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Half-life

Time

Figure 2.2. As isotopic decay curve. Knowing the amount of unstable isotope that was originally present, as well as the amount of unstable isotope now present and the rate of decay of the unstable isotope, it is possible to determine the age of a rock with that isotope in it.The diagram shows the half-life; that is, the amount of time that it takes for 50% of an unstable isotope to decay.

(87Rb/87Sr), with its half-life of 48.8 billion years, would hardly be the ideal isotopic system. This would be a bit like timing a 100 m dash with a sundial. Likewise, dating dinosaur bones (ages that would be in the hundreds of millions of years) using 14C, with a half-life of 5,730 years, would be like giving your own age in milliseconds.

Radiogenic isotopes are powerful tools but they can't be used directly on dinosaur bone. There has to be a source of radioactive isotopes, which occur commonly in igneous rocks, such as those blasted from a volcano. Then the age of formation of the igneous rock can be obtained. Is it any wonder that artists always seem to depict dinosaurs with active volcanoes in the background? Still, no dinosaur lived within an active volcano, and so the challenge is to correlate in time the dinosaur bone with the volcanic event that produced the datable igneous rock. This - the relationships of one body of rock to another - is the province of lithostratigraphy.

Sediments - sand, silt, mud, dust, and other less-familiar materials -are deposited in time and in space (geographically). Deposition occurs in strata that can be broad and sheet like or narrow or ribbon shaped, and wedged at the edges, where each layer becomes progressively thinner until it is said to "pinch out." These shapes occur on a scales of meters to hundreds of kilometers, and are the direct result of sedimentation such as flowing water, wind, or explosion from a volcano, to name a few more or less common processes. Virtually every geographical location we can think of - a river, a desert, a lake, an estuary, a mountain, the bottom of the ocean, the pampas - has sedimentary processes peculiar to it that will produce distinctive sediments and, with time and burial, distinctive sedimentary rocks.

Lithostratigraphy Sedimentation and sedimentary rocks

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