1. 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

2. 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

3. Principles of relative dating - The law of superposition

4. 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

5. 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.

6. 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

7. 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
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8. Formation of an angular unconformity

9. 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

10. 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!

11. 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!

12. Example of missing layers from a disconformity

13. 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.

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

16. 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.