Facts of the Matter
Earth rocks on thanks to diverse geology
GEOLOGY is the science of the solid Earth and the processes that modify it.
The science of geology began in the latter 18th century as a study of rock formations and their origins when James Hutton, a Scottish gentleman farmer, took an interest in the formations on his farm. In particular, he noticed something in the rock layers along the seashore at Siccar point.
The Siccar rocks are made of fine-grained sand that has been solidified into sedimentary rock. Two groups of parallel layers lie at an angle to one another with the lower layer tilted, truncated, and the upper layer lying atop it horizontally.
Having watched sediments accumulate in low-lying areas after a rainstorm, Hutton saw that sediments are laid down in flat, horizontal layers. It seemed reasonable that sediments had always been deposited in a like manner, and so something must have happened to the tilted layers to disturb them before the upper layers were deposited on top of them.
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This 80-foot-high section of exposed rock in Valaste, Estonia, Eastern Europe, shows layers dating back to the Lower Cambrian epoch, roughly 542 million years ago, a time when invertebrate sea life dominated the planet.
Hutton reckoned that it required a great amount of time to deposit the many tens of feet of sediment that were exposed in the lower layer, to solidify it, tilt it, uplift it, erode it nearly flat, deposit layer after layer on top of it, then bury it, convert it to rock, then lift it up once again and erode it to form the craggy shoreline of Siccar Point.
This stimulated in Hutton not only the beginnings of the science that would become geology, but also the first recorded conceptualization of the concept of geologic time.
Hutton went on to state two self-evident postulates upon which the science of geology has been built, and which still serve as axioms for all of the Earth sciences.
First is that geologic processes have taken place in the same way throughout time. Since geological processes are really physical and chemical processes, this is equivalent to saying that the laws of physics and chemistry have not changed over time. It is the basis for studies in astronomy and cosmology as well as geology.
The second postulate is that sediments are deposited in flat, continuous, horizontal layers with the oldest on the bottom.
From these humble beginnings, the science of geology evolved into a complex science involving virtually all of the physical, chemical, and biological processes on Earth and other planets.
Consider that except for an insignificant amount of space dust, every atom on Earth has been here from the birth of the planet 4 1/2 billion years ago. The various atoms have been combined and recombined with other atoms to concentrate certain substances such as ores through the rock cycle.
The rock cycle is a way of modeling the processes and materials of Earth's crust. Since it is a cycle, there is no absolute starting point. Melting is the most primitive process, but there are igneous rocks that have been formed from melting of previously solid rocks.
Molten rock is called magma when it is below the surface and lava if it erupts. Upon solidification, whether underground or on the surface, it becomes igneous rock. The type and texture of the resulting igneous rock depends on the composition of the magma, how fast it cools, and chemical reactions that take place during cooling.
As soon as igneous rock is exposed at the surface, it is subject to weathering. Physical weathering breaks the rock into smaller pieces, while chemical weathering alters the minerals that constitute the rock, changing them to forms that are in equilibrium with surface conditions of temperature, pressure and moisture.
Chemical weathering is a response to a change in environment from that in which the rock formed, either from molten material or at depth under pressure and temperatures much higher than where it finds itself at the surface. It always involves the chemical breakdown of the mineral components of the rock and recombination of their atoms with oxygen from the atmosphere through the action of the weak acid that forms when carbon dioxide from the atmosphere dissolves in rainwater.
Weathered rock is vulnerable to erosion by gravity, wind, running water, ice, or waves. Once it is eroded, those same agents transport it as mineral grains or in solution and eventually deposit it as sediment.
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This optical photomicrograph of a rough diamond shows the natural diamond growth surface. Below the surface and at the center is a brass-colored hexagonal shaped grain of iron sulfide surrounded by an irregular black rim. This rim is caused by internal fracture of the diamond on its ascent to Earth's surface via explosive volcanism.
Erosion of land that is continually built up by tectonic uplift or volcanic eruptions creates the landscape of hills and valleys that characterize a particular region of Earth's surface. Although the landscape does not appear to change much in our lifetime, a human lifetime is less than the blink of an eye in Earth years and the landscape changes significantly over millions of years.
Sediment becomes sedimentary rock if it is buried deeply enough to become compacted and cemented by dissolved minerals that are deposited by water trapped between the sediment grains.
Tectonic processes may eventually uplift the sedimentary rock as erosion exposes it over millions or hundreds of millions of years. Once exposed, it is subject to the weathering and erosion processes and may become part of a new series of sedimentary rocks after being transported and deposited at a new site.
If rocks are buried deeply enough they may undergo metamorphism, which is a chemical transformation that takes place at high temperature and high pressures without melting and without addition of new atoms, as is the case in chemical weathering. At great depths in Earth's crust, the temperature is high enough that the thermal motions of atoms shakes them loose from their crystal lattices, allowing them to migrate and form new minerals that are in equilibrium with the new conditions.
Metamorphic rocks form in the core of folded mountain ranges, which typically consist of sediments that were deposited offshore along with volcanic igneous rocks that got folded and deformed as they were uplifted by the collision of two tectonic plates.
As one tectonic plate slides beneath another during subduction, some rock may melt and work its way to the surface as magma, either to erupt as lava or to solidify underground.
It may seem obvious or overstated to say that everything we need such as water and air, everything we use in daily life, everything we are likely to get in the near future, and everything we see around us are all in the realm of geology.
Yet we don't think much about where it all came from when we walk on a marble floor inside a concrete and steel building wearing shoes containing carbon-based polymers, when we type on a plastic computer keyboard to issue commands to a silicon chip, or drive a car made from metal, plastic, and rubber.
However you choose to look at it, geology is everywhere!
Richard Brill, professor of science at Honolulu Community College, teaches earth and physical science and investigates life and the universe. E-mail questions and comments to firstname.lastname@example.org