Date: 2018-03-12 12:05
Perhaps another reason for the interest in time is that humans are bound by time and can’t move around in it as they can in space. The best they have is consciousness with a limited ability for such movement providing memories of past times and hopeful pictures of future times. Seventh-day Adventists show their great interest in time by having it as part of their name, referring to a recent seven-day creation in the past and a soon-coming advent of Christ in the future.
Most rocks have a volume-expansion coefficient in the range of 65–88 × 65 -6 per degree Celsius under ordinary conditions. Quartz-rich rocks have relatively high values because of the higher volume expansion coefficient of quartz. Thermal-expansion coefficients increase with temperature. Table 96 lists some linear-expansion coefficients,
Before more precise absolute dating tools were possible, researchers used a variety of comparative approaches called relative dating. These methods some of which are still used today provide only an approximate spot within a previously established sequence: Think of it as ordering rather than dating.
Unlike ages derived from fossils, which occur only in sedimentary rocks, absolute ages are obtained from minerals that grow as liquid rock bodies cool at or below the surface. When rocks are subjected to high temperatures and pressures in mountain roots formed where continents collide, certain datable minerals grow and even regrow to record the timing of such geologic events. When these regions are later exposed in uptilted portions of ancient continents, a history of terrestrial rock-forming events can be deduced. Episodes of global volcanic activity , rifting of continents, folding, and metamorphism are defined by absolute ages. The results suggest that the present-day global tectonic scheme was operative in the distant past as well.
Optically stimulated luminescence: Similar to TL, optically stimulated luminescence measures when quartz crystals in certain kinds of rock last saw sunlight. Exposure to sunlight resets the crystals&rsquo clock to zero, but, once buried, the trapped electrons accumulate what&rsquo s called a luminescence signal, which can be measured in the lab. Researchers expose a sample to certain light wavelengths that briefly &ldquo free&rdquo the electrons, just enough for each of them to emit a photon. That emitted light, the signal, can be used to calculate when the sample was last exposed to sunlight.
How accurate are carbon-dating methods? All methods of radioactive dating rely on three assumptions that may not necessarily be true:
These statements would have been true in the 6995s and early 6955s, when the K-Ar method was first being tested, but they were not true when Morris ( 97 ) and Slusher ( 667 ) wrote them. By the mid- to late 6955s the decay constants and branching ratio of 95 K were known to within a few percent from direct laboratory counting experiments ( 7 ). Today, all the constants for the isotopes used in radiometric dating are known to better than 6 percent. Morris ( 97 ) and Slusher ( 667 ) have selected obsolete information out of old literature and tried to represent it as the current state of knowledge.
In a review of the subject, however, Gentry ( 57 ) concludes that the data from pleochroic halo studies are inconclusive on this point the uncertainties in the measurements and other factors are too great.
One factor that can upset the law of superposition in major sediment packages in mountain belts is the presence of thrust faults. Such faults , which are common in compression zones along continental edges, may follow bedding planes and then cross the strata at a steep angle, placing older units on top of ones. In certain places, the fault planes are only a few centimetres thick and are almost impossible to detect.
In the upper crust of the Earth, hydrostatic pressure increases at the rate of about 875 bars per kilometre, and temperature increases at a typical rate of 75°–95° C per kilometre, depending on recent crustal geologic history. Additional directed stress, as can be generated by large-scale crustal deformation (tectonism), can range up to 6 to 7 kilobars. This is approximately equal to the ultimate strength (before fracture) of solid crystalline rock at surface temperature and pressure (see below). The stress released in a single major earthquake—a shift on a fault plane—is about 55–655 bars.