Historical geology or palaeogeology is a discipline that uses the principles and methods of geology to reconstruct the geological history of Earth.[1] Historical geology examines the vastness of geologic time, measured in billions of years, and investigates changes in the Earth, gradual and sudden, over this deep time. It focuses on geological processes, such as plate tectonics, that have changed the Earth's surface and subsurface over time and the use of methods including stratigraphy, structural geology, paleontology, and sedimentology to tell the sequence of these events. It also focuses on the evolution of life during different time periods in the geologic time scale.[2]
18th-century geologist James Hutton contributed to an early understanding of the Earth's history by proposing the theory of uniformitarianism, which is now a basic principle in all branches of geology. Uniformitarianism describes an Earth formed by the same natural phenomena that are at work today, the product of slow and continuous geological changes.[4][5] The theory can be summarized by the phrase "the present is the key to the past."[6] Hutton also described the concept of deep time. The prevailing conceptualization of Earth history in 18th-century Europe, grounded in a literal interpretation of Christian scripture, was that of a young Earth shaped by catastrophic events. Hutton, however, depicted a very old Earth, shaped by slow, continuous change.[7]Charles Lyell further developed the theory of uniformitarianism in the 19th century.[8] Modern geologists have generally acknowledged that Earth's geological history is a product of both sudden, cataclysmic events (such as meteorite impacts and volcanic eruptions) and gradual processes (such as weathering, erosion, and deposition).
The discovery of radioactive decay in the late 19th century and the development of radiometric dating techniques in the 20th century provided a means of deriving absolute ages of events in geological history.
Use and importance
Geology is considered a historical science; accordingly, historical geology plays a prominent role in the field.[9]
Historical geology covers much of the same subject matter as physical geology, the study of geological processes and the ways in which they shape the Earth's structure and composition. Historical geology extends physical geology into the past.[1]
Fossils are organic traces of Earth's history. In a historical geology context, paleontological methods can be used to study fossils and their environments, including surrounding rocks, and place them within the geologic time scale.
Sedimentology
Sedimentology is the study of the formation, transport, deposition, and diagenesis of sediments. Sedimentary rocks, including limestone, sandstone, and shale, serve as a record of Earth's history: they contain fossils and are transformed by geological processes, such as weathering, erosion, and deposition, through deep time.
Historical geology makes use of relative dating in order to establish the sequence of geological events in relation to each another, without determining their specific numerical ages or ranges.[10]
The theory of plate tectonics explains how the movement of lithospheric plates has structured the Earth throughout its geological history.[12]
Weathering, erosion, and deposition
Weathering, erosion, and deposition are examples of gradual geological processes, taking place over large sections of the geologic time scale. In the rock cycle, rocks are continually broken down, transported, and deposited, cycling through three main rock types: sedimentary, metamorphic, and igneous.
Paleoclimatology
Paleoclimatology is the study of past climates recorded in geological time.
Geology is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth sciences, including hydrology. It is integrated with Earth system science and planetary science.
The geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy and geochronology. It is used primarily by Earth scientists to describe the timing and relationships of events in geologic history. The time scale has been developed through the study of rock layers and the observation of their relationships and identifying features such as lithologies, paleomagnetic properties, and fossils. The definition of standardised international units of geologic time is the responsibility of the International Commission on Stratigraphy (ICS), a constituent body of the International Union of Geological Sciences (IUGS), whose primary objective is to precisely define global chronostratigraphic units of the International Chronostratigraphic Chart (ICC) that are used to define divisions of geologic time. The chronostratigraphic divisions are in turn used to define geochronologic units.
James Hutton was a Scottish geologist, agriculturalist, chemical manufacturer, naturalist and physician. Often referred to as the "Father of Modern Geology," he played a key role in establishing geology as a modern science.
Uniformitarianism, also known as the Doctrine of Uniformity or the Uniformitarian Principle, is the assumption that the same natural laws and processes that operate in our present-day scientific observations have always operated in the universe in the past and apply everywhere in the universe. It refers to invariance in the metaphysical principles underpinning science, such as the constancy of cause and effect throughout space-time, but has also been used to describe spatiotemporal invariance of physical laws. Though an unprovable postulate that cannot be verified using the scientific method, some consider that uniformitarianism should be a required first principle in scientific research. Other scientists disagree and consider that nature is not absolutely uniform, even though it does exhibit certain regularities.
Neptunism is a superseded scientific theory of geology proposed by Abraham Gottlob Werner (1749–1817) in the late 18th century, who proposed that rocks formed from the crystallisation of minerals in the early Earth's oceans.
In geology, catastrophism is the theory that the Earth has largely been shaped by sudden, short-lived, violent events, possibly worldwide in scope. This contrasts with uniformitarianism, according to which slow incremental changes, such as erosion, brought about all the Earth's geological features. The proponents of uniformitarianism held that the present was "the key to the past", and that all geological processes throughout the past resembled those that can be observed today. Since the 19th-century disputes between catastrophists and uniformitarians, a more inclusive and integrated view of geologic events has developed, in which the scientific consensus accepts that some catastrophic events occurred in the geologic past, but regards these as explicable as extreme examples of natural processes which can occur.
Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is provided by tools such as paleomagnetism and stable isotope ratios. By combining multiple geochronological indicators the precision of the recovered age can be improved.
Deep time is a term introduced and applied by John McPhee to the concept of geologic time in his book Basin and Range (1981), parts of which originally appeared in the New Yorker magazine.
Denudation is the geological processes in which moving water, ice, wind, and waves erode the Earth's surface, leading to a reduction in elevation and in relief of landforms and landscapes. Although the terms erosion and denudation are used interchangeably, erosion is the transport of soil and rocks from one location to another, and denudation is the sum of processes, including erosion, that result in the lowering of Earth's surface. Endogenous processes such as volcanoes, earthquakes, and tectonic uplift can expose continental crust to the exogenous processes of weathering, erosion, and mass wasting. The effects of denudation have been recorded for millennia but the mechanics behind it have been debated for the past 200 years and have only begun to be understood in the past few decades.
Relative dating is the science of determining the relative order of past events, without necessarily determining their absolute age. In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another. Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating, archaeologists and geologists used relative dating to determine ages of materials. Though relative dating can only determine the sequential order in which a series of events occurred, not when they occurred, it remains a useful technique. Relative dating by biostratigraphy is the preferred method in paleontology and is, in some respects, more accurate. The Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.
Plutonism is the geologic theory that the igneous rocks forming the Earth originated from intrusive magmatic activity, with a continuing gradual process of weathering and erosion wearing away rocks, which were then deposited on the sea bed, re-formed into layers of sedimentary rock by heat and pressure, and raised again. It proposes that basalt is solidified molten magma. The theory lead to plutonic (intrinsic) rock classification, which includes intrinsic igneous rocks such as gabbro, diorite, granite and pegmatite. The name plutonism references Pluto, the classical ruler of the underworld and the Roman god of wealth. A main reason Pluto was incorporated into the classification was due to the plutonic rocks commonly being present in gold and silver ore deposits (veins).
Siccar Point is a rocky promontory in the county of Berwickshire on the east coast of Scotland. It is famous in the history of geology for Hutton's Unconformity found in 1788, which James Hutton regarded as conclusive proof of his uniformitarian theory of geological development.
Time's Arrow, Time's Cycle: Myth and Metaphor in the Discovery of Geological Time is a 1987 history of geology by the paleontologist Stephen Jay Gould, in which the author offers a historical account of the conceptualization of Deep Time and uniformitarianism using the works of the English theologian Thomas Burnet, and the Scottish geologists James Hutton and Charles Lyell.
In geology, a bed is a layer of sediment, sedimentary rock, or volcanic rock "bounded above and below by more or less well-defined bedding surfaces". Specifically in sedimentology, a bed can be defined in one of two major ways. First, Campbell and Reineck and Singh use the term bed to refer to a thickness-independent layer comprising a coherent layer of sedimentary rock, sediment, or pyroclastic material bounded above and below by surfaces known as bedding planes. By this definition of bed, laminae are small beds that constitute the smallest (visible) layers of a hierarchical succession and often, but not always, internally comprise a bed.
The history of geology is concerned with the development of the natural science of geology. Geology is the scientific study of the origin, history, and structure of the Earth.
Principles of Geology: Being an Attempt to Explain the Former Changes of the Earth's Surface, by Reference to Causes Now in Operation is a book by the Scottish geologist Charles Lyell that was first published in 3 volumes from 1830 to 1833. Lyell used the theory of uniformitarianism to describe how the Earth's surface was changing over time. This theory was in direct contrast to the geological theory of catastrophism.
Earth Revealed: Introductory Geology, originally titled Earth Revealed, is a 26-part video instructional series covering the processes and properties of the physical Earth, with particular attention given to the scientific theories underlying geological principles. The telecourse was produced by Intelecom and the Southern California Consortium, was funded by the Annenberg/CPB Project, and first aired on PBS in 1992 with the title Earth Revealed. All 26 episodes are hosted by Dr. James L. Sadd, professor of environmental science at Occidental College in Los Angeles, California.
Hutton's Unconformity is a name given to various notable geological sites in Scotland identified by the 18th-century Scottish geologist James Hutton as places where the junction between two types of rock formations can be seen. This geological phenomenon marks the location where rock formations created at different times and by different forces adjoin. For Hutton, such an unconformity provided evidence for his Plutonist theories of uniformitarianism and the age of Earth.
A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, although backgrounds in physics, chemistry, biology, and other sciences are also useful. Field research is an important component of geology, although many subdisciplines incorporate laboratory and digitalized work. Geologists can be classified in a larger group of scientists, called geoscientists.
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