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21 Jul

Superposition of rock units is a very simple and straightforward method of relative age determination.The principle states that in a sequence of undeformed sedimentary rocks the oldest beds are at the bottom and the youngest ones are at the top. Angular unconformity – tilted rocks are overlain by flat-lying rocks b. William Smith (late1700s–early 1800s) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content 2.Disconformity – strata on either side are parallel c. Principle of fossil succession – fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content a.Relative time can not determine the actual year a material was deposited or how long deposition lasted; it simply tell us which events came first.is a way to use geometric relationships between rock bodies to determine the sequence of geologic events in an area.

By using the principles of relative geologic age, the sequence of geologic events -- what happened first, what happened next, what happened last -- can be established.

Absolute geologic age refers to how long ago a geologic event occurred or a rock formed, in numeric terms, such as 65.5 million years ago.

Some rocks and minerals can have their absolute age directly measured by analyzing the ratios of certain radioactive and non-radioactive isotopes they contain.

The units commonly used for geologic age are mega-annum (Ma) for millions of years, giga-annum (Ga) for billions of years, and kiloannum (ka) ka for thousands of years.

Because these units are used according to the rules of the metric system, the M in Ma and the G in Ga must be capitalized, and the k in ka must not be capitalized.