Geology is the science and study of the solid matter of a celestial body, its composition, structure, physical properties, history and the processes that shape it. It is one of the Earth sciences. Geologists have helped establish the age of the Earth at about 4.6 billion (4.6x109) years, and have determined that the Earth's lithosphere, which includes the crust, is fragmented into tectonic plates that move over a rheic upper mantle (asthenosphere) via processes that are collectively referred to as plate tectonics. Geologists help locate and manage the earth's natural resources, such as petroleum and coal, as well as metals such as iron, copper, and uranium. Additional economic interests include gemstones and many minerals such as asbestos, perlite, mica, phosphates, zeolites, clay, pumice, quartz, and silica, as well as elements such as sulphur, chlorine, and helium.

Astrogeology refers to the application of geologic principles to other bodies of the solar system. However, specialised terms such as selenology (studies of the Moon), areology (of Mars), etc., are also in use.

The word "geology" was first used by Jean-Andrˇ Deluc in the year 1778 and introduced as a fixed term by Horace-Bˇnˇdict de Saussure in the year 1779. An older meaning of the word was first used by Richard de Bury. He used it to distinguish between earthly and theological jurisprudence.

In China, the polymath Shen Kua (1031 - 1095) formulated a hypothesis for the process of land formation: based on his observation of fossil shells in a geological stratum in a mountain hundreds of miles from the ocean, he inferred that the land was formed by erosion of the mountains and by deposition of silt.

The work Peri lithon (On Stones) by Theophrastus (372 - 287 BC), a student of Aristotle, remained authoritative for millennia. Its interpretation of fossils was not overturned until after the Scientific Revolution. It was translated into Latin and the other languages of Europe such as French.

Georg Agricola (1494-1555)), a physician, wrote the first systematic treatise about mining and smelting works, De re metallica libri XII, with an appendix Buch von den Lebewesen unter Tage (Book of the Creatures Beneath the Earth). He covered subjects like wind energy, hydrodynamic power, melting cookers, transport of ores, extraction of soda, sulfur and alum, and administrative issues. The book was published in 1556.

Nicolaus Steno (1638-1686) is credited with the law of superposition, the principle of original horizontality, and the principle of lateral continuity: three defining principles of stratigraphy.

By the 1700s Jean-Etienne Guettard and Nicolas Desmarest hiked central France and recorded their observations on geological maps; Guettard recorded the first observation of the volcanic origins of this part of France.

William Smith (1769-1839) drew some of the first geological maps and began the process of ordering rock strata (layers) by examining the fossils contained in them.

James Hutton is often viewed as the first modern geologist. In 1785 he presented a paper entitled Theory of the Earth to the Royal Society of Edinburgh. In his paper, he explained his theory that the Earth must be much older than had previously been supposed in order to allow enough time for mountains to be eroded and for sediment to form new rocks at the bottom of the sea, which in turn were raised up to become dry land. Hutton published a two-volume version of his ideas in 1795.

Followers of Hutton were known as Plutonists because they believed that some rocks were formed by vulcanism which is the deposition of lava from volcanoes, as opposed to the Neptunists, who believed that all rocks had settled out of a large ocean whose level gradually dropped over time.

In 1811 Georges Cuvier and Alexandre Brongniart published their explanation of the antiquity of the Earth, inspired by Cuvier's discovery of fossil elephant bones in Paris. To prove this, they formulated the principle of stratigraphic succession of the layers of the earth. They were independently anticipated by William Smith's stratigraphic studies on England and Scotland.

Sir Charles Lyell first published his famous book, Principles of Geology, in 1830 and continued to publish new revisions until he died in 1875. He successfully promoted the doctrine of uniformitarianism. This theory states that slow geological processes have occurred throughout the Earth's history and are still occurring today. In contrast, catastrophism is the theory that Earth's features formed in single, catastrophic events and remained unchanged thereafter. Though Hutton believed in uniformitarianism, the idea was not widely accepted at the time.

By 1827 Charles Lyell's Principles of Geology reiterated Hutton's uniformitarianism, which influenced the thought of Charles Darwin.

19th Century geology revolved around the question of the Earth's exact age. Estimates varied from a few 100,000 to billions of years. The most significant advance in 20th century geology has been the development of the theory of plate tectonics in the 1960s. Plate tectonic theory arose out of two separate geological observations: seafloor spreading and continental drift. The theory revolutionized the Earth sciences.

The theory of continental drift was proposed by Alfred Wegener in 1912 and by Arthur Holmes, but wasn't broadly accepted until the 1960s when the theory of plate tectonics was developed.

There are a number of important principles in geology. Many of these involve the ability to provide the relative ages of strata or the manner in which they were formed.

The Principle of Intrusive Relationships concerns crosscutting intrusions. In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.
• The Principle of Cross-cutting Relationships pertains to the formation of faults and the age of the sequences through which they cut. Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault.

• The Principle of Inclusions and Components states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them.

• The Principle of Uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time. A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "The Present is the Key to the Past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now."The Principle of Original Horizontality states that the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and nonmarine sediments in a wide variety of environments supports this generalization (although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal).

• The Principle of Superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. Logically a younger layer cannot slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.

• The Principle of Faunal Succession is based on the appearance of fossils in sedimentary rocks. As organisms exist at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found. Based on principles laid out by William Smith almost a hundred years before the publication of Charles Darwin's theory of evolution, the principles of succession were developed independently of evolutionary thought. The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat (facies change in sedimentary strata), and that not all fossils may be found globally at the same time.

Geology Wikipedia

Vredefort Crater in South Africa
The largest verified impact crater on Earth

Hoodoo Sky Bryce Canyon NASA -July 3, 2008

Hoodoos Bryce Canyon

Hoodoos are tall thin spires of rock that protrude from the bottom of arid basins and badlands. They are composed of soft sedimentary rock and are topped by a piece of harder, less easily-eroded stone that protects the column from the elements. They are mainly located in the desert in dry, hot areas. In common usage, the difference between hoodoos and pinnacles or spires is that hoodoos have a variable thickness often described as having a "totem pole-shaped body." A spire, on the other hand, has a smoother profile or uniform thickness that tapers from the ground upward. (Geology purists do note that only a tall formation should be called a hoodoo; any other shape is called a 'hoodoo rock'.) Hoodoos are most commonly found in the High Plateaus region of the Colorado Plateau and in the Badlands regions of the Northern Great Plains (both in North America). While hoodoos are scattered throughout these areas, nowhere in the world are they as abundant as in the northern section of Bryce Canyon National Park. WED Enterprises (now Walt Disney Imagineering) notably based the design of the Disneyland version of the popular Big Thunder Mountain Railroad around a series of hoodoos, although these were constructed out of steel and concrete.

Hoodoos range in size from that of an average human to heights exceeding a 10-story building. Formed in sedimentary rock, hoodoo shapes are affected by the erosional patterns of alternating hard and softer rock layers. Minerals deposited within different rock types cause hoodoos to have different colors throughout their height. Good examples of hoodoos are found at Bryce Canyon National Park, located in the U.S. state of Utah (see geology of the Bryce Canyon area).

'Meteor Crater' in Arizona helps unlock planetary history   MSNBC - December 8, 2011
The Barringer meteorite crater - known popularly as "Meteor Crater" - near Winslow, Ariz., was formed some 50,000 years ago in the flat-lying sedimentary rocks of the Southern Colorado Plateau in Arizona. Now, scientists are using the crater to study mysteries near and far. This out-of-the-blue geological feature is considered a prime example of a young, well-preserved and well-documented simple impact crater.

  Undersea mountains march into the abyss   BBC - December 6, 2011

Startling new images from the depths of the Pacific Ocean reveal one of Earth's most violent processes: the destruction of massive underwater mountains. The pictures were created by sonar in waters up to 6km (4mi) deep. They expose how tectonic action is dragging giant volcanoes into a chasm in the seabed. The volcanoes are strung across several thousand kilometres of ocean floor and are moving westward on the Pacific tectonic plate at up to 6cm per year. The extraordinary scene was captured along the Tonga Trench during a research expedition last summer.

Lost landscape discovered off Scottish coast   BBC - July 14, 2011
Geologists have discovered a vast new landscape that rose above the north Atlantic waves 56 million years ago. It was caused by a sudden up-welling from the Earth's mantle and may explain rapid climate change that took place at about that time.

Ancient rock under Haiti came from 1,000+ miles away, 1 billion years older than previously thought   PhysOrg - July 12, 2011
Earthquakes and volcanoes are known for their ability to transform Earth's surface, but new research in the Caribbean has found they can also move ancient Earth rock foundations more than 1,000 miles.

Geologist's discoveries resolve debate about oxygen in Earth's mantle   PhysOrg - December 15, 2010
Analysis of erupted rock from Agrigan volcano in the western Pacific near Guam found it to be highly oxidized as a result of its exposure to oxygen when it formed in the Earth's mantle. When, over millions of years, seafloor rocks are transported back into the Earth's mantle at subduction zones Š sites on the seafloor where tectonic plates have collided, forcing one plate beneath the other Š they deliver more oxygen into the mantle.

Scottish rocks record ancient oxygen clues   BBC - November 11, 2010
Oxygen levels on Earth reached a critical threshold to enable the evolution of complex life much earlier than thought, say scientists. The evidence is found in 1.2-billion-year-old rocks from Scotland. These rocks retain signatures of bacterial activity known to occur when there is copious atmospheric oxygen. The microbes' behavior is seen 400 million years further back in time than any previous discovery.

Infant, Magma-Ball Earth Glimpsed Via Newfound Rocks   National Geographic - August 12, 2010

Canada's Baffin Island, home to perhaps the world's oldest rocks

Arctic rocks may contain oldest remnants of Earth   BBC - August 11, 2010

Scientists have found Arctic rocks that may preserve the earliest remnants of Earth. Over billions of years, much of the material that made up the early Earth was modified by processes such as melting and mixing. But the Arctic rocks seem to contain chemical signatures that date from just after the Earth's violent origin.

Canyon Carved in Just Three Days in Texas Flood: Insight Into Ancient Flood Events on Earth and Mars   Science Daily - June 21, 2010

Geologist investigates canyon carved in just three days in 2002 Central Texas flood   PhysOrg - June 21, 2010

Earth's Gooey Insides Ooze Faster Than Thought   Live Science - May 20, 2010
Earth's mantle flows fast   PhysOrg - May 20, 2010

Silver tells a volatile story of Earth's origin: Water was present during its birth   PhysOrg - May 13, 2010

Quantum mechanics reveals new details of deep earth   PhysOrg - May 11, 2010

The age of Aquarius? Nope, it's the Anthropocene epoch   PhysOrg - April 15, 2010
The geologic time scale might be amended in the future to include a new period called the "Anthropocene epoch" that is marked by stunning population growth and unprecedented changes in human activity.

Geologist discovers pattern in Earth's long-term climate record   PhysOrg - April 6, 2010

Study reveals ancient rocks linked to old Earth's crust   PhysOrg - February 24, 2010

Britain cut off from Europe by 'super-river' as little as 30,000 years ago   Telegraph.co.uk - November 30, 2009

A glimpse at the Earth's crust deep below the Atlantic   PhysOrg - November 12, 2009

Central Africa's Tropical Congo Basin Was Arid, Treeless In Late Jurassic   Science Daily - November 11, 2009

Giant Crack in Africa Will Create a New Ocean   Live Science - November 3, 2009

The rift in Afar, Ethiopia, that researchers say will eventually become a new ocean.
African Desert Rift Confirmed As New Ocean In The Making   Science Daily - November 3, 2009

Deep-sea Rocks Point To Early Oxygen On Earth   Science Daily - March 26, 2009

21st Century Detective Work Reveals How Ancient Rock Got Off To A Hot Start Science Daily - October 28, 2008

Quebec: Oldest Rocks on Earth Found Live Science - September 25, 2008

Diamonds hint at 'earliest life' on Earth BBC - July 2, 2008
Tiny slivers of diamond forged on an infant Earth may contain the earliest traces of life, a study has shown. Analysis of the crystals showed they contain a form of carbon often associated with plants and bacteria.

Researchers confirm discovery of Earth's inner, innermost core Science Daily - March 11, 2008

Ancient oil points to 'cradle of life' BBC - August 4, 2000

Mysterious Tremors' Strength Ebbs With Tides National Geographic - November 22, 2007
Undersea slide set off giant flow BBC - November 22, 2007

How The Discovery Of Geologic Time Changed Our View Of The World Science Daily - September 18, 2007

Without Hot Rock, Much Of North America Would Be Underwater Science Daily - June 26, 2007

Magma Wikipedia
Thick Layer of Magma Found Under American Southwest Live Science - June 23, 2007
Scientists have spotted a thick layer of melted rock beneath the EarthÕs crust that could be part of a fluid band of hot magma circling the globe. The magma ring has until now remained a theory.

Thick Layer of Magma Found Under American Southwest Live Science - June 23, 2007
Scientists have spotted a thick layer of melted rock beneath the EarthÕs crust that could be part of a fluid band of hot magma circling the globe. The magma ring has until now remained a theory.

Oldest Known Ocean Crust Found on Greenland National Geographic - March 26, 2007

Scientists have discovered a 3.8-billion-year-old rock formation in Greenland that they say is the earliest example of oceanic crust ever to be discovered.
Sea floor records ancient Earth BBC - March 26, 2007

A sliver of four-billion-year-old sea floor has offered a glimpse into the inner workings of an adolescent Earth. The baked and twisted rocks, now part of Greenland, show the earliest evidence of plate tectonics, colossal movements of the planet's outer shell. Until now, researchers were unable to say when the process, which explains how oceans and continents form, began. Plate tectonics is a geological theory used to explain the observed large-scale motions of the Earth's surface.

Mining heat from the earth? New technology shows promise Christian Science Monitor - February 7, 2007

Ancient Rocks Show How Young Earth Avoided Becoming Giant Snowball Science Daily - February 6, 2007

Weird Australia Rocks Are Earliest Signs of Life, Study Says National Geographic - June 8, 2006

Ancient rocks 'built by microbes' BBC - June 7, 2006

Drilled Core Exposes Hitherto Unseen Layer of Earth's Crust Scientific American - April 21, 2006
Scientists Find the Elusive Gabbro Live Science - April 21, 2006
It's not quite the center of the Earth, but scientists have drilled nearly a mile into the planet's ocean crust, retrieving samples from the pristine layer of igneous rock for the first time.

Redefining the geological time scale Science Daily - November 2004

Geological time gets a new period BBC - May 2004

Geologists have added a new period to their official calendar of Earth's history - the first in 120 years. The Ediacaran Period covers some 50 million years of ancient time on our planet from 600 million years ago to about 542 million years ago.

Quakes reveal 'core within a core' October 2002 - BBC

Earth's early battering revealed July 2002 - BBC
The first convincing evidence that the Earth was bombarded by a devastating and prolonged storm of meteoroids and asteroids four billion years ago has been found in the Earth's oldest rocks.

Ancient rock points to life's origin BBC - July 2002

The continents were moving across the face of the Earth much sooner than had been thought, according to new evidence from China. The new data come from a huge chunk of the rock that lay beneath the sea floor 2.5 billion years ago. Tim Kusky, of St Louis University, US, says it is the first large intact piece of oceanic mantle ever found from our planet's earliest period, the Archean. Located not far from the Great Wall of China, the ancient mantle rocks are preserved in a highly faulted belt 100 kilometres (62 miles) long.