Wednesday, April 7, 2010

love







Love is any of a number of emotions related to a sense of strong affection[1] and attachment. The word love can refer to a variety of different feelings, states, and attitudes, ranging from generic pleasure ("I loved that meal") to intense interpersonal attraction ("I love my wife"). This diversity of uses and meanings, combined with the complexity of the feelings involved, makes love unusually difficult to consistently define, even compared to other emotional states.

As an abstract concept, love usually refers to a deep, ineffable feeling of tenderly caring for another person. Even this limited conception of love, however, encompasses a wealth of different feelings, from the passionate desire and intimacy of romantic love to the nonsexual emotional closeness of familial and platonic love[2] to the profound oneness or devotion of religious love.[3] Love in its various forms acts as a major facilitator of interpersonal relationships and, owing to its central psychological importance, is one of the most common themes in the creative arts.

Contents

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Definitions

Part of a series on Love
Emblem-favorites.svg
Basic Aspects
Charity
Human bonding
Chemical basis
Religious views
Philosophy of love
Historically
Courtly love
Types of emotion
Eroticism
Platonic love
Familial love
Romance
See also
Limerence
Love sickness
Human sexuality
Unrequited love
Valentine's Day
Sexual intercourse
Interpersonal relationship
Fraternal love (Prehispanic sculpture from 250–900 A.D., of Huastec origin). Museum of Anthropology in Xalapa, Veracruz, Mexico.

The English word "love" can have a variety of related but distinct meanings in different contexts. Often, other languages use multiple words to express some of the different concepts that English relies mainly on "love" to encapsulate; one example is the plurality of Greek words for "love." Cultural differences in conceptualizing love thus make it doubly difficult to establish any universal definition.[4]

Although the nature or essence of love is a subject of frequent debate, different aspects of the word can be clarified by determining what isn't love. As a general expression of positive sentiment (a stronger form of like), love is commonly contrasted with hate (or neutral apathy); as a less sexual and more emotionally intimate form of romantic attachment, love is commonly contrasted with lust; and as an interpersonal relationship with romantic overtones, love is commonly contrasted with friendship, although other definitions of the word love may be applied to close friendships in certain contexts.

When discussed in the abstract, love usually refers to interpersonal love, an experience felt by a person for another person. Love often involves caring for or identifying with a person or thing, including oneself (cf. narcissism). In addition to cross-cultural differences in understanding love, ideas about love have also changed greatly over time. Some historians date modern conceptions of romantic love to courtly Europe during or after the Middle Ages, although the prior existence of romantic attachments is attested by ancient love poetry.[5]

Two hands forming the outline of a heart shape.

Because of the complex and abstract nature of love, discourse on love is commonly reduced to a thought-terminating cliché, and there are a number of common proverbs regarding love, from Virgil's "Love conquers all" to the Beatles' "All you need is love." St. Thomas Aquinas, following Aristotle, defines love as "to will the good of another."[6] Bertrand Russell describes love as a condition of "absolute value," as opposed to relative value. Philosopher Gottfried Leibniz said that love is "to be delighted by the happiness of another."[7]

Love is sometimes referred to as being the "international language", overriding cultural and linguistic divisions.

Impersonal love

A person can be said to love an object, principle, or goal if they value it greatly and are deeply committed to it. Similarly, compassionate outreach and volunteer workers' "love" of their cause may sometimes be borne not of interpersonal love, but impersonal love coupled with altruism and strong political convictions. People can also "love" material objects, animals, or activities if they invest themselves in bonding or otherwise identifying with those things. If sexual passion is also involved, this condition is called paraphilia.[8]

Interpersonal love

Interpersonal love refers to love between human beings. It is a more potent sentiment than a simple liking for another. Unrequited love refers to those feelings of love that are not reciprocated. Interpersonal love is most closely associated with interpersonal relationships. Such love might exist between family members, friends, and couples. There are also a number of psychological disorders related to love, such as erotomania.

Throughout history, philosophy and religion have done the most speculation on the phenomenon of love. In the last century, the science of psychology has written a great deal on the subject. In recent years, the sciences of evolutionary psychology, evolutionary biology, anthropology, neuroscience, and biology have added to the understanding of the nature and function of love.

Chemical basis

Simplified overview of the chemical basis of love.

Biological models of sex tend to view love as a mammalian drive, much like hunger or thirst.[9] Helen Fisher, a leading expert in the topic of love, divides the experience of love into three partly overlapping stages: lust, attraction, and attachment. Lust exposes people to others; romantic attraction encourages people to focus their energy on mating; and attachment involves tolerating the spouse (or indeed the child) long enough to rear a child into infancy.

Lust is the initial passionate sexual desire that promotes mating, and involves the increased release of chemicals such as testosterone and estrogen. These effects rarely last more than a few weeks or months. Attraction is the more individualized and romantic desire for a specific candidate for mating, which develops out of lust as commitment to an individual mate forms. Recent studies in neuroscience have indicated that as people fall in love, the brain consistently releases a certain set of chemicals, including pheromones, dopamine, norepinephrine, and serotonin, which act in a manner similar to amphetamines, stimulating the brain's pleasure center and leading to side effects such as increased heart rate, loss of appetite and sleep, and an intense feeling of excitement. Research has indicated that this stage generally lasts from one and a half to three years.[10]

Since the lust and attraction stages are both considered temporary, a third stage is needed to account for long-term relationships. Attachment is the bonding that promotes relationships lasting for many years and even decades. Attachment is generally based on commitments such as marriage and children, or on mutual friendship based on things like shared interests. It has been linked to higher levels of the chemicals oxytocin and vasopressin to a greater degree than short-term relationships have.[10] Enzo Emanuele and coworkers reported the protein molecule known as the nerve growth factor (NGF) has high levels when people first fall in love, but these return to previous levels after one year. [11]

Psychological basis

Grandmother and grandchild,
Sri Lanka

Psychology depicts love as a cognitive and social phenomenon. Psychologist Robert Sternberg formulated a triangular theory of love and argued that love has three different components: intimacy, commitment, and passion. Intimacy is a form in which two people share confidences and various details of their personal lives, and is usually shown in friendships and romantic love affairs. Commitment, on the other hand, is the expectation that the relationship is permanent. The last and most common form of love is sexual attraction and passion. Passionate love is shown in infatuation as well as romantic love. All forms of love are viewed as varying combinations of these three components. American psychologist Zick Rubin seeks to define love by psychometrics. His work states that three factors constitute love: attachment, caring, and intimacy.[12] [13]

Following developments in electrical theories such as Coulomb's law, which showed that positive and negative charges attract, analogs in human life were developed, such as "opposites attract." Over the last century, research on the nature of human mating has generally found this not to be true when it comes to character and personality—people tend to like people similar to themselves. However, in a few unusual and specific domains, such as immune systems, it seems that humans prefer others who are unlike themselves (e.g., with an orthogonal immune system), since this will lead to a baby that has the best of both worlds.[14] In recent years, various human bonding theories have been developed, described in terms of attachments, ties, bonds, and affinities.

Some Western authorities disaggregate into two main components, the altruistic and the narcissistic. This view is represented in the works of Scott Peck, whose work in the field of applied psychology explored the definitions of love and evil. Peck maintains that love is a combination of the "concern for the spiritual growth of another," and simple narcissism.[15] In combination, love is an activity, not simply a feeling.

Comparison of scientific models

Biological models of love tend to see it as a mammalian drive, similar to hunger or thirst.[9] Psychology sees love as more of a social and cultural phenomenon. There are probably elements of truth in both views. Certainly love is influenced by hormones (such as oxytocin), neurotrophins (such as NGF), and pheromones, and how people think and behave in love is influenced by their conceptions of love. The conventional view in biology is that there are two major drives in love: sexual attraction and attachment. Attachment between adults is presumed to work on the same principles that lead an infant to become attached to its mother. The traditional psychological view sees love as being a combination of companionate love and passionate love. Passionate love is intense longing, and is often accompanied by physiological arousal (shortness of breath, rapid heart rate); companionate love is affection and a feeling of intimacy not accompanied by physiological arousal.

Studies have shown that brain scans of those infatuated by love display a resemblance to those with a mental illness. Love creates activity in the same area of the brain where hunger, thirst, and drug cravings create activity. New love, therefore, could possibly be more physical than emotional. Over time, this reaction to love mellows, and different areas of the brain are activated, primarily ones involving long-term commitments.

Cultural views

Persian

Rumi, Hafez and Sa'di are icons of the passion and love that the Persian culture and language present. The Persian word for love is eshgh, deriving from the Arabic ishq. In the Persian culture, everything is encompassed by love and all is for love, starting from loving friends and family, husbands and wives, and eventually reaching the divine love that is the ultimate goal in life. Over seven centuries ago, Sa'di wrote:

The children of Adam are limbs of one body
Having been created of one essence.
When the calamity of time afflicts one limb
The other limbs cannot remain at rest.
If you have no sympathy for the troubles of others
You are not worthy to be called by the name of "man."

Chinese and other Sinic cultures


"Ai," the traditional Chinese character for love (愛) consists of a heart (middle) inside of "accept," "feel," or "perceive," which shows a graceful emotion. It can also be interpreted as a hand offering ones heart to another hand.

Two philosophical underpinnings of love exist in the Chinese tradition, one from Confucianism which emphasized actions and duty while the other came from Mohism which championed a universal love. A core concept to Confucianism is Ren ("benevolent love", 仁), which focuses on duty, action and attitude in a relationship rather than love itself. In Confucianism, one displays benevolent love by performing actions such as filial piety from children, kindness from parent, loyalty to the king and so forth.

The concept of Ai (愛) was developed by the Chinese philosopher Mozi in the 4th century BC in reaction to Confucianism's benevolent love. Mozi tried to replace what he considered to be the long-entrenched Chinese over-attachment to family and clan structures with the concept of "universal love" (jiān'ài, 兼愛). In this, he argued directly against Confucians who believed that it was natural and correct for people to care about different people in different degrees. Mozi, by contrast, believed people in principle should care for all people equally. Mohism stressed that rather than adopting different attitudes towards different people, love should be unconditional and offered to everyone without regard to reciprocation, not just to friends, family and other Confucian relations. Later in Chinese Buddhism, the term Ai (愛) was adopted to refer to a passionate caring love and was considered a fundamental desire. In Buddhism, Ai was seen as capable or being either selfish or selfless, the latter being a key element towards enlightenment.

In contemporary Chinese, Ai (愛) is often used as the equivalent of the Western concept of love. Ai is used as both a verb (e.g. wo ai ni 我愛你, or "I love you") and a noun (such as aiqing 愛情, or "romantic love"). However, due to the influence of Confucian Ren, the phrase ‘Wo ai ni’ (I love you) carries with it a very specific sense of responsibility, commitment and loyalty. Instead of frequently saying "I love you" as in some Western societies, the Chinese are more likely to express feelings of affection in a more casual way. Consequently, "I like you" (Wo xihuan ni, 我喜欢你) is a more common way of expressing affection in Chinese; it is more playful and less serious.[16] This is also true in Japanese (suki da, 好きだ). The Chinese are also more likely to say "I love you" in English or other foreign languages than they would in their mother tongue.

Japanese

In Japanese Buddhism, ai (愛) is passionate caring love, and a fundamental desire. It can develop towards either selfishness or selflessness and enlightenment. Amae (甘え), a Japanese word meaning "indulgent dependence," is part of the child-rearing culture of Japan. Japanese mothers are expected to hug and indulge their children, and children are expected to reward their mothers by clinging and serving. Some sociologists have suggested that Japanese social interactions in later life are modeled on the mother-child amae.

Ancient Greek

Greek distinguishes several different senses in which the word "love" is used. For example, Ancient Greek has the words philia, eros, agape, storge, and xenia. However, with Greek (as with many other languages), it has been historically difficult to separate the meanings of these words totally. At the same time, the Ancient Greek text of the Bible has examples of the verb agapo having the same meaning as phileo.

Agape (ἀγάπη agápē) means love in modern-day Greek. The term s'agapo means I love you in Greek. The word agapo is the verb I love. It generally refers to a "pure," ideal type of love, rather than the physical attraction suggested by eros. However, there are some examples of agape used to mean the same as eros. It has also been translated as "love of the soul."

Eros (ἔρως érōs) (from the Greek deity Eros) is passionate love, with sensual desire and longing. The Greek word erota means in love. Plato refined his own definition. Although eros is initially felt for a person, with contemplation it becomes an appreciation of the beauty within that person, or even becomes appreciation of beauty itself. Eros helps the soul recall knowledge of beauty and contributes to an understanding of spiritual truth. Lovers and philosophers are all inspired to seek truth by eros. Some translations list it as "love of the body."

Philia (φιλία philía), a dispassionate virtuous love, was a concept developed by Aristotle. It includes loyalty to friends, family, and community, and requires virtue, equality, and familiarity. Philia is motivated by practical reasons; one or both of the parties benefit from the relationship. It can also mean "love of the mind."

Storge (στοργή storgē) is natural affection, like that felt by parents for offspring.

Xenia (ξενία xenía), hospitality, was an extremely important practice in Ancient Greece. It was an almost ritualized friendship formed between a host and his guest, who could previously have been strangers. The host fed and provided quarters for the guest, who was expected to repay only with gratitude. The importance of this can be seen throughout Greek mythology—in particular, Homer's Iliad and Odyssey.

Turkish (Shaman & Islamic)

In Turkish, the word "love" comes up with several meanings. A person can love a god, a person, parents, or family. But that person can "love" just one person from the opposite sex, which they call the word "aşk." Aşk is a feeling for to love, or being "in love" (Aşık), as it still is in Turkish today. The Turks used this word just for their loves in a romantic or sexual sense. If a Turk says that he is in love (Aşık) with somebody, it is not a love that a person can feel for his or her parents; it is just for one person, and it indicates a huge infatuation. The word is also common for Turkic languages, such as Azerbaijani (eşq) and Kazakh (ғашық).

Ancient Roman (Latin)

The Latin language has several different verbs corresponding to the English word "love." Amāre is the basic word for to love, as it still is in Italian today. The Romans used it both in an affectionate sense as well as in a romantic or sexual sense. From this verb come amans—a lover, amator, "professional lover," often with the accessory notion of lechery—and amica, "girlfriend" in the English sense, often as well being applied euphemistically to a prostitute. The corresponding noun is amor (the significance of this term for the Romans is well illustrated in the fact, that the name of the City, Rome—in Latin: Roma—can be viewed as an anagram for amor, which was used as the secret name of the City in wide circles in ancient times),[17] which is also used in the plural form to indicate love affairs or sexual adventures. This same root also produces amicus—"friend"—and amicitia, "friendship" (often based to mutual advantage, and corresponding sometimes more closely to "indebtedness" or "influence"). Cicero wrote a treatise called On Friendship (de Amicitia), which discusses the notion at some length. Ovid wrote a guide to dating called Ars Amatoria (The Art of Love), which addresses, in depth, everything from extramarital affairs to overprotective parents.

Complicating the picture somewhat, Latin sometimes uses amāre where English would simply say to like. This notion, however, is much more generally expressed in Latin by placere or delectāre, which are used more colloquially, the latter used frequently in the love poetry of Catullus. Diligere often has the notion "to be affectionate for," "to esteem," and rarely if ever is used for romantic love. This word would be appropriate to describe the friendship of two men. The corresponding noun diligentia, however, has the meaning of "diligence" or "carefulness," and has little semantic overlap with the verb. Observare is a synonym for diligere; despite the cognate with English, this verb and its corresponding noun, observantia, often denote "esteem" or "affection." Caritas is used in Latin translations of the Christian Bible to mean "charitable love"; this meaning, however, is not found in Classical pagan Roman literature. As it arises from a conflation with a Greek word, there is no corresponding verb.

Religious views

Abrahamic religions

Robert Indiana's 1977 "LOVE sculpture" spelling ahava in Israel

Judaism

In Hebrew, Ahava is the most commonly used term for both interpersonal love and love of God. Judaism employs a wide definition of love, both among people and between man and the Deity. Regarding the former, the Torah states, "Love your neighbor like yourself" (Leviticus 19:18). As for the latter, one is commanded to love God "with all your heart, with all your soul and with all your might" (Deuteronomy 6:5), taken by the Mishnah (a central text of the Jewish oral law) to refer to good deeds, willingness to sacrifice one's life rather than commit certain serious transgressions, willingness to sacrifice all of one's possessions, and being grateful to the Lord despite adversity (tractate Berachoth 9:5). Rabbinic literature differs as to how this love can be developed, e.g., by contemplating divine deeds or witnessing the marvels of nature. As for love between marital partners, this is deemed an essential ingredient to life: "See life with the wife you love" (Ecclesiastes 9:9). The biblical book Song of Solomon is considered a romantically phrased metaphor of love between God and his people, but in its plain reading, reads like a love song. The 20th-century Rabbi Eliyahu Eliezer Dessler is frequently quoted as defining love from the Jewish point of view as "giving without expecting to take" (from his Michtav me-Eliyahu, Vol. 1).

Christianity

Sacred Love Versus Profane Love (1602–03) by Giovanni Baglione

The Christian understanding is that love comes from God. The love of man and woman—eros in Greek—and the unselfish love of others (agape), are often contrasted as "ascending" and "descending" love, respectively, but are ultimately the same thing.[18]

There are several Greek words for "love" that are regularly referred to in Christian circles.

  • Agape: In the New Testament, agapē is charitable, selfless, altruistic, and unconditional. It is parental love, seen as creating goodness in the world; it is the way God is seen to love humanity, and it is seen as the kind of love that Christians aspire to have for one another.
  • Phileo: Also used in the New Testament, phileo is a human response to something that is found to be delightful. Also known as "brotherly love."
  • Two other words for love in the Greek language, eros (sexual love) and storge (child-to-parent love), were never used in the New Testament.

Christians believe that to Love God with all your heart, mind, and strength and Love your neighbor as yourself are the two most important things in life (the greatest commandment of the Jewish Torah, according to Jesus; cf. Gospel of Mark chapter 12, verses 28–34). Saint Augustine summarized this when he wrote "Love God, and do as thou wilt."

The Apostle Paul glorified love as the most important virtue of all. Describing love in the famous poem in 1 Corinthians, he wrote, "Love is patient, love is kind. It does not envy, it does not boast, it is not proud. It is not rude, it is not self-seeking, it is not easily angered, it keeps no record of wrongs. Love does not delight in evil but rejoices with the truth. It always protects, always trusts, always hopes, and always perseveres." (1 Cor. 13:4–7, NIV)

The Apostle John wrote, "For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life. For God did not send his Son into the world to condemn the world, but to save the world through him. Whoever believes in him is not condemned, but whoever does not believe stands condemned already because he has not believed in the name of God's one and only Son." (John 3:16–18, NIV) John also wrote, "Dear friends, let us love one another for love comes from God. Everyone who loves has been born of God and knows God. Whoever does not love does not know God, because God is love." (1 John 4:7–8, NIV)

Saint Augustine says that one must be able to decipher the difference between love and lust. Lust, according to Saint Augustine, is an overindulgence, but to love and be loved is what he has sought for his entire life. He even says, “I was in love with love.” Finally, he does fall in love and is loved back, by God. Saint Augustine says the only one who can love you truly and fully is God, because love with a human only allows for flaws such as “jealousy, suspicion, fear, anger, and contention.” According to Saint Augustine, to love God is “to attain the peace which is yours.” (Saint Augustine's Confessions)

Christian theologians see God as the source of love, which is mirrored in humans and their own loving relationships. Influential Christian theologian C.S. Lewis wrote a book called The Four Loves. Benedict XVI wrote his first encyclical on "God is love." He said that a human being, created in the image of God, who is love, is able to practice love; to give himself to God and others (agape) and by receiving and experiencing God's love in contemplation (eros). This life of love, according to him, is the life of the saints such as Teresa of Calcutta and the Blessed Virgin Mary and is the direction Christians take when they believe that God loves them.[18]

Islam and Arab

In a sense, love does encompass the Islamic view of life as universal brotherhood that applies to all who hold the faith. There are no direct references stating that God is love, but amongst the 99 names of God (Allah), there is the name Al-Wadud, or "the Loving One," which is found in Surah 11:90 as well as Surah 85:14. It refers to God as being "full of loving kindness." All who hold the faith have God's love, but to what degree or effort he has pleased God depends on the individual itself.

Ishq, or divine love, is the emphasis of Sufism. Sufis believe that love is a projection of the essence of God to the universe. God desires to recognize beauty, and as if one looks at a mirror to see oneself, God "looks" at itself within the dynamics of nature. Since everything is a reflection of God, the school of Sufism practices to see the beauty inside the apparently ugly. Sufism is often referred to as the religion of love. God in Sufism is referred to in three main terms, which are the Lover, Loved, and Beloved, with the last of these terms being often seen in Sufi poetry. A common viewpoint of Sufism is that through love, humankind can get back to its inherent purity and grace. The saints of Sufism are infamous for being "drunk" due to their love of God; hence, the constant reference to wine in Sufi poetry and music.

Eastern religions

Buddhism

In Buddhism, Kāma is sensuous, sexual love. It is an obstacle on the path to enlightenment, since it is selfish. Karuṇā is compassion and mercy, which reduces the suffering of others. It is complementary to wisdom and is necessary for enlightenment. Adveṣa and mettā are benevolent love. This love is unconditional and requires considerable self-acceptance. This is quite different from ordinary love, which is usually about attachment and sex and which rarely occurs without self-interest. Instead, in Buddhism it refers to detachment and unselfish interest in others' welfare.

The Bodhisattva ideal in Mahayana Buddhism involves the complete renunciation of oneself in order to take on the burden of a suffering world. The strongest motivation one has in order to take the path of the Bodhisattva is the idea of salvation within unselfish, altruistic love for all sentient beings.

Hinduism

In Hinduism, kāma is pleasurable, sexual love, personified by the god Kamadeva. For many Hindu schools, it is the third end (artha) in life. Kamadeva is often pictured holding a bow of sugar cane and an arrow of flowers; he may ride upon a great parrot. He is usually accompanied by his consort Rati and his companion Vasanta, lord of the spring season. Stone images of Kaama and Rati can be seen on the door of the Chenna Keshava temple at Belur, in Karnataka, India. Maara is another name for kāma.

In contrast to kāma, prema – or prem – refers to elevated love. Karuna is compassion and mercy, which impels one to help reduce the suffering of others. Bhakti is a Sanskrit term, meaning "loving devotion to the supreme God." A person who practices bhakti is called a bhakta. Hindu writers, theologians, and philosophers have distinguished nine forms of bhakti, which can be found in the Bhagavata Purana and works by Tulsidas. The philosophical work Narada Bhakti Sutras, written by an unknown author (presumed to be Narada), distinguishes eleven forms of love.

geological

Earth (or, "the earth") is the only planet presently known to support life, and its natural features are the subject of many fields of scientific research. Within the solar system, it is third nearest to the sun; it is the largest terrestrial planet and the fifth largest overall. Its most prominent climatic features are its two large polar regions, two relatively narrow temperate zones, and a wide equatorial tropical to subtropical region.[4] Precipitation varies widely with location, from several metres of water per year to less than a millimetre. 71 percent of the Earth's surface is covered by salt-water oceans. The remainder consists of continents and islands, with most of the inhabited land in the Northern Hemisphere.

Earth has evolved through geological and biological processes that have left traces of the original conditions. The outer surface is divided into several gradually migrating tectonic plates, which have changed relatively quickly several times. The interior remains active, with a thick layer of molten mantle and an iron-filled core that generates a magnetic field.

The atmospheric conditions have been significantly altered from the original conditions by the presence of life-forms,[5] which create an ecological balance that stabilizes the surface conditions. Despite the wide regional variations in climate by latitude and other geographic factors, the long-term average global climate is quite stable during interglacial periods,[6] and variations of a degree or two of average global temperature have historically had major effects on the ecological balance, and on the actual geography of the Earth.[7][8]

[edit] Historical perspective

Plankton inhabit oceans, seas and lakes, and have existed on the Earth for at least 2 billion years.[9]

Earth is estimated to have formed 4.54 billion years ago from the solar nebula, along with the Sun and other planets.[10] The moon formed roughly 20 million years later. Initially molten, the outer layer of the planet cooled, resulting in the solid crust. Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, most or all of which came from ice delivered by comets, produced the oceans and other water sources.[11] The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago.[12]

Continents formed, then broke up and reformed as the surface of Earth reshaped over hundreds of millions of years, occasionally combining to make a supercontinent. Roughly 750 million years ago, the earliest known supercontinent Rodinia, began to break apart. The continents later recombined to form Pannotia which broke apart about 540 million years ago, then finally Pangaea, which broke apart about 180 million years ago.[13]

An animation of the Earth's hypothesized Pangaea separation.

There is significant evidence, still being discussed among scientists, that a severe glacial action during the Neoproterozoic era covered much of the planet in a sheet of ice. This hypothesis has been termed the "Snowball Earth", and it is of particular interest as it precedes the Cambrian explosion in which multicellular life forms began to proliferate about 530–540 million years ago.[14]

Since the Cambrian explosion there have been five distinctly identifiable mass extinctions.[15] The last mass extinction occurred some 65 million years ago, when a meteorite collision probably triggered the extinction of the non-avian dinosaurs and other large reptiles, but spared small animals such as mammals, which then resembled shrews. Over the past 65 million years, mammalian life diversified.[16]

Several million years ago, a species of small African ape gained the ability to stand upright.[17] The subsequent advent of human life, and the development of agriculture and further civilization allowed humans to affect the Earth more rapidly than any previous life form, affecting both the nature and quantity of other organisms as well as global climate. By comparison, the oxygen catastrophe, produced by the proliferation of algae during the Siderian period, required about 300 million years to culminate.)

The present era is classified as part of a mass extinction event, the Holocene extinction event, the fastest ever to have occurred.[18][19] Some, such as E. O. Wilson of Harvard University, predict that human destruction of the biosphere could cause the extinction of one-half of all species in the next 100 years.[20] The extent of the current extinction event is still being researched, debated and calculated by biologists.[21]

[edit] Geology

The Earth's layered structure. (1) inner core; (2) outer core; (3) lower mantle; (4) upper mantle; (5) lithosphere; (6) crust
Three types of geological plate tectonic boundaries.

Geology is the science and study of the solid and liquid matter that constitutes the Earth. The field of geology encompasses the study of the composition, structure, physical properties, dynamics, and history of Earth materials, and the processes by which they are formed, moved, and changed. The field is a major academic discipline, and is also important for mineral and hydrocarbon extraction, knowledge about and mitigation of natural hazards, some engineering fields, and understanding past climates and environments.

[edit] Geological evolution

The geology of an area evolves through time as rock units are deposited and inserted and deformational processes change their shapes and locations.

Rock units are first emplaced either by deposition onto the surface or intrude into the overlying rock. Deposition can occur when sediments settle onto the surface of the Earth and later lithify into sedimentary rock, or when as volcanic material such as volcanic ash or lava flows, blanket the surface. Igneous intrusions such as batholiths, laccoliths, dikes, and sills, push upwards into the overlying rock, and crystallize as they intrude.

After the initial sequence of rocks has been deposited, the rock units can be deformed and/or metamorphosed. Deformation typically occurs as a result of horizontal shortening, horizontal extension, or side-to-side (strike-slip) motion. These structural regimes broadly relate to convergent boundaries, divergent boundaries, and transform boundaries, respectively, between tectonic plates.

[edit] Atmosphere, climate, and weather

A snow avalanche
Blue light is scattered more than other wavelengths by the gases in the atmosphere, giving the Earth a blue halo when seen from space

The atmosphere of the Earth serves as a key factor in sustaining the planetary ecosystem. The thin layer of gases that envelops the Earth is held in place by the planet's gravity. Dry air consists of 78% nitrogen, 21% oxygen, 1% argon and other inert gases, carbon dioxide, etc.; but air also contains a variable amount of water vapor. The atmospheric pressure declines steadily with altitude, and has a scale height of about 8 kilometres at the Earth's surface: the height at which the atmospheric pressure has declined by a factor of e (a mathematical constant equal to 2.71...).[22][23] The ozone layer of the Earth's atmosphere plays an important role in depleting the amount of ultraviolet (UV) radiation that reaches the surface. As DNA is readily damaged by UV light, this serves to protect life at the surface. The atmosphere also retains heat during the night, thereby reducing the daily temperature extremes.

Terrestrial weather occurs almost exclusively in the lower part of the atmosphere, and serves as a convective system for redistributing heat. Ocean currents are another important factor in determining climate, particularly the major underwater thermohaline circulation which distributes heat energy from the equatorial oceans to the polar regions. These currents help to moderate the differences in temperature between winter and summer in the temperate zones. Also, without the redistributions of heat energy by the ocean currents and atmosphere, the tropics would be much hotter, and the polar regions much colder.

Weather can have both beneficial and harmful effects. Extremes in weather, such as tornadoes or hurricanes and cyclones, can expend large amounts of energy along their paths, and produce devastation. Surface vegetation has evolved a dependence on the seasonal variation of the weather, and sudden changes lasting only a few years can have a dramatic effect, both on the vegetation and on the animals which depend on its growth for their food.

A tornado in central Oklahoma.

The planetary climate is a measure of the long-term trends in the weather. Various factors are known to influence the climate, including ocean currents, surface albedo, greenhouse gases, variations in the solar luminosity, and changes to the planet's orbit. Based on historical records, the Earth is known to have undergone drastic climate changes in the past, including ice ages.

The climate of a region depends on a number of factors, especially latitude. A latitudinal band of the surface with similar climatic attributes forms a climate region. There are a number of such regions, ranging from the tropical climate at the equator to the polar climate in the northern and southern extremes. Weather is also influenced by the seasons, which result from the Earth's axis being tilted relative to its orbital plane. Thus, at any given time during the summer or winter, one part of the planet is more directly exposed to the rays of the sun. This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the northern and southern hemispheres experience opposite seasons.

Weather is a chaotic system that is readily modified by small changes to the environment, so accurate weather forecasting is currently limited to only a few days. Overall, two things are currently happening worldwide: (1) temperature is increasing on the average; and (2) regional climates have been undergoing noticeable changes.[24]

[edit] Water on Earth

Water is a chemical substance that is composed of hydrogen and oxygen and is vital for all known forms of life.[25] In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71% of the Earth's surface[26]. On Earth, it is found mostly in oceans and other large water bodies, with 1.6% of water below ground in aquifers and 0.001% in the air as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation.[27] Oceans hold 97% of surface water, glaciers and polar ice caps 2.4%, and other land surface water such as rivers, lakes and ponds 0.6%. Additionally, a minute amount of the Earth's water is contained within biological bodies and manufactured products.

[edit] Oceans

A view of the Atlantic ocean from Leblon, Rio de Janeiro.
Earth's oceans
(World Ocean)

An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface (an area of some 361 million square kilometers) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas. More than half of this area is over 3,000 meters (9,800 ft) deep. Average oceanic salinity is around 35 parts per thousand (ppt) (3.5%), and nearly all seawater has a salinity in the range of 30 to 38 ppt. Though generally recognized as several 'separate' oceans, these waters comprise one global, interconnected body of salt water often referred to as the World Ocean or global ocean.[28][29] This concept of a global ocean as a continuous body of water with relatively free interchange among its parts is of fundamental importance to oceanography.[30]

The major oceanic divisions are defined in part by the continents, various archipelagos, and other criteria: these divisions are (in descending order of size) the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean and the Arctic Ocean. Smaller regions of the oceans are called seas, gulfs, bays and other names. There are also salt lakes, which are smaller bodies of landlocked saltwater that are not interconnected with the World Ocean. Two notable examples of salt lakes are the Aral Sea and the Great Salt Lake.

[edit] Lakes

The Lácar Lake is a lake of glacial origin in the province of Neuquén, Argentina.

A lake (from Latin lacus) is a terrain feature (or physical feature), a body of liquid on the surface of a world that is localized to the bottom of basin (another type of landform or terrain feature; that is, it is not global) and moves slowly if it moves at all. On Earth, a body of water is considered a lake when it is inland, not part of the ocean, is larger and deeper than a pond, and is fed by a river.[31][32] The only world other than Earth known to harbor lakes is Titan, Saturn's largest moon, which has lakes of ethane, most likely mixed with methane. It is not known if Titan's lakes are fed by rivers, though Titan's surface is carved by numerous river beds. Natural lakes on Earth are generally found in mountainous areas, rift zones, and areas with ongoing or recent glaciation. Other lakes are found in endorheic basins or along the courses of mature rivers. In some parts of the world, there are many lakes because of chaotic drainage patterns left over from the last Ice Age. All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them.

[edit] Ponds

The Westborough Reservoir (Mill Pond) in Westborough, Massachusetts.

A pond is a body of standing water, either natural or man-made, that is usually smaller than a lake. A wide variety of man-made bodies of water are classified as ponds, including water gardens designed for aesthetic ornamentation, fish ponds designed for commercial fish breeding, and solar ponds designed to store thermal energy. Ponds and lakes are distinguished from streams via current speed. While currents in streams are easily observed, ponds and lakes possess thermally driven microcurrents and moderate wind driven currents. These features distinguish a pond from many other aquatic terrain features, such as stream pools and tide pools.

[edit] Rivers

The Macal River in Belize at San Ignacio, in November, 2001.

A river is a natural watercourse[33], usually freshwater, flowing toward an ocean, a lake, a sea or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including stream, creek, brook, rivulet, and rill; there is no general rule that defines what can be called a river. Many names for small rivers are specific to geographic location; one example is Burn in Scotland and North-east England. Sometimes a river is said to be larger than a creek,[34] but this is not always the case, due to vagueness in the language.[35] A river is part of the hydrological cycle. Water within a river is generally collected from precipitation through surface runoff, groundwater recharge, springs, and the release of stored water in natural ice and snowpacks (i.e., from glaciers).

A rocky stream in Hawaii.

[edit] Streams

A stream is a flowing body of water with a current, confined within a bed and stream banks. In the United States a stream is classified as a watercourse less than 60 feet (18 metres) wide. Streams are important as conduits in the water cycle, instruments in groundwater recharge, and they serve as corridors for fish and wildlife migration. The biological habitat in the immediate vicinity of a stream is called a riparian zone. Given the status of the ongoing Holocene extinction, streams play an important corridor role in connecting fragmented habitats and thus in conserving biodiversity. The study of streams and waterways in general is known as surface hydrology and is a core element of environmental geography.[36]

[edit] Ecosystems

Loch Lomond in Scotland forms a relatively isolated ecosystem. The fish community of this lake has remained unchanged over a very long period of time.[37]
An aerial view of a human ecosystem. Pictured is the city of Chicago

Ecosystems are composed of a variety of abiotic and biotic components that function in an interrelated way.[38] The structure and composition is determined by various environmental factors that are interrelated. Variations of these factors will initiate dynamic modifications to the ecosystem. Some of the more important components are: soil, atmosphere, radiation from the sun, water, and living organisms.

Central to the ecosystem concept is the idea that living organisms interact with every other element in their local environment. Eugene Odum, a founder of ecology, stated: "Any unit that includes all of the organisms (ie: the "community") in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (ie: exchange of materials between living and nonliving parts) within the system is an ecosystem."[39] Within the ecosystem, species are connected and dependent upon one another in the food chain, and exchange energy and matter between themselves as well as with their environment.[40] The human ecosystem concept is grounded in the deconstruction of the human/nature dichotomy and the premise that all species are ecologically integrated with each other, as well as with the abiotic constituents of their biotope.[citation needed]