A rocky shore is an intertidal area of seacoasts where solid rock predominates. Rocky shores are biologically rich environments, are a useful "natural laboratory" for studying intertidal ecology and other biological processes. Due to their high accessibility, they have been well studied for a long time and their species are well known. There are a large number of factors. Temperate coastal waters are mixed by waves and convection, maintaining adequate availability of nutrients; the sea brings plankton and broken organic matter in with each tide. The high availability of light and nutrient levels means that primary productivity of seaweeds and algae can be high. Human actions can benefit rocky shores due to nutrient runoff. Despite these favourable factors, there are a number of challenges to marine organisms associated with the rocky shore ecosystem; the distribution of benthic species is limited by salinity, wave exposure, temperature and general stress. The constant threat of desiccation during exposure at low tide can result in dehydration.
Hence, many species have developed adaptations to prevent this drying out, such as the production of mucous layers and shells. Many species use holdfasts to provide stability against strong wave actions. There are a variety of other challenges such as temperature fluctuations due to tidal flow, changes in salinity and various ranges of illumination. Other threats include predation from birds and other marine organisms, as well as the effects of pollution; the Ballantine Scale is a biologically defined scale for measuring the degree of exposure level of wave action on a rocky shore. Devised in 1961 by W. J. Ballantine at the zoology department of Queen Mary College, London, U. K. the scale is based on the observation that where shoreline species are concerned "Different species growing on rocky shores require different degrees of protection from certain aspects of the physical environment, of which wave action is the most important." The species present in the littoral zone therefore indicate the degree of the shore's exposure.
The scale runs from an "extremely exposed" shore, to an "extremely sheltered" shore. Tidal movements of water creates zonation patterns along rocky shores from high to low-tide; the area above the high-tide mark is the supralittoral zone, a terrestrial environment. The area around the high-tide mark is known as the intertidal fringe. Between the high and low-tide marks is the littoral zone. Below the low-tide mark is the subtidal zone; the presence and abundance of different animals and algae vary in different zones along the rocky shore due to differing adaptations to the varying levels of exposure to sun and desiccation along the rocky shore. Rocky shores are exposed in particular pollution related to oil spills. Prominent spills are the Torrey Canyon spill, The Amoco Cadiz spill outside the Brittany coast in France and the Exxon Valdez spill in Prince William Sound, Alaska, USA. Garbage such as plastics and metals being left behind by people is a problem among many rocky coastlines that attract tourists.
Intertidal ecology Intertidal zone Rock pool Seashore wildlife Cruz-Motta J. J. Miloslavich P. Palomo G. Iken K. Konar B. et al.. "Patterns of Spatial Variation of Assemblages Associated with Intertidal Rocky Shores: A Global Perspective". PLoS ONE 5: e14354. Doi:10.1371/journal.pone.0014354
San Andreas Fault
The San Andreas Fault is a continental transform fault that extends 1,200 kilometers through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, its motion is right-lateral strike-slip; the fault divides into three segments, each with different characteristics and a different degree of earthquake risk. The slip rate along the fault ranges from 20 to 35 mm /yr; the fault was identified in 1895 by Professor Andrew Lawson of UC Berkeley, who discovered the northern zone. It is described as having been named after San Andreas Lake, a small body of water, formed in a valley between the two plates. However, according to some of his reports from 1895 and 1908, Lawson named it after the surrounding San Andreas Valley. Following the 1906 San Francisco earthquake, Lawson concluded that the fault extended all the way into southern California. In 1953, geologist Thomas Dibblee concluded that hundreds of miles of lateral movement could occur along the fault. A project called the San Andreas Fault Observatory at Depth near Parkfield, Monterey County, was drilled through the fault during 2004 – 2007 to collect material and make physical and chemical observations to better understand fault behavior.
The northern segment of the fault runs from Hollister, through the Santa Cruz Mountains, epicenter of the 1989 Loma Prieta earthquake up the San Francisco Peninsula, where it was first identified by Professor Lawson in 1895 offshore at Daly City near Mussel Rock. This is the approximate location of the epicenter of the 1906 San Francisco earthquake; the fault returns onshore at Bolinas Lagoon just north of Stinson Beach in Marin County. It returns underwater through the linear trough of Tomales Bay which separates the Point Reyes Peninsula from the mainland, runs just east of Bodega Head through Bodega Bay and back underwater, returning onshore at Fort Ross. From Fort Ross, the northern segment continues overland, forming in part a linear valley through which the Gualala River flows, it goes back offshore at Point Arena. After that, it runs underwater along the coast until it nears Cape Mendocino, where it begins to bend to the west, terminating at the Mendocino Triple Junction; the central segment of the San Andreas Fault runs in a northwestern direction from Parkfield to Hollister.
While the southern section of the fault and the parts through Parkfield experience earthquakes, the rest of the central section of the fault exhibits a phenomenon called aseismic creep, where the fault slips continuously without causing earthquakes. The southern segment begins near California. Box Canyon, near the Salton Sea, contains upturned strata associated with that section of the fault; the fault runs along the southern base of the San Bernardino Mountains, crosses through the Cajon Pass and continues northwest along the northern base of the San Gabriel Mountains. These mountains are a result of movement along the San Andreas Fault and are called the Transverse Range. In Palmdale, a portion of the fault is examined at a roadcut for the Antelope Valley Freeway; the fault continues northwest alongside the Elizabeth Lake Road to the town of Elizabeth Lake. As it passes the towns of Gorman, Tejon Pass and Frazier Park, the fault begins to bend northward, forming the "Big Bend"; this restraining bend is thought to be where the fault locks up in Southern California, with an earthquake-recurrence interval of 140–160 years.
Northwest of Frazier Park, the fault runs through the Carrizo Plain, a long, treeless plain where much of the fault is plainly visible. The Elkhorn Scarp defines the fault trace along much of its length within the plain; the southern segment, which stretches from Parkfield in Monterey County all the way to the Salton Sea, is capable of an 8.1-magnitude earthquake. At its closest, this fault passes about 35 miles to the northeast of Los Angeles; such a large earthquake on this southern segment would kill thousands of people in Los Angeles, San Bernardino and surrounding areas, cause hundreds of billions of dollars in damage. The Pacific Plate, to the west of the fault, is moving in a northwest direction while the North American Plate to the east is moving toward the southwest, but southeast under the influence of plate tectonics; the rate of slippage averages about 33 to 37 millimeters a year across California. The southwestward motion of the North American Plate towards the Pacific is creating compressional forces along the eastern side of the fault.
The effect is expressed as the Coast Ranges. The northwest movement of the Pacific Plate is creating significant compressional forces which are pronounced where the North American Plate has forced the San Andreas to jog westward; this has led to the formation of the Transverse Ranges in Southern California, to a lesser but still significant extent, the Santa Cruz Mountains. Studies of the relative motions of the Pacific and North American plates have shown that only about 75 percent of the motion can be accounted for in the movements of the San Andreas and its various branch faults; the rest of the motion has been found in an area east of the Sierra Nevada mountains called the Walker Lane or Eastern California Shear Zone. The reason for this is not clear. Several hypotheses have been offered and research is ongoing. One hypothesis – which gained interest following the Landers earthquake in 1992 – suggests the plate boundary may be shifting eastward aw
Quartz is a mineral composed of silicon and oxygen atoms in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Quartz is the second most abundant mineral behind feldspar. Quartz exists in two forms, the normal α-quartz and the high-temperature β-quartz, both of which are chiral; the transformation from α-quartz to β-quartz takes place abruptly at 573 °C. Since the transformation is accompanied by a significant change in volume, it can induce fracturing of ceramics or rocks passing through this temperature threshold. There are many different varieties of quartz. Since antiquity, varieties of quartz have been the most used minerals in the making of jewelry and hardstone carvings in Eurasia; the word "quartz" is derived from the German word "Quarz", which had the same form in the first half of the 14th century in Middle High German in East Central German and which came from the Polish dialect term kwardy, which corresponds to the Czech term tvrdý.
The Ancient Greeks referred to quartz as κρύσταλλος derived from the Ancient Greek κρύος meaning "icy cold", because some philosophers believed the mineral to be a form of supercooled ice. Today, the term rock crystal is sometimes used as an alternative name for the purest form of quartz. Quartz belongs to the trigonal crystal system; the ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end. In nature quartz crystals are twinned, distorted, or so intergrown with adjacent crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal faces altogether and appear massive. Well-formed crystals form in a'bed' that has unconstrained growth into a void. However, doubly terminated crystals do occur where they develop without attachment, for instance within gypsum. A quartz geode is such a situation where the void is spherical in shape, lined with a bed of crystals pointing inward. Α-quartz crystallizes in the trigonal crystal system, space group P3121 or P3221 depending on the chirality.
Β-quartz belongs to space group P6222 and P6422, respectively. These space groups are chiral. Both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks; the transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, without change in the way they are linked. Although many of the varietal names arose from the color of the mineral, current scientific naming schemes refer to the microstructure of the mineral. Color is a secondary identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. Pure quartz, traditionally called rock crystal or clear quartz, is colorless and transparent or translucent, has been used for hardstone carvings, such as the Lothair Crystal. Common colored varieties include citrine, rose quartz, smoky quartz, milky quartz, others; these color differentiation's arise from chromophores which have been incorporated into the crystal structure of the mineral.
Polymorphs of quartz include: α-quartz, β-quartz, moganite, cristobalite and stishovite. The most important distinction between types of quartz is that of macrocrystalline and the microcrystalline or cryptocrystalline varieties; the cryptocrystalline varieties are either translucent or opaque, while the transparent varieties tend to be macrocrystalline. Chalcedony is a cryptocrystalline form of silica consisting of fine intergrowths of both quartz, its monoclinic polymorph moganite. Other opaque gemstone varieties of quartz, or mixed rocks including quartz including contrasting bands or patterns of color, are agate, carnelian or sard, onyx and jasper. Amethyst is a form of quartz that ranges from a dull purple color; the world's largest deposits of amethysts can be found in Brazil, Uruguay, France and Morocco. Sometimes amethyst and citrine are found growing in the same crystal, it is referred to as ametrine. An amethyst is formed. Blue quartz contains inclusions of fibrous crocidolite. Inclusions of the mineral dumortierite within quartz pieces result in silky-appearing splotches with a blue hue, shades giving off purple and/or grey colors additionally being found.
"Dumortierite quartz" will sometimes feature contrasting light and dark color zones across the material. Interest in the certain quality forms of blue quartz as a collectible gemstone arises in India and in the United States. Citrine is a variety of quartz whose color ranges from a pale yellow to brown due to ferric impurities. Natural citrines are rare. However, a heat-treated amethyst will have small lines in the crystal, as opposed to a natural citrine's cloudy or smokey appearance, it is nearly impossible to differentiate between cut citrine and yellow topaz visually, but they differ in hardness. Brazil is the leading producer of citrine, with much
Elephant seals are large, oceangoing earless seals in the genus Mirounga. The two species, the northern elephant seal and the southern elephant seal, were both hunted to the brink of extinction by the end of the 19th century, but the numbers have since recovered; the northern elephant seal, somewhat smaller than its southern relative, ranges over the Pacific coast of the U. S. Canada and Mexico; the most northerly breeding location on the Pacific Coast is at Race Rocks, at the southern tip of Vancouver Island in the Strait of Juan de Fuca. The southern elephant seal is found in the Southern Hemisphere on islands such as South Georgia and Macquarie Island, on the coasts of New Zealand, South Africa, Argentina in the Peninsula Valdés. In southern Chile, there is a small colony of 120 animals at Jackson Bay, Admiralty Sound, Tierra del Fuego; the oldest known unambiguous elephant seal fossils are fragmentary fossils of an unnamed member of the tribe Miroungini described from the late Pliocene Petane Formation of New Zealand.
Teeth identified as representing an unnamed species of Mirounga have been found in South Africa, dated to the Miocene epoch. Elephant seals breed annually and are faithful to colonies that have established breeding areas. Elephant seals are marine mammals classified under the order Pinnipedia, which in Latin, means feather or fin footed. Elephant seals are considered true seals, fall under the family Phocidae. Phocids are characterized by having reduced limbs; the reduction of their limbs helps them be more streamlined and move in the water. However, it makes navigating on land a bit difficult because they cannot turn their hind flippers forward to walk like the Otariids. In addition, the hind flipper of elephant seals have a lot of surface area, which helps propel them in the water. Elephant seals spend the majority of their time underwater in search of food, can cover 60 miles a day when they head out to sea; when elephant seals are born, they can reach lengths up to 4 feet. Sexual dimorphism is prominently seen in elephant seals due to the fact that male elephant seals can weigh up to 10 times more than females.
The large proboscis, considered a secondary sexual characteristic, helps males assert dominance during mating season. Elephant seals take their name from the large proboscis of the adult male, which resembles an elephant's trunk; the bull's proboscis is used in producing extraordinarily loud roaring noises during the mating season. More however, the nose acts as a sort of rebreather, filled with cavities designed to reabsorb moisture from their exhalations; this is important during the mating season when the seals do not leave the beach to feed, must conserve body moisture as there is no incoming source of water. They are colossally large in comparison with other pinnipeds, with southern elephant seal bulls reaching a length of 5 m and a weight of 3,000 kg, are much larger than the adult females, with some exceptionally large males reaching up to 6 m in length and weighing 4,000 kg. Northern elephant seal bulls reach the heaviest weigh about 2,500 kg; the northern and southern elephant seal can be distinguished by looking at various external features.
On average, the southern elephant seal tends to be larger than the northern species. Adult male elephant seals belonging to the northern species tend to have a larger proboscis, thick chest area with a red coloration compared to the southern species. Females do not have the large proboscis and can be distinguished between species by looking at their nose characteristics. Southern females tend to have a smaller, blunt nose compared to northern females. Elephant seals spend up to 80% of their lives in the ocean, they can hold their breath for more than 100 minutes – longer than any other noncetacean mammal. Elephant seals dive to 1,550 m beneath the ocean's surface; the average depth of their dives is about 300 to 600 m for around 20 minutes for females and 60 minutes for males, as they search for their favorite foods, which are skates, squid, eels, small sharks and large fish. Their stomachs often contain gastroliths, they spend only brief amounts of time at the surface to rest in between dives.
Females tend to dive a bit deeper due to their prey source. Elephant seals are shielded from extreme cold more so than by fur, their hair and outer layers of skin molt in large patches. The skin has to be regrown by blood vessels reaching through the blubber; when molting occurs, the seal is susceptible to the cold, must rest on land, in a safe place called a "haul out". Northern males and young adults haul out during June to July to molt. Elephant seals have a large volume of blood, allowing them to hold a large amount of oxygen for use when diving, they have large sinuses in their abdomens to hold blood and can store oxygen in their muscles with increased myoglobin concentrations in muscle. In addition, they have a larger proportion of oxygen-carrying red blood cells; these adaptations allow elephant seals to dive to such depths and remain underwater for up to two hours. Elephant seals are able to slow down their heartbeat and divert blood f
The United States of America known as the United States or America, is a country composed of 50 states, a federal district, five major self-governing territories, various possessions. At 3.8 million square miles, the United States is the world's third or fourth largest country by total area and is smaller than the entire continent of Europe's 3.9 million square miles. With a population of over 327 million people, the U. S. is the third most populous country. The capital is Washington, D. C. and the largest city by population is New York City. Forty-eight states and the capital's federal district are contiguous in North America between Canada and Mexico; the State of Alaska is in the northwest corner of North America, bordered by Canada to the east and across the Bering Strait from Russia to the west. The State of Hawaii is an archipelago in the mid-Pacific Ocean; the U. S. territories are scattered about the Pacific Ocean and the Caribbean Sea, stretching across nine official time zones. The diverse geography and wildlife of the United States make it one of the world's 17 megadiverse countries.
Paleo-Indians migrated from Siberia to the North American mainland at least 12,000 years ago. European colonization began in the 16th century; the United States emerged from the thirteen British colonies established along the East Coast. Numerous disputes between Great Britain and the colonies following the French and Indian War led to the American Revolution, which began in 1775, the subsequent Declaration of Independence in 1776; the war ended in 1783 with the United States becoming the first country to gain independence from a European power. The current constitution was adopted in 1788, with the first ten amendments, collectively named the Bill of Rights, being ratified in 1791 to guarantee many fundamental civil liberties; the United States embarked on a vigorous expansion across North America throughout the 19th century, acquiring new territories, displacing Native American tribes, admitting new states until it spanned the continent by 1848. During the second half of the 19th century, the Civil War led to the abolition of slavery.
By the end of the century, the United States had extended into the Pacific Ocean, its economy, driven in large part by the Industrial Revolution, began to soar. The Spanish–American War and World War I confirmed the country's status as a global military power; the United States emerged from World War II as a global superpower, the first country to develop nuclear weapons, the only country to use them in warfare, a permanent member of the United Nations Security Council. Sweeping civil rights legislation, notably the Civil Rights Act of 1964, the Voting Rights Act of 1965 and the Fair Housing Act of 1968, outlawed discrimination based on race or color. During the Cold War, the United States and the Soviet Union competed in the Space Race, culminating with the 1969 U. S. Moon landing; the end of the Cold War and the collapse of the Soviet Union in 1991 left the United States as the world's sole superpower. The United States is the world's oldest surviving federation, it is a representative democracy.
The United States is a founding member of the United Nations, World Bank, International Monetary Fund, Organization of American States, other international organizations. The United States is a developed country, with the world's largest economy by nominal GDP and second-largest economy by PPP, accounting for a quarter of global GDP; the U. S. economy is post-industrial, characterized by the dominance of services and knowledge-based activities, although the manufacturing sector remains the second-largest in the world. The United States is the world's largest importer and the second largest exporter of goods, by value. Although its population is only 4.3% of the world total, the U. S. holds 31% of the total wealth in the world, the largest share of global wealth concentrated in a single country. Despite wide income and wealth disparities, the United States continues to rank high in measures of socioeconomic performance, including average wage, human development, per capita GDP, worker productivity.
The United States is the foremost military power in the world, making up a third of global military spending, is a leading political and scientific force internationally. In 1507, the German cartographer Martin Waldseemüller produced a world map on which he named the lands of the Western Hemisphere America in honor of the Italian explorer and cartographer Amerigo Vespucci; the first documentary evidence of the phrase "United States of America" is from a letter dated January 2, 1776, written by Stephen Moylan, Esq. to George Washington's aide-de-camp and Muster-Master General of the Continental Army, Lt. Col. Joseph Reed. Moylan expressed his wish to go "with full and ample powers from the United States of America to Spain" to seek assistance in the revolutionary war effort; the first known publication of the phrase "United States of America" was in an anonymous essay in The Virginia Gazette newspaper in Williamsburg, Virginia, on April 6, 1776. The second draft of the Articles of Confederation, prepared by John Dickinson and completed by June 17, 1776, at the latest, declared "The name of this Confederation shall be the'United States of America'".
The final version of the Articles sent to the states for ratification in late 1777 contains the sentence "The Stile of this Confederacy shall be'The United States of America'". In June 1776, Thomas Jefferson wrote the phrase "UNITED STATES OF AMERICA" in all capitalized letters in the headline of his "original Rough draught" of the Declaration of Independence; this draft of the document did not surface unti
Natural history is a domain of inquiry involving organisms including animals and plants in their environment. A person who studies natural history is called natural historian. Natural history is not limited to it, it involves the systematic study of any category of natural organisms. So while it dates from studies in the ancient Greco-Roman world and the mediaeval Arabic world, through to European Renaissance naturalists working in near isolation, today's natural history is a cross discipline umbrella of many specialty sciences; the meaning of the English term "natural history" has narrowed progressively with time. In antiquity, "natural history" covered anything connected with nature, or which used materials drawn from nature, such as Pliny the Elder's encyclopedia of this title, published circa 77 to 79 AD, which covers astronomy, geography and their technology and superstition, as well as animals and plants. Medieval European academics considered knowledge to have two main divisions: the humanities and divinity, with science studied through texts rather than observation or experiment.
The study of nature revived in the Renaissance, became a third branch of academic knowledge, itself divided into descriptive natural history and natural philosophy, the analytical study of nature. In modern terms, natural philosophy corresponded to modern physics and chemistry, while natural history included the biological and geological sciences; the two were associated. During the heyday of the gentleman scientists, many people contributed to both fields, early papers in both were read at professional science society meetings such as the Royal Society and the French Academy of Sciences – both founded during the seventeenth century. Natural history had been encouraged by practical motives, such as Linnaeus' aspiration to improve the economic condition of Sweden; the Industrial Revolution prompted the development of geology to help find useful mineral deposits. Modern definitions of natural history come from a variety of fields and sources, many of the modern definitions emphasize a particular aspect of the field, creating a plurality of definitions with a number of common themes among them.
For example, while natural history is most defined as a type of observation and a subject of study, it can be defined as a body of knowledge, as a craft or a practice, in which the emphasis is placed more on the observer than on the observed. Definitions from biologists focus on the scientific study of individual organisms in their environment, as seen in this definition by Marston Bates: "Natural history is the study of animals and Plants – of organisms.... I like to think of natural history as the study of life at the level of the individual – of what plants and animals do, how they react to each other and their environment, how they are organized into larger groupings like populations and communities" and this more recent definition by D. S. Wilcove and T. Eisner: "The close observation of organisms—their origins, their evolution, their behavior, their relationships with other species"; this focus on organisms in their environment is echoed by H. W. Greene and J. B. Losos: "Natural history focuses on where organisms are and what they do in their environment, including interactions with other organisms.
It encompasses changes in internal states insofar as they pertain to what organisms do". Some definitions go further, focusing on direct observation of organisms in their environment, both past and present, such as this one by G. A. Bartholomew: "A student of natural history, or a naturalist, studies the world by observing plants and animals directly; because organisms are functionally inseparable from the environment in which they live and because their structure and function cannot be adequately interpreted without knowing some of their evolutionary history, the study of natural history embraces the study of fossils as well as physiographic and other aspects of the physical environment". A common thread in many definitions of natural history is the inclusion of a descriptive component, as seen in a recent definition by H. W. Greene: "Descriptive ecology and ethology". Several authors have argued for a more expansive view of natural history, including S. Herman, who defines the field as "the scientific study of plants and animals in their natural environments.
It is concerned with levels of organization from the individual organism to the ecosystem, stresses identification, life history, distribution and inter-relationships. It and appropriately includes an esthetic component", T. Fleischner, who defines the field more broadly, as "A practice of intentional, focused attentiveness and receptivity to the more-than-human world, guided by honesty and accuracy"; these definitions explicitly include the arts in the field of natural history, are aligned with the broad definition outlined by B. Lopez, who defines the field as the "Patient interrogation of a landscape" while referring to the natural history knowledge of the Eskimo. A different framework for natural history, covering a similar range of themes, is implied in the scope of work encompassed by many leading natural history museums, which include elements of anthropology, geology and astronomy along with botany and zoology, or include both cultural and natural components of the world; the pl
Soil is a mixture of organic matter, gases and organisms that together support life. Earth's body of soil, called the pedosphere, has four important functions: as a medium for plant growth as a means of water storage and purification as a modifier of Earth's atmosphere as a habitat for organismsAll of these functions, in their turn, modify the soil; the pedosphere interfaces with the lithosphere, the hydrosphere, the atmosphere, the biosphere. The term pedolith, used to refer to the soil, translates to ground stone in the sense "fundamental stone". Soil consists of a solid phase of minerals and organic matter, as well as a porous phase that holds gases and water. Accordingly, soil scientists can envisage soils as a three-state system of solids and gases. Soil is a product of several factors: the influence of climate, relief and the soil's parent materials interacting over time, it continually undergoes development by way of numerous physical and biological processes, which include weathering with associated erosion.
Given its complexity and strong internal connectedness, soil ecologists regard soil as an ecosystem. Most soils have a dry bulk density between 1.1 and 1.6 g/cm3, while the soil particle density is much higher, in the range of 2.6 to 2.7 g/cm3. Little of the soil of planet Earth is older than the Pleistocene and none is older than the Cenozoic, although fossilized soils are preserved from as far back as the Archean. Soil science has two basic branches of study: pedology. Edaphology studies the influence of soils on living things. Pedology focuses on the formation and classification of soils in their natural environment. In engineering terms, soil is included in the broader concept of regolith, which includes other loose material that lies above the bedrock, as can be found on the Moon and on other celestial objects as well. Soil is commonly referred to as earth or dirt. Soil is a major component of the Earth's ecosystem; the world's ecosystems are impacted in far-reaching ways by the processes carried out in the soil, from ozone depletion and global warming to rainforest destruction and water pollution.
With respect to Earth's carbon cycle, soil is an important carbon reservoir, it is one of the most reactive to human disturbance and climate change. As the planet warms, it has been predicted that soils will add carbon dioxide to the atmosphere due to increased biological activity at higher temperatures, a positive feedback; this prediction has, been questioned on consideration of more recent knowledge on soil carbon turnover. Soil acts as an engineering medium, a habitat for soil organisms, a recycling system for nutrients and organic wastes, a regulator of water quality, a modifier of atmospheric composition, a medium for plant growth, making it a critically important provider of ecosystem services. Since soil has a tremendous range of available niches and habitats, it contains most of the Earth's genetic diversity. A gram of soil can contain billions of organisms, belonging to thousands of species microbial and in the main still unexplored. Soil has a mean prokaryotic density of 108 organisms per gram, whereas the ocean has no more than 107 procaryotic organisms per milliliter of seawater.
Organic carbon held in soil is returned to the atmosphere through the process of respiration carried out by heterotrophic organisms, but a substantial part is retained in the soil in the form of soil organic matter. Since plant roots need oxygen, ventilation is an important characteristic of soil; this ventilation can be accomplished via networks of interconnected soil pores, which absorb and hold rainwater making it available for uptake by plants. Since plants require a nearly continuous supply of water, but most regions receive sporadic rainfall, the water-holding capacity of soils is vital for plant survival. Soils can remove impurities, kill disease agents, degrade contaminants, this latter property being called natural attenuation. Soils maintain a net absorption of oxygen and methane and undergo a net release of carbon dioxide and nitrous oxide. Soils offer plants physical support, water, temperature moderation and protection from toxins. Soils provide available nutrients to plants and animals by converting dead organic matter into various nutrient forms.
A typical soil is about 50% solids, 50% voids of which half is occupied by water and half by gas. The percent soil mineral and organic content can be treated as a constant, while the percent soil water and gas content is considered variable whereby a rise in one is balanced by a reduction in the other; the pore space allows for the infiltration and movement of air and water, both of which are critical for life existing in soil. Compaction, a common problem with soils, reduces this space, preventing air and water from reaching plant roots and soil organisms. Given sufficient time, an undifferentiated soil will evolve a soil profile which consists of two or more layers, referred to as soil horizons, that differ in one or more properties such as in their texture, density, consistency, temperature and reactivity; the horizons differ in thickness and gene