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With this in mind geologist have long known that the deeper a sedimentary rock layer is the older it is, but how old?
Although there might be some mineral differences due to the difference in source rock, most sedimentary rock deposited year after year look very similar to one another.
Using this time scale, geologists can place all events of Earth history in order without ever knowing their numerical ages.
The specific events within Earth history are discussed in Chapter 8.
via Wikimedia Commons' src=" alt="Chart showing the names of the unit of the Geooogic Time Scale" width="300" height="300" srcset=" sizes="(max-width: 300px) 85vw, 300px"/ is the process of determining if one rock or geologic event is older or younger than another, without knowing their specific ages—i.e., how many years ago the object was formed.
The principles of relative time are simple, even obvious now, but were not generally accepted by scholars until the scientific revolution of the 17th and 18th centuries via Wikimedia Commons' src=" Superposition-228x300.jpg" alt="Photo of superposed strata with the younger on top of the older" width="270" height="355" srcset=" Superposition-228x300228w, Superposition-768x1009768w, Superposition-780x1024780w, Superposition-1200x15761200w, 1392w" sizes="(max-width: 270px) 85vw, 270px"/CC BY 2.0], via Wikimedia Commons' src=" alt="Photo of rock outcrop with a dike cutting through an older rock and another dike cutting across that one." width="215" height="287" srcset=" sizes="(max-width: 215px) 85vw, 215px"/ Wikimedia Commons, via Wikimedia Commons' src=" alt="Photo of the Grand Canyon showing expanse of canyon and the various rock layers" width="392" height="261" srcset=" sizes="(max-width: 392px) 85vw, 392px"/CC BY-SA 3.0], via Wikimedia Commons' src=" alt="The red rocks are layered, the dark rocks are not." width="300" height="225" srcset=" sizes="(max-width: 300px) 85vw, 300px"/All three of these formations have a disconformity at the two contacts between them.
CC BY-SA 4.0], via Wikimedia Commons' src=" alt="It shows two paths" width="254" height="300" srcset=" sizes="(max-width: 254px) 85vw, 254px"/ is when a proton in the nucleus captures an electron from one of the electron shells and becomes a neutron.
This produces one of two different effects: 1) an electron jumps in to fill the missing spot of the departed electron and emits an X-ray, or 2) in what is called the Auger process, another electron is released and changes the atom into an An atom that has different number of neutrons but the same number of protons.
This also happens to be the nucleus of a helium atom; helium gas may get trapped in the crystal lattice of a CC BY-SA 4.0], via Wikimedia Commons' src=" alt="Simplified Periodic Table of the Elements" width="477" height="254" srcset=" sizes="(max-width: 477px) 85vw, 477px"/An atom that has different number of neutrons but the same number of protons.
Using this process geologists are able to assign actual ages with known degrees of error to specific geologic events.
By combining knowledge gained using both relative and absolute dating processes geologists have been able to produce the geologic time scale.
Unlike relative time, absolute time assigns specific ages to events or formations and is typically recorded in years before present.
This process requires much more sophisticated chemical analysis and, although other processes have been developed, often utilizes the decay rates of radioactive isotopes to determine the age of a given material.
Common accessory mineral in igneous and metamorphic rocks, as well as detrital sediments.