Who “dun” it?

Relative Time: Using layers of sequenced rock to find out which is older and younger Ages of events are placed in order of occurrence. No exact date is identified. Miss Miller is older than her students. Edison is an older building than Pfeiffer. WWI occurred before WWII. List one more example of relative age: ___________________________________

Finding age with relative time: 1. The Law of Superposition – In a sequence of undisturbed sedimentary rocks, the oldest rocks will be at the bottom.

The law of cross-cutting relationships: Anything that cuts across layers of rock is younger than the rocks that it has intruded into. This applies to faults and igneous intrusions. Magma that rises thru to the surface and hardens into rock is younger than the rock it is cutting through. This is usually igneous rock, or faults– breaks in rocks.

The Law of Folds or Tilts: Tilts in rocks are younger than the rocks themselves. Original Horizontality: Rocks are usually deposited flat and level.

Original horizontality

Other Guidelines for determining a sequence: Sedimentary rocks are usually formed under water. Weathering and erosion usually happen above water (on dry land). Contact metamorphism shows that the rock that was changed was there first when the intrusion happened.

Formation of an Unconformity

Unconformities This is a rough/wavy surface that represents “missing” time, separating young rock from old rock.

Layers are formed according to superposition.

Something happens to uplift the area folding faulting, etc.

Erosion wears away the uppermost layers.

Area submerges and deposition begins again. Here’s the unconformity!

The law of included fragments : The fragments that make up a rock are older than the entire sample.

Absolute Time: Identifies the exact date of an event. 65 million years ago 1995 two years older

Absolute Time Tree rings- every year a tree grows a new ring. When the tree dies, the rings can be counted and events such as forest fires can be dated. (Evidence of the same fire may be seen as a layer of ash in the rocks to correlate a rock layer with the tree rings). Tree rings can be used to date events up to 3000 years ago. Radioactive Dating Radioactive elements decay (fall apart) at steady rates. One "half- life" of time has passed when exactly half of the element remains. Half lives differ from element to element. When they fall apart they form a different material. A ratio between the original material (parent material) and the decay product (daughter material) can be used to determine how many half-lives the material has undergone.

Sequence 1: Uplift & Erosion 1. Limestone deposited 2. Sandstone deposited 3. Shale Deposited 4. Uplift 5. Erosion

Sequence 2: Faulting 1. Limestone deposited 2. Sandstone deposited 3. Shale deposited 4. Faulting

Sequence 3: Folding 1. Limestone deposited 2. Sandstone deposited 3. Shale deposited 4. Folding

Sequence 4: Igneous Intrusion 1. Limestone deposited 2. Sandstone deposited 3. Shale deposited 4. Igneous Intrusion Note: contact metamorphism is the same event as the intrusion.