Determining geological ages Relative age dates – placing rocks and events in their proper sequence of formation Numerical dates – specifying the actual number of years that have passed since an event occurred (known as absolute age dating, radiometric dating) Determining age is difficult

Principles of relative dating Law of superposition Developed by Nicolaus Steno in 1669 In an undeformed or altered sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom

Principles of relative dating - The law of superposition

Principles of relative dating Principle of original horizontality Layers of sediment are generally deposited in a horizontal position Rock layers that are flat have not been disturbed

Principles of relative dating Principle of cross-cutting relationships Younger features cut across older feature Inclusions - a piece of rock that is enclosed within another rock Fault - A break in a rock mass along which movement has occurred Dike - A tubular-shaped intrusive igneous feature that cuts through the surrounding rock.

UNCONFORMITIES An unconformity is a break in the rock record produced by erosion and/or nondeposition of rock units. These time gaps in the rock layers, are typically on the order of tens of millions of years or more. Nondeposition - means that no sediments were deposited for an interval of time. uplift and erosion can remove layers deposited at an earlier time. Unconformities indicate: Major sea level changes Major tectonic events have occurred

Types of unconformities Angular unconformity – tilted rocks are overlain by flat-lying rocks Disconformity – strata on either side of the unconformity are parallel Nonconformity – metamorphic or igneous rocks in contact with sedimentary strata These sound ways in which freshman students rebell against strict teachers

Formation of an angular unconformity

Nonconformity A nonconformity is a place where younger sedimentary layers lie on top of older igneous or metamorphic rocks. The older igneous or metamorphic rocks indicate active igneous and/or tectonic period, followed by uplift and erosion The overlying sedimentary rocks indicate a time of deposition of sedimentary layers

Nonconformity Nonconformity, Grand Canyon, Arizona. In this photo, Cambrian sandstone overlies Proterozoic schist (Metamorphic rock high in mica), the erosional surface between the two represents more than a billion years of geologic time!

DISCONFORMITY A disconformity is a type of gap in the rock layers where the above and below an erosional boundary have the same orientation (often horizontal) Disconformities may be difficult to recognize However, as with all unconformities, disconformities involve a significant time gap-- typically on the order of tens of millions of years!

Example of missing layers from a disconformity

DISCONFORMITY All of the units above the Grand Canyon Series are horizontal. Yet, a significant disconformity exists between the Cambrian Muav Limestone and the Mississippian Redwall Limestone.

A, B, C = Intrusive - molten rocks that cuts through the pre existing rocks.

1. Disconformity-are irregular erosional surfaces between parallel layers of rock. 2. Angular unconformity- formed when rock layers are tilted or folded before being eroded. 3. Nonconformity- places where sedimentary layers lie on top of igneous or metamorphic rocks.