Geologic Principles:
Nicolas Steno proposed several basic geologic principles in 1669 that were promoted in the 1700's by the Scottish "Father of geology"
James Hutton, who added
"The present is the key to the past."All was cast in bronze by
Charles Lyell in his 1830 book "Principles of Geology."
These basic principles are easily observed in geologic outcrops with layered rocks that illustrate the relative age of rocks and the order in which they formed as geologic events occurred.
Steno's three laws:
Law of Original Horizontality —this law states that most sediments, when originally deposited, were laid down horizontally. But many layered rocks are no longer horizontal. Faulting, tectonic forces and volcanic activity can deform rock layers.
Law of Superposition —this law states that in any undisturbed sequence of rocks deposited in layers, the youngest layer is on top and the oldest on bottom, each layer being younger than the one beneath it and older than the one above it.
Law of Cross-Cutting Relationships —this law states that a body of igneous rock (an intrusion), a fault, or other geologic feature must be younger than any rock across which it cuts through.
The Principles of Geology as accepted Today
UniformitarianismThe principle of uniformitarianism states that processes that alter the earth’s crust are the same processes that occurred millions of years ago. The results of processes today are the same as the results of the same processes millions of years ago. "The Present is the KEY to the Past."
Original HorizontalityThe principle of original horizontality states that sediment is deposited horizontally. The rocks remain horizontal until a force acts on them, pushing or pulling them out of their original orientation. Of course rocks falling on to a slanted hillside may remain for a time on a slant.
SuperpositionThis principle states that a sequence of rocks in their original orientation will have the oldest rock on the bottom and the youngest rock on the top. In order to deposit a sandstone on top of a limestone, the limestone has to already be there.
Cross-Cutting RelationshipsA rock must already be in place to be cut by a fault, igneous intrusion or erosion. By carefully examining which rock units are cut by faults or intrusions, or which rock units have been weathered, geologists can further determine the
relative ages of rocks.
Walther's Law"Sedimentary environments that started out side-by-side will end up overlapping one another over time due to transgressions and regressions."Walther’s law deals with relative space through time. It states that depositional environments that are laterally adjacent on the surface of the earth will also appear in succession in a stratigraphic sequence. If there is something missing, there is missing time, or an unconformity.
Changes in depositional environments are driven by changes in base level, or the elevation of the terminal body of water (Lake, sea or ocean level). When base level changes, the depositional environments shift to achieve a new equilibrium. If sea level falls the depositional environment shifts toward it. Where there once was a meandering river there is now a delta. If, instead,the sea level rises, it shortens the environment around it. If the beach moves towards where the ocean used to be, sea level has fallen and your sediments prograde. If the beach moves away from where the ocean used to be, sea level has risen and your sediments regress.
Unconformities Erosional boundaries between rock layers are gaps in the geologic record. An unconformity is a surface between successive strata that represents a missing interval in the geologic record of time. Unconformities show evidence that the Earth's surface may have been exposed to erosion for long or short periods of time. Unconformities are produced either by an interruption in deposition or by the erosion of per-existing strata followed by renewed deposition. Note that unconformities can be complex. For example, erosion may be taking place in one location in geologic time, where nearby or elsewhere sediments may have continued to be deposited and preserved. As a result, an unconformity in one location may span a different amount of time than that of a nearby location.
Several types of unconformity boundaries are recognized:
Nonconformity - an unconformity between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock.
Angular unconformity - an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers.
Disconformity - an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.
Conformable boundary - an arrangement where layers of sedimentary strata are parallel, but there is little apparent erosion and the boundary between two rock layer surfaces resemble a simple bedding plane. There is typically little evidence to support a significant passage of time occurred at a conformable boundary.
How Do Unconformities Form?
Unconformities are caused by Impacts or Weather, relative changes in Wind Patterns, Ice Flows, and Sea Levels over time.
A transgression occurs when a shoreline migrates landward as sea level (or lake level) rises.
A regression occurs when a shoreline migrates seaward as sea level (or lake level) falls.
Wave erosion wears away materials exposed along coastlines, scouring surfaces smooth. On scales of thousands to millions of years, shorelines may move across entire regions. Erosion strips away materials exposed to waves and currents. New (younger) material can be deposited on the scoured surface. Shallow seas may flood in and then withdrawal repeatedly. Long-lasting transgressions can erode away entire mountain ranges with enough time.
Thrust FaultsA thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks.
Faunal SuccessionNew Fossils appear over time, often exhibiting identifiable patterns or characteristics which progressively change over time. In almost every case, rock containing a certain fossil was deposited during the time that creature existed on earth.
Organic ExtinctionSpecies of plants and animals may go extinct for a variety of reasons.
Mass extinction events with widespread and rapid termination of organisms, often with following rapid increases in diversity of lifeforms, are used as major divisions in the geologic time scale. The extinction of dinosaurs at the end of the Mesozoic Era is presently the best explained extinction event.
Geochronology is the branch of earth sciences concerned with determining the age of earth materials and events through geologic time.
Relative Dating:In relative dating, we determine which things are older or younger based on their relationships, the science of determining the relative order of past events without necessarily determining their absolute age. Relative dating involves the order andstudy of fossils and the correlation or comparison of fossils of similar ages but from different regions where their age is known. Microfossils derived from sediments and cores from wells help in the subsurface exploration for oil and gas.
In many cases - for instance, igneous rocks and most metamorphic rocks do not contain fossils - the exact age of rock units is unknown until it can be confirmed by other means involving absolute dating methods.
Relative dating can only be used to sort the exposed visible features in the order that they formed
Sometimes fossils preserved in sedimentary layers are very useful for correlating rock layers from one area or region to another. Certain "index fossils" are both abundant and widely distributed through sedimentary rocks of relatively limited geologic time ranges. Care must be taken because sedimentary rock layers of different ages often look very similar. Also, sedimentary rock layers of similar ages may appear very different in other locations.
Absolute Dating:Absolute dating methods measure the physical properties of an object itself and use these measurements to calculate its age.
Tree Ring Dating is useful because ring growth changes with the weather and dead trees can be compared with live ones to get accurate dates. As of 2020, securely dated tree-ring data for the Northern Hemisphere are available going back 13,910 years. It is also used as a check on radiocarbon dating to calibrate radiocarbon ages.
Varve Dating is an absolute dating technique using thin sedimentary layers of clays called varves. The varves, have alternate light and dark bands corresponding to winter and summer deposition. By counting varves it is possible to establish an absolute time scale for fossils up to about 20 000 years ago.
One of the most useful absolute dating methods for archaeologists is called radiocarbon dating. It works by measuring carbon isotopes, which are versions of the element carbon. All isotopes of carbon have 6 protons but different numbers of neutrons. One of the carbon isotopes that occurs in nature is radioactive; it has 8 neutrons and is called carbon-14.
Developed in the late 1940s at the University of Chicago by Willard Libby, it is based on the fact that radiocarbon (14
C) is constantly being created in the Earth's atmosphere by the interaction of cosmic rays with atmospheric nitrogen. The resulting 14C combines with atmospheric oxygen to form radioactive carbon dioxide, which is incorporated into plants by photosynthesis; animals then acquire 14C by eating the plants. When the animal or plant dies, it stops exchanging carbon with its environment, and thereafter the amount of 14C it contains begins to decrease as the 14C undergoes radioactive decay.
Radiocarbon dating is generally limited to dating samples no more than 50,000 years old, as samples older than that have insufficient 14C to be measurable.
Radiometric dating:Most absolute dates for rocks are obtained with radiometric methods. These use radioactive minerals in rocks as geological clocks.
The atoms of some chemical elements have different forms, called isotopes. These break down over time in a process scientists call radioactive decay. Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter. Each isotope is identified with what is called a ‘mass number’. When ‘parent’ uranium-238 decays, for example, it produces subatomic particles, energy and ‘daughter’ lead-206.
Isotopes are important to geologists because each radioactive element decays at a constant rate, which is unique to that element. These rates of decay are known, so if you can measure the proportion of parent and daughter isotopes in rocks now, you can calculate when the rocks were formed.
Because of their unique decay rates, different elements are used for dating different age ranges. For example, the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years, and the decay of uranium-238 to lead-206 is used for rocks older than 1 million years.
At present,
zircon crystals are choice specimens for the information provided as their interior potassium-40 breaks down to argon-40 for age dating.
Yet to be accepted:
Every atom on earth came from outer space, but not all at the same time.
The most common geologic action in the universe is impact.
There have been brief catastrophes in the past.
The past is the key to understanding the future.
