| Geology (from Greek γη- (ge-, "the earth")
and λογος (logos, "word", "reason")) is the science
and study of the Earth, its composition, structure, physical properties, history, and the
processes that shape it.
Geologists have helped establish the age of the Earth at about 4.5 billion (4.5x109) years, and have determined that the Earth's
crust is fragmented into tectonic plates that move over a semi-molten 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 oil and coal, as well as metals such as iron, copper, and
uranium. Additional economic interests include minerals such as asbestos, perlite, mica, phosphates, zeolites, clay, pumice, quartz, and silica, as well as elements such as
sulfur, 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-André Deluc in the
year 1778 and introduced as a fixed term by Horace-Bénédict 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.
History
Georg Agricola (1494-1555) 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.
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.
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.
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.
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.
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.
Important Principles of Geology
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 sedementary 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 occurrs 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 fundemental 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 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 independent 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.
Fields or related disciplines
Regional geology
National geology
Planetary geology
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