The Geologic Time Scale contains a chapter about planetary time scales, written by Ken Tanaka and Bill Hartmann. Paleontologists have examined layered sequences of fossil-bearing rocks all over the world, and noted where in those sequences certain fossils appear and disappear. Even though it is far away from the nearside basalts, geologists can use crater statistics to determine whether it erupted before, concurrently with, or after nearside maria did. The Orientale impact happened shortly after the Imbrium impact, and that was pretty much it for major basin-forming impacts on the Moon. The other way we use craters to age-date surfaces is simply to count the craters. But it's really not nearly as neat as the crisp lines on this diagram make it seem. The chapter draws on five decades of work going right back to the origins of planetary geology.
The science of paleontology, and its use for relative age dating, was well-established before the science of isotopic age-dating was developed. When lava flows overlap, it's not too hard to use the law of superposition to tell which one is older and which one is younger. Missing time that was not recorded in the layers of the rock. There are other people who argue that the rocks we think are from the Nectaris are either actually from Imbrium or were affected by Imbrium, so that we don't actually know when Nectaris happened and consequently can't say for sure whether the Late Heavy Bombardment happened. Relative age dating has given us the names we use for the major and minor geologic time periods we use to split up the history of Earth and all the other planets. Relative datingThe process of determining whether an event or object is older or younger than other events or objects. Paleontologists have used major appearances and disappearances of different kinds of fossils on Earth to divide Earth's history -- at least the part of it for which there are lots of fossils -- into lots of eras and periods and epochs.
The Copernican period is the most recent one; Copernican-age craters have visible rays. He concluded, after studying rocksat many outcrops, that each layer represented a specific interval of geologic time. Tanaka and Hartmann suggest that the decline in mare volcanism -- and whatever impact crater density is associated with the last gasps of mare volcanism -- would be a better marker than any one impact crater. From the results of studies on the origins of the various kinds of rocks petrology , coupled with studies of rock layering stratigraphy and the evolution of life paleontology , geologists reconstruct the sequence of events that has shaped the Earth's surface. We have no idea how much older thing B is, we just know that it's older.
Older literature divides the Tertiary into epochs from oldest to newest : Paleocene, Eocene, Oligocence, Miocene, and Pliocene. Volcanoes can spew out large lava deposits that cover up old cratered surfaces, obliterating the cratering record and resetting the crater-age clock. Such recurring events as mountain building and sea encroachment, of which the rocks themselves are records, comprise units of geologic time even though the actual dates of the events are unknown. Of course the real world is never quite so simple. Put simply, if a geologist says to be younger than his or her colleague, that is a relative dating. Further, he proposed that wherever uncontorted layers were exposed, the bottom layer was deposited first and was, therefore, the oldest layer exposed; each succeeding layer, up to the topmost one, was progressively younger. Technically, relative dating is the determining of past events without taking into account the absolute scientific estimated age.
There are some uncertainties in the positions of the boundaries of the units. SuperpositionPrinciple that states that younger rocks lie above older rocks, if the layers have not been disturbed. Source text and graphic : University of Calgary, Geology and Geophysics Department. During the same interval of time in another part of the world, the land surface subsided and was covered by the seas. I absolutely hate absolute ages in planetary science, because their precision is illusory, even for a place like the Moon for which we have quite a lot of returned samples.
Geologists use the geologic column to:Interpret rock sequences Identify the layers in puzzling rock sequences. So the Imbrian period is divided into the Early Imbrian epoch -- when Imbrium and Orientale formed -- and the Late Imbrian epoch -- when most mare volcanism happened. Red marks individual impact basins. The oldest couple of chronostratigraphic boundaries are defined according to when two of the Moon's larger impact basins formed: older Nectaris and younger Imbrium. Any surface that has Imbrium ejecta lying on top of it is older than Imbrium.
Geological Survey examining the fossil bones ofPaleoparadoxia, an aquatic mammal that lived about 14 million years ago. Some rock sequences have been disturbed by forces within the Earth. These subdivisions are given names, most of which can be recognized globally, usually on the basis of fossils. Imbrium is therefore a stratigraphic marker -- something we can use to divide the chronostratigraphic history of the Moon. And we can pretty confidently date mare volcanism for each of the Apollo and Luna landing sites -- that was happening around 3.
This geological time scale is based upon Harland et al. Few discussions in geology can occur without reference to geologic time. Here is the same diagram I showed above, but this time I've squished and stretched parts of it to fit a linear time scale on the right. Most of the events on the list could move up and down the absolute time scale quite a lot as we improve our calibration of the relative time scale. When you find the same fossils in rocks far away, you know that the sediments those rocks must have been laid down at the same time. Hutton,a Scottish geologist, first proposed formally the fundamental principle used toclassify rocks according to their relative ages. By comparison, the history of mankind is similarly organized into relative units of time.
If the ages are so uncertain for the Moon, what about the ages of Mars and Mercury? Most Earth geologists don't talk about that much. The Moon's history is divided into pre-Nectarian, Nectarian, Imbrian, Eratosthenian, and Copernican periods from oldest to youngest. Identify two events and two features that disrupt rock layers. Vast quantities of lava erupted onto the Moon's nearside, filling many of the older basins with dark flows. As you move from the top to the bottom in layers of sedimentary rock, the lower layers are older. Almost all the cratering happened in the bottom bit of the diagram. After the uplift of the land, the forces of erosion attacked the highlands and the eroded rock debris was transported and redeposited in the lowlands.