Formation and Dating of Rocks

General term for layering of sedimentary rocks. It serves as a reflection of changing conditions during deposition where every strata or layer represents an interval of time where conditions have remained uniform. A change in color can reflect differences in grain size and/or composition.

• Relative Dating cannot tell us how long ago something took place, only that it followed one event and preceded another. • It means that relative dating establishes what rock is older than what rock. • Absolute Dating refers to dates that are expressed in millions and billions of years. • It establishes how old the rock actually is (eg. This rock is over a million years now.).

• Principle of Superposition • Principle of Original Horizontality • Principle of Lateral Continuity • Principle of Cross-Cutting Relationship • Principle of Inclusions • Unconformities

Nicolas Steno, a Danish anatomist, geologist, and priest (1638-1686)..

• The principle simply states that in an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below. • The younger rock is on top, while the older rocks slips below.

• It states that layers of sediment are generally deposited in a horizontal position. • Rocks in a horizontal position are considered undisturbed, while rocks that are slant were affected by deposition.

• It refers to the fact that sedimentary beds originate as continuous layers that extend in all directions until they eventually grade into a different type of sediment. • Rock layers were once connected to each other, but something big shookt them.

• It states that geologic features that cut across rocks must form after the rocks they cut through. • Primarily, Igneous rocks (dikes) were the ones who intruded. • Dikes are usually younger because they are intrusions, therefore they disturbed the older rocks.

• Inclusions are fragments of one rock unit that have been enclosed within another. • For example, when a magma intrudes a surround rock blocks of that rocks may be incorporated to the magma making them the inclusions. • Inclusions aka Xenoliths.

An unconformity represents a long period during which deposition ceased, erosion removed previously formed rocks, and then deposition resumed.