Relative Dating

Relative Dating Relative Age: the age of something compared to something else. Absolute Age: the actual age of a rock or an event Relative age = older, younger, etc. Absolute age = in years

The Principal of Superposition In undisturbed sedimentary rock layers (strata), the oldest rock is at the bottom. YOUNGEST rock layer Exceptions: deformed rock layers (folded layers, faults, etc.) – draw pictures OLDEST rock layer

Dating Intrusions, Extrusions, & Inclusions Intrusion: when magma squeezes into preexisting rocks and cools. YOUNGER than any rock it cuts through Intrusion = cm to 100s of kilometers

Dating Intrusions, Extrusions, & Inclusions Extrusion: when lava flows on the SURFACE and cools. YOUNGER than rocks beneath it OLDER than rocks above it. Lava flows, volcanoes,

Dating Intrusions, Extrusions, & Inclusions Inclusion: a piece of older rock within igneous rock. Magma rises to surface – pieces of rock that it is intruding into can break off and fall into magma. If magma is cool enough, rock will not melt, and stay a part of the magma.

Cross-Cutting Relationships Types of Cross-cutting relationships: Intrusions, faults, veins, etc. These features are YOUNGER than the rocks they cut across.

Cross-Cutting Relationships A fault is younger than the rock layers it cuts across An igneous intrusion is younger than the rock it penetrates

Dating Rock Features The folding and tilting of rock layers are events that are younger than the rock layers they effect.

Unconformity – A surface of erosion between rock layers that represents a missing gap in Earth’s history. Uplifting and folding Deposition Unconformity Subsidence Erosion

Correlation The process of showing that rock layers, or geologic events, from different places are the same, or similar in age. Types of Correlation: Exposed bedrock Similarities in Rocks Use of Index Fossils Volcanic Ash or Meteorite Deposits Helps to determine the sequence of geologic events in an area. Helps to find certain mineral resources, such as fossil fuels, bc they’re only found in rocks of specific age.

1. Correlation by Exposed Bedrock Bedrock is usually covered with soil, but can occasionally be exposed. Correlation can be accomplished by directly following the rock layers. River valley, road cut, etc. OUTCROPS – exposed layers of bedrock

2. Correlation by Similarities in Rocks Rocks can be correlated by having similar appearance, color, mineral composition, and rock sequence. Only valid in small areas May be incorrect – similar rocks can form in similar envt millions of years apart.

3. Correlation by Index Fossils Index Fossil: a fossil that is used to date the rock layers in which it is found. An organism that lived during a relatively short, well-defined time span. Have a wide distribution geographically.

Which of these is an INDEX FOSSIL?

4. Correlation by Volcanic Ash and Meteorite Deposits Volcanic Ash: ash from each volcanic eruption has unique composition Ash deposits are widely distributed, and represent a small period of time. Meteorite Deposits: rock particles and debris produced by an asteroid, comet, or meteorite impact can cover large areas of Earth. Specific ash deposits can be detected in rock layers. Specific age markers in rocks and glacial ice 1000s of km apart. EX: 65mya, meteorite hit Yucatan Peninsula in Mexico – Debris can be found in ALL of earth’s continents

“SEQUENCE OF EVENTS” PRACTICE WORKSHEETS

Sequence 1: Uplift & Erosion Limestone deposited Sandstone deposited Shale Deposited Uplift Erosion

Sequence 2: Faulting Limestone deposited Sandstone deposited Shale deposited Faulting

Sequence 3: Folding Limestone deposited Sandstone deposited Shale deposited Folding

6 5 7 4 6(erosion) 4 4 Folding 5(tilting) 3 3 3 2 2 2 1 1 1 Oldest- Shale Youngest- Folding Oldest- Shale Youngest- Siltstone Oldest- Sandstone Youngest- Shale Oldest- Limestone Youngest- Igneous Intrusion Oldest- Sandstone Youngest- Igneous Intrusion Oldest- Shale Youngest- Fault 4 4 4 6 3 3 5 3 2 2 2 5 6 5 1 1 12 1

5 4 6 3 2 7 1 Oldest- Siltstone Youngest- Igneous Intrusion link unconformity formation 13