and is generally believed to be composed primarily of iron and some nickel. Mantle[edit] World map showing the position of the Moho. Crust[edit] See also[edit] References[edit]

Four elements Segment of the macrocosm showing the elemental spheres of terra (earth), aqua (water), aer (air), and ignis (fire). Robert Fludd. 1617. Many philosophies and worldviews have a set of classical elements believed to reflect the simplest essential parts and principles of which anything can consist or upon which the constitution and fundamental powers of everything are based. Most frequently, classical elements refer to ancient concepts which some science writers compare to the modern states of matter, relating earth to the solid state, water to liquid, air to gaseous and fire to plasma.[1][2] Historians trace the evolution of modern theory pertaining to the chemical elements, as well as chemical compounds and mixtures of chemical substances to medieval, and Greek models. Many concepts once thought to be analogous, such as the Chinese Wu Xing, are now understood more figuratively. Ancient[edit] Cosmic elements in Babylonia[edit] Greece[edit] Medieval alchemy[edit] Egypt[edit] India[edit]

Element Element or elements may refer to: Arts[edit] Film[edit] Elements trilogy, three films by Indian film-maker Deepa Mehta Literature[edit] Lower Elements, a fictional underground city in the Artemis Fowl world, created by Eoin Colfer Magazines[edit] Element Magazine, a men's lifestyle and fashion digital magazine published in Singapore since 2013 Music[edit] Automobiles[edit] Honda Element, a car Business[edit] Element by Westin, a brand of Starwood Hotels and Resorts WorldwideElement Skateboards, a skateboard manufacturer Chemistry and science[edit] Chemical element, a pure substance consisting of one type of atomElectrical element, an abstract part of a circuitHeating element, a device that generates heat by electrical resistanceOrbital elements, the parameters required to uniquely identify a specific orbit of one body around anotherWeather, sometimes referred to as "the elements" Computing[edit] Law[edit] Mathematics[edit] Philosophy[edit] Places[edit] See also[edit]