Portal:Earth sciences/Selected articles

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1

Aerial view of the caldera of Mount Tambora.
Mount Tambora (or Tamboro) is an active stratovolcano on the island of Sumbawa, Indonesia. Sumbawa is flanked both to the north and south by oceanic crust, and Tambora was formed by the active subduction zone beneath it. This raised Mount Tambora as high as 4,300 m (14,100 ft), making it, in the 18th century, one of the tallest peaks in the Indonesian archipelago. After a large magma chamber inside the mountain filled over the course of several decades, volcanic activity reached a historic climax in the eruption of 10 April 1815.

With an estimated ejecta volume of 160 km3 (38 cu mi), Tambora's 1815 outburst was the largest volcanic eruption in recorded history. The explosion was heard on Sumatra island more than 2,000 km (1,200 mi) away. Heavy volcanic ash falls were observed as far away as Borneo, Sulawesi, Java and Maluku islands. Most deaths from the eruption were from starvation and disease, as the eruptive fallout ruined agricultural productivity in the local region. The death toll was at least 71,000 people, of whom 11,000–12,000 were killed directly by the eruption.

The eruption caused global climate anomalies that included the phenomenon known as "volcanic winter": 1816 became known as the "Year Without a Summer" because of the effect on North American and European weather. Crops failed and livestock died in much of the Northern Hemisphere, resulting in the worst famine of the 19th century.

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2

Map of Chicxulub Crater
The Chicxulub Crater is an ancient impact crater buried underneath the Yucatán Peninsula, with its center located near the town of Chicxulub, Yucatán, Mexico. The crater is over 180 kilometers (110 mi) in diameter, making the feature one of the largest confirmed impact structures in the world; the asteroid or comet whose impact formed the crater was at least 10 km (6 mi) in diameter. The crater was named for the nearby town, as well as for the literal Maya translation of the name: "tail of the devil." The crater was discovered by Glen Penfield, a geophysicist who had been working in the Yucatán while looking for oil during the late 1970s. The presence of tektites, shocked quartz and gravity anomalies, as well as the age of the rocks and isotope analysis, show that this impact structure dates from the late Cretaceous Period, roughly 65 million years ago. The impact associated with the crater is implicated in causing the extinction of the dinosaurs as suggested by the K–T boundary, although some critics disagree that the impact was the sole reason and also debate whether there was a single impact or whether the Chicxulub impactor was one of several that may have struck the Earth at around the same time. Recent evidence suggests that the impactor was a piece of a much larger asteroid which broke up in a collision more than 160 million years ago.
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3

Hurricane Isabel (2003) as seen from orbit during Expedition 7 of the International Space Station.
A tropical cyclone is a rapidly-rotating storm system characterized by a low-pressure center, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by names such as hurricane (/ˈhʌrɨkn/ or /ˈhʌrɨkən/, typhoon /tˈfn/, tropical storm, cyclonic storm, tropical depression, and simply cyclone.

Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy from the evaporation of water from the ocean surface, which ultimately recondenses into clouds and rain when moist air rises and cools to saturation. This energy source differs from that of mid-latitude cyclonic storms, such as nor'easters and European windstorms, which are fueled primarily by horizontal temperature contrasts. The strong rotating winds of a tropical cyclone are a result of the (partial) conservation of angular momentum imparted by the Earth's rotation as air flows inwards toward the axis of rotation. As a result, they rarely form within 5° of the equator. Tropical cyclones are typically between 100 and 4,000 km (62 and 2,485 mi) in diameter.

In addition to strong winds and rain, tropical cyclones are capable of generating high waves, damaging storm surge, and tornadoes. They typically weaken rapidly over land where they are cut off from their primary energy source. For this reason, coastal regions are particularly vulnerable to damage from a tropical cyclone as compared to inland regions. Heavy rains, however, can cause significant flooding inland, and storm surges can produce extensive coastal flooding up to 40 kilometres (25 mi) from the coastline. Though their effects on human populations are often devastating, tropical cyclones can relieve drought conditions. They also carry heat energy away from the tropics and transport it toward temperate latitudes, which may play an important role in modulating regional and global climate.

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4

Ground fire at Grant Village
The Yellowstone fires of 1988 together formed the largest wildfire in the recorded history of Yellowstone National Park. Starting as many smaller individual fires, the flames spread quickly out of control with increasing winds and drought and combined into one large conflagration, which burned for several months. It was finally extinguished by moist weather in the late fall. A total of 793,880 acres (3,213 km2), or roughly 36 percent of the park was affected by the wildfires. Thousands of firefighters fought the fires, assisted by dozens of helicopters and airplanes which were used for water and fire retardant drops. At the peak of the effort, over 9,000 firefighters were assigned to the park. With fires raging throughout the Greater Yellowstone Ecosystem and other areas in the western United States, the staffing levels of the National Park Service and other land management agencies were inadequate to the situation. Over 4,000 U.S. Military personnel were soon assisting in fire suppression efforts. The fire fighting effort cost $120 million. No firefighters died while fighting the fires in Yellowstone, though there were two fire-related deaths outside the park. The Yellowstone fires of 1988 were unprecedented in the history of the National Park Service, and many questioned existing fire management policies. Media accounts of mismanagement were often sensational and inaccurate, sometimes wrongly reporting that most of the park was being destroyed. While there were temporary declines in air quality during the fires, no adverse long-term health effects have been recorded in the ecosystem. Contrary to initial reports, few large mammals were killed by the fires, though there has been a reduction in the number of moose which has yet to rebound. Losses to structures were minimized by concentrating fire fighting efforts near major visitor areas, keeping property damage down to $3 million.
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5

Yellowstonefire.jpg
Yellowstone National Park became the world's first national park on March 1, 1872. Located mostly in the U.S. state of Wyoming, the park extends into Montana and Idaho. The park is known for its wildlife and geothermal features, especially Old Faithful Geyser, one of the most popular areas in the park. Native Americans have lived in the Yellowstone region for at least 11,000 years. The region was bypassed during the Lewis and Clark Expedition in the early 1800s. Aside from visits by mountain men during the early to mid-1800s, organized exploration did not begin until the late 1860s. The U.S. Army was commissioned to oversee the park just after its establishment. In 1917, administration of the park was transferred to the National Park Service, which had been created the previous year. Hundreds of structures have been built and are protected for their architectural and historical significance, and researchers have examined more than 1,000 archaeological sites. Yellowstone National Park spans an area of 8,987 square kilometres (3,470 sq mi), comprising lakes, canyons, rivers and mountain ranges. Yellowstone Lake is the largest high-altitude lake in North America and is centered over the Yellowstone Caldera, the largest supervolcano on the continent. The caldera is considered an active volcano; it has erupted with tremendous force several times in the last two million years. Half of the world's geothermal features are in Yellowstone, fueled by this ongoing volcanism. Lava flows and rocks from volcanic eruptions cover most of the land area of Yellowstone. The park is the centerpiece of the Greater Yellowstone Ecosystem, the largest remaining, nearly-intact ecosystem in the Earth's northern temperate zone.
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6

Plates tect2 en.svg
Plate tectonics is a theory of geology that has been developed to explain the observed evidence for large scale motions of the Earth's lithosphere. The theory encompassed and superseded the older theory of continental drift from the first half of the 20th century and the concept of seafloor spreading developed during the 1960s. The outermost part of the Earth's interior is made up of two layers: above is the lithosphere, comprising the crust and the rigid uppermost part of the mantle. Below the lithosphere lies the asthenosphere. Although solid, the asthenosphere has relatively low viscosity and shear strength and can flow like a liquid on geological time scales. The deeper mantle below the asthenosphere is more rigid again. This is, however, not due to cooler temperatures but due to high pressure. The lithosphere is broken up into what are called tectonic plates—in the case of Earth, there are seven major and many minor plates (see list below). The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries: convergent or collision boundaries, divergent or spreading boundaries, and transform boundaries. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 0.65 to 8.50 centimeters per year (the speed at which human nails grow).
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7

Amchitka Underground Test Site
Amchitka is a volcanic, tectonically unstable island in the Rat Islands group of the Aleutian Islands in southwest Alaska. It is about 68 kilometres (42 mi) long, and varies from 3 to 6 km (2–3.75 mi) in width. It has a maritime climate, with many storms, and mostly overcast skies. The island was populated for more than 2,500 years by the Aleut people, but has had no permanent population since 1832. It was included in the Alaska Purchase of 1867, and has since been part of the United States. During World War II, it was used as an airfield by US forces in the Battle of the Aleutian Islands. Amchitka was selected by the United States Atomic Energy Commission to be the site for underground detonations of nuclear weapons. Three such tests were carried out: Long Shot, an 80 kiloton blast in 1965; Milrow, a 1 megaton blast in 1969; and Cannikin in 1971 — at "under 5 megatons", the largest underground test ever conducted by the United States. The tests were highly controversial, with environmental groups fearing that the Cannikin explosion, in particular, would cause severe earthquakes and tsunamis. Amchitka is no longer used for nuclear testing, although it is monitored for the leakage of radioactive materials.
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8

India climatic zone
The climate of India comprises a wide range of weather conditions across a large geographic scale and varied topography, making generalisations difficult. Analysed according to the Köppen system, India hosts six major climatic subtypes, ranging from desert in the west, to alpine tundra and glaciers in the north, to humid tropical regions supporting rainforests in the southwest and the island territories. Many regions have starkly different microclimates. The nation has four seasons: winter (January and February), summer (March to May), a monsoon (rainy) season (June to September), and a post-monsoon period (October to December). India's unique geography and geology strongly influence its climate; this is particularly true of the Himalayas in the north and the Thar Desert in the northwest. The Himalayas act as a barrier to the frigid katabatic winds flowing down from Central Asia. Thus, North India is kept warm or only mildly cold during winter; in summer, the same phenomenon makes India relatively hot. Although the Tropic of Cancer—the boundary between the tropics and subtropics—passes through the middle of India, the whole country is considered to be tropical. As in much of the tropics, monsoonal and other weather conditions in India are unstable: major droughts, floods, cyclones and other natural disasters are sporadic, but have killed or displaced millions. India's long-term climatic stability is further threatened by global warming. Climatic diversity in India makes the analysis of these issues complex.
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9

Fossil of Kimberella quadrata
The Ediacara biota are ancient lifeforms of the Ediacaran Period, which represent the earliest known complex multicellular organisms. They appeared soon after the Earth thawed from the Cryogenian period's extensive glaciers, and largely disappeared soon before the rapid appearance of biodiversity known as the Cambrian explosion, which saw the first appearance in the fossil record of the basic patterns and body-plans that would go on to form the basis of modern animals. Little of the diversity of the Ediacara biota would be incorporated in this new scheme, with a distinct Cambrian biota arising and usurping the organisms that dominated the Ediacaran fossil record. The organisms of the Ediacaran Period first appeared around 580 million years ago and flourished until the cusp of the Cambrian 542 million years ago, when the characteristic communities of fossils vanished. While rare fossils that may represent survivors have been found as late as the Middle Cambrian (510 to 500 million years ago), the earlier fossil communities disappear from the record at the end of the Ediacaran, leaving only controversial fragments of once-thriving ecosystems, if anything. Multiple hypotheses exist to explain this disappearance, including preservation bias, a changing environment, the advent of predators, and competition from other lifeforms.
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10

Global Warming Map
Global warming is the increase in the average temperature of the Earth's near-surface air and oceans since the mid-twentieth century and its projected continuation. The average global air temperature near the Earth's surface increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the hundred years ending in 2005. The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-twentieth century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations" via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward. These basic conclusions have been endorsed by at least thirty scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with some findings of the IPCC, the overwhelming majority of scientists working on climate change agree with the IPCC's main conclusions.
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11

Yosemite National Park
Yosemite National Park (/jˈsɛmɨt/ yo-sem-it-ee) is a national park located largely in Mariposa and Tuolumne Counties, California, United States. The park covers an area of 761,266 acres or 1,189 square miles (3,081 km²) and reaches across the western slopes of the Sierra Nevada mountain chain. Yosemite is visited by over 3.5 million people each year, many of whom only spend time in the seven square miles (18 km²) of Yosemite Valley. Designated a World Heritage Site in 1984, Yosemite is internationally recognized for its spectacular granite cliffs, waterfalls, clear streams, Giant Sequoia groves, and biological diversity. Almost 95% of the park is designated wilderness. Although not the first designated national park, Yosemite was a focal point in the development of the national park idea, largely owing to the work of people like John Muir. Yosemite is one of the largest and least fragmented habitat blocks in the Sierra Nevada, and the park supports a diversity of plants and animals. The park has an elevation range from 2,000 to 13,114 feet (600 to 4,000 m) and contains five major vegetation zones: chaparral/oak woodland, lower montane, upper montane, subalpine, and alpine. Of California's 7,000 plant species, about 50% occur in the Sierra Nevada and more than 20% within Yosemite. There is suitable habitat or documentation for more than 160 rare plants in the park, with rare local geologic formations and unique soils characterizing the restricted ranges many of these plants occupy.
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12

Hurricane Gloria (1985)
Hurricane Gloria was a powerful Cape Verde-type hurricane that formed during the 1985 Atlantic hurricane season and prowled the Atlantic Ocean from September 16 to September 28. Gloria reached Category 4 on the Saffir-Simpson Hurricane Scale near the Bahamas, but weakened significantly by the time it made landfall on North Carolina's Outer Banks. From there, Gloria closely followed the Mid-Atlantic coastline and made a second landfall on Long Island, New York, and, after crossing the Long Island Sound, it made a third landfall in Connecticut. Overall, the storm caused extensive damage along the East Coast of the United States, amounting to $900 million ($1.6 billion in 2005 USD), and was responsible for 8 fatalities. The storm was the first significant system to impact the northeastern United States since Hurricane Agnes in 1972 and the first major storm to affect New York and Long Island directly since Hurricane Donna in 1960. It was the last storm to hit the northeast until Hurricane Bob in 1991.
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13

Geyser
A geyser is a hot spring characterized by intermittent discharge of water ejected turbulently and accomplished by a vapor phase. The name geyser comes from Geysir, the name of an erupting spring at Haukadalur, Iceland; that name, in turn, comes from the Icelandic verb gjósa, “to gush”. The formation of geysers requires a favourable hydrogeology which exists in only a few places on Earth, and so they are fairly rare phenomena. There must be a volcanic heat source. Generally all geyser field sites are located near active volcanic areas. The surface water works its way down to an average depth of around 2,134 metres (7,001 ft) where it meets up with the hot rocks. About a thousand exist worldwide, with about half of these in Yellowstone National Park, USA. A geyser's eruptive activity may change or cease due to ongoing mineral deposition within the geyser plumbing, exchange of functions with nearby hot springs, earthquake influences, and human intervention. Erupting fountains of liquefied nitrogen have been observed on Neptune's moon Triton, as have possible signs of carbon dioxide eruptions from Mars' south polar ice cap. These phenomena are also often referred to as geysers. Instead of being driven by geothermal energy, they seem to rely on solar heating aided by a kind of solid-state greenhouse effect. On Triton, the nitrogen may erupt to heights of 8 kilometres (5 mi).
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14

Geologic timescale
The geological history of Earth began 4.57 billion years ago when the planets of the Solar System were formed out of the solar nebula, a disk-shaped mass of dust and gas left over from the formation of the Sun. Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterwards, possibly as the result of a Mars-sized object with about 10% of the Earth's mass, known as Theia, impacting the Earth in a glancing blow. Some of this object's mass merged with the Earth and a portion was ejected into space, but enough material survived to form an orbiting moon. Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice delivered by comets, produced the oceans. As the surface continually reshaped itself, over hundreds of millions of years, continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750  Ma (million years ago) (ICS 2004), the earliest known supercontinent Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540  Ma (ICS 2004), then finally Pangaea, which broke apart 180 Ma (ICS 2004). The present pattern of ice ages began about 40 Ma (ICS 2004), then intensified during the Pleistocene about 3 Ma (ICS 2004). The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40,000–100,000 years. The last glacial period of the current ice age ended about 10,000 years ago.
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15

Terrestrial planet size comparisons
The geology of solar terrestrial planets mainly deals with the geological aspects of four planets of the Solar system namely, Mercury, Venus, Earth and Mars and one terrestrial dwarf planet, Ceres. Objects like Pluto are similar to terrestrial planets in the fact that they do have a solid surface, but are composed of more icy materials (see Ice dwarf). During the formation of the solar system, there were probably many more (planetesimals), but they have all merged with or been destroyed by the four remaining worlds in the solar nebula. Only one terrestrial planet, Earth, is known to have an active hydrosphere. Terrestrial planets are substantially different from gas giants, which might not have solid surfaces and are composed mostly of some combination of hydrogen, helium, and water existing in various physical states. These planets have a compact, rocky surfaces, with the last three also having an atmosphere. Their size, radius, and density are all similar.Terrestrial planets all have roughly the same structure- a central metallic core, mostly iron, with a surrounding silicate mantle. The Moon is similar, but lacks an iron core. Three of the four solar terrestrial planets (Venus, Earth and Mars) have substantial atmospheres; all have impact craters and tectonic surface features such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets which are closer to the Sun than Earth is (i.e. Mercury and Venus).
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16

Crater Lake
Crater Lake is a caldera lake located in the U.S. state of Oregon. It is the main feature of Crater Lake National Park and famous for its deep blue color and water clarity. The lake partly fills a nearly 4,000 feet (1,220 m) deep caldera that was formed around 5,677 (± 150) BC by the collapse of the volcano Mount Mazama. On June 12, 1853, John Wesley Hillman was reportedly the first European American to see what he named "Deep Blue Lake" in Oregon. The lake was renamed at least three times, as Blue Lake, Lake Majesty, and finally Crater Lake. Crater Lake is known for its famous piece of driftwood named the "Old Man of the Lake." It is a full-sized tree that has been bobbing vertically in the lake for more than a century. Due to the cold water of the lake, the tree has been rather well preserved. While having no indigenous fish population, the lake was stocked from 1888 to 1941 with a variety of fish. Several species have formed self sustaining populations. The commemorative Oregon State Quarter, which was released by the United States Mint in 2005, features an image of Crater Lake on its reverse side.
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17

Hurricane Lili (1984)
Hurricane Lili was one of only four Atlantic tropical cyclones on record to reach hurricane status in the month of December. The final of thirteen tropical storms in the 1984 Atlantic hurricane season, Lili developed as a subtropical cyclone which originated from a frontal trough to the south of Bermuda on December 12. It tracked southeastward, then northward, slowly attaining tropical characteristics and becoming a hurricane on December 20. Lili turned to the south and southwest, briefly threatening the northern Caribbean islands before weakening and dissipating near the coast of the Dominican Republic. Lili was the longest lasting tropical cyclone outside of the Atlantic hurricane season, as well as the strongest hurricane to form during the month of December. It briefly threatened to pass through the Lesser Antilles as a minimal hurricane, though upon passing through the area as a dissipating tropical depression Lili produced light rainfall and no reported damage.
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18

Turquoise
Turquoise is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminium, with the chemical formula CuAl6(PO4)4(OH)8·4H2O. It is rare and valuable in finer grades and has been prized as a gem and ornamental stone for thousands of years owing to its unique hue. In recent times turquoise, like most other opaque gems, has been devalued by the introduction of treatments, imitations, and synthetics onto the market, some difficult to detect even by experts. The substance has been known by many names, but the word turquoise was derived around 16th century from the French language either from the word for Turkish (Turquois) or dark-blue stone (pierre turquin). This may have arisen from a misconception: turquoise does not occur in Turkey but was traded at Turkish bazaars to Venetian merchants who brought it to Europe. The colour, however, has been employed extensively in the decorative tiles adorning Turkish places of worship and homes for hundreds of years, beginning with the Seljuks, and the association quite possibly has caused the name to take root.
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19

April 6-8, 2006 Tornado Outbreak
The April 6-8, 2006 Tornado Outbreak was a major tornado outbreak in the Central and parts of the Southern United States that began on April 6, 2006 in the Great Plains and continued until April 8 in South Carolina, with most of the activity on April 7. The hardest-hit area was Middle Tennessee where several strong tornadoes devastated entire neighborhoods and left nine people dead. The worst damage took place in Gallatin, Tennessee. Other communities north of Nashville were also hard hit. There were 73 tornadoes confirmed across 13 states, with the bulk of them coming on the afternoon and evening of April 7 across the South, particularly in Tennessee. In total, 13 deaths were reported as a result of the severe weather (12 of them in Tennessee) and over $1.5 billion in damage was reported,[1] of which over $630 million was in Middle Tennessee.[2] It was the third major outbreak of 2006, hot on the heels of a major outbreak on April 2. It was also considered to be the worst disaster event in Middle Tennessee since the Nashville tornado outbreak on April 16, 1998.
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20

Major volcanoes in Indonesia
The geography of Indonesia is dominated by volcanoes that are formed due to subduction zones between the Eurasian plate and the Indo-Australian plate. Some of the volcanoes are notable for their eruptions, for instance, Krakatau for its global effects in 1883, Lake Toba for its supervolcanic eruption estimated to have occurred 74,000 Before Present which was responsible for six years of volcanic winter, and Mount Tambora for the most violent eruption in recorded history in 1815. Volcanoes in Indonesia are a part of the Pacific Ring of Fire. The 150 entries in the list below are grouped into six geographical regions, four of which belong to the volcanoes of the Sunda Arc trench system. The remaining two groups are volcanoes of Halmahera, including its surrounding volcanic islands, and volcanoes of Sulawesi and the Sangihe Islands. The latter group is in one volcanic arc together with the Philippines volcanoes. The most active volcanoes are Kelut and Merapi on Java island which have been responsible for thousands of deaths in the region. Since AD 1000, Kelut has erupted more than 30 times, of which the largest eruption was at scale 5 on the Volcanic Explosivity Index (VEI), while Merapi has erupted more than 80 times. The International Association of Volcanology and Chemistry of the Earth's Interior has named Merapi as a Decade Volcano since 1995 because of its high volcanic activity.
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21

"The Blue Marble" photograph of Earth,taken from Apollo 17
Earth is the third planet from the Sun. It is the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the world or the Blue Planet.

Earth formed approximately 4.54 billion years ago, and life appeared on its surface within its first billion years. Earth's biosphere then significantly altered the atmospheric and other basic physical conditions, which enabled the proliferation of organisms as well as the formation of the ozone layer, which together with Earth's magnetic field blocked harmful solar radiation, and permitted formerly ocean-confined life to move safely to land. The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist. Estimates on how much longer the planet will be able to continue to support life range from 500 million years (myr), to as long as 2.3 billion years (byr).

Earth's lithosphere is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with ice that is the solid ice of the Antarctic ice sheet and the sea ice that is the polar ice packs. The planet's interior remains active, with a solid iron inner core, a liquid outer core that generates the magnetic field, and a thick layer of relatively solid mantle.

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22

Darwin, aged 45 in 1854
Charles Robert Darwin, FRS (12 February 1809 – 19 April 1882) was an English naturalist and geologist, best known for his contributions to evolutionary theory. He established that all species of life have descended over time from common ancestors, and in a joint publication with Alfred Russel Wallace introduced his scientific theory that this branching pattern of evolution resulted from a process that he called natural selection, in which the struggle for existence has a similar effect to the artificial selection involved in selective breeding.

Darwin published his theory of evolution with compelling evidence in his 1859 book On the Origin of Species, overcoming scientific rejection of earlier concepts of transmutation of species. By the 1870s the scientific community and much of the general public had accepted evolution as a fact. However, many favoured competing explanations and it was not until the emergence of the modern evolutionary synthesis from the 1930s to the 1950s that a broad consensus developed in which natural selection was the basic mechanism of evolution. In modified form, Darwin's scientific discovery is the unifying theory of the life sciences, explaining the diversity of life.

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23

Artist's rendering of a bolide impact
The Cretaceous–Paleogene (K–Pg) extinction event, formerly known as the Cretaceous–Tertiary (K–T) extinction, was a mass extinction of some three-quarters of plant and animal species on Earth—including all non-avian dinosaurs—that occurred over a geologically short period of time 66 million years (Ma) ago. It marked the end of the Cretaceous period and with it, the entire Mesozoic Era, opening the Cenozoic Era which continues today.

In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows high levels of the metal iridium, which is rare in the Earth's crust but abundant in asteroids.

It is generally believed that the K–Pg extinction was triggered by a massive comet/asteroid impact and its catastrophic effects on the global environment, including a lingering impact winter that made it impossible for plants and plankton to carry out photosynthesis. The impact hypothesis was bolstered by the discovery of the 180-kilometre-wide (112 mi) Chicxulub crater in the Gulf of Mexico in the late 1970s, which provided conclusive evidence that the K–Pg boundary clay represented debris from an asteroid impact. The fact that the extinctions occurred at the same time as the impact provides strong situational evidence that the K–Pg extinction was caused by the asteroid. However, some scientists maintain the extinction was caused or exacerbated by other factors, such as volcanic eruptions, climate change, and/or sea level change.

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24

Fossil of complete Archaeopteryx, including indentations of feathers on wings and tail
Archaeopteryx (/ˌɑrkˈɒptərɨks/ AR-kee-OP-tər-iks), sometimes referred to by its German name Urvogel ("original bird" or "first bird"), is a genus of early bird that is transitional between feathered dinosaurs and modern birds. The name derives from the ancient Greek ἀρχαῖος (archaīos) meaning "ancient", and πτέρυξ (ptéryx), meaning "feather" or "wing". Since the late nineteenth century, it had been generally accepted by palaeontologists, and celebrated in lay reference works, as being the oldest known bird (member of the group Avialae). However, older potential avialans have since been identified, including Anchiornis, Xiaotingia, and Aurornis.

Archaeopteryx lived in the Late Jurassic period around 150 million years ago, in what is now southern Germany during a time when Europe was an archipelago of islands in a shallow warm tropical sea, much closer to the equator than it is now. Similar in shape to a European Magpie, with the largest individuals possibly attaining the size of a raven, Archaeopteryx could grow to about 0.5 m (1 ft 8 in) in length. Despite its small size, broad wings, and inferred ability to fly or glide, Archaeopteryx has more in common with other small Mesozoic dinosaurs than it does with modern birds. In particular, it shares the following features with the deinonychosaurs (dromaeosaurs and troodontids): jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), feathers (which also suggest homeothermy), and various skeletal features.

These features make Archaeopteryx a clear candidate for a transitional fossil between dinosaurs and birds. Thus, Archaeopteryx plays an important role, not only in the study of the origin of birds, but in the study of dinosaurs. It was named from a feather in 1861. That same year, the first complete specimen of Archaeopteryx was announced. Over the years, ten more fossils of Archaeopteryx have surfaced. Despite variation among these fossils, most experts regard all the remains that have been discovered as belonging to a single species, although this is still debated.

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25

Mary Anning with her dog, Tray, painted before 1833
Mary Anning (21 May 1799 – 9 March 1847) was a British fossil collector, dealer, and paleontologist who became known around the world for a number of important finds she made in the Jurassic marine fossil beds at Lyme Regis in Dorset, where she lived. Her work contributed to fundamental changes that occurred during her lifetime in scientific thinking about prehistoric life and the history of the Earth.

Anning searched for fossils in the area's Blue Lias cliffs, particularly during the winter months when landslides exposed new fossils that had to be collected quickly before they were lost to the sea. It was dangerous work, and she nearly lost her life in 1833 during a landslide that killed her dog, Tray. Her discoveries included the first ichthyosaur skeleton to be correctly identified, which she and her brother Joseph found when she was just twelve years old; the first two plesiosaur skeletons ever found; the first pterosaur skeleton located outside Germany; and some important fish fossils. Her observations played a key role in the discovery that coprolites, known as bezoar stones at the time, were fossilised faeces. She also discovered that belemnite fossils contained fossilised ink sacs like those of modern cephalopods. When geologist Henry De la Beche painted Duria Antiquior, the first widely circulated pictorial representation of a scene from prehistoric life derived from fossil reconstructions, he based it largely on fossils Anning had found, and sold prints of it for her benefit.

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  1. ^ Cite error: The named reference billiondollar was invoked but never defined (see the help page).
  2. ^ "Storms' losses linger". Gallatin News-Examiner. 2006-10-07. Retrieved 2006-11-11.