Relative Dating

Warm up List and describe as many ways that you think scientists used to figure out the age of the earth and what happened in the history of Earth, before they were able to date the rocks and fossils with technology. Think about what kinds of things they found in the rocks and what clues they look for in rock formations around the world.

Vocabulary Erosion- The process known as weathering breaks up rocks so that they can be carried away by the process known as erosion. Water, wind, ice, and waves are the agents of erosion that wear away at the surface of the earth. Deposition- when previously eroded material is carried by wind or water or moved by gravity to another location and is deposited. This builds layers of sedimentary rocks

Do now Describe and/or diagram, in your notebook on the left hand side, a place or system on the Earth where there would be erosion followed by the eroded material being transported and deposited in another area.

Relative Dating of Rock Layers To figure out the age of rocks before we had the technology, we needed to find other means. We call this relative dating. By using the law of superposition and original horizontality we can assume the sequence would be as follows: Layer 1 was laid down Layer 2 was laid down Layer 3 was laid down Layer 4 was laid down

Law of Superpostion In a layered, series of sedimentary rock beds or lava flows that have been deposited on top of each other, the material being deposited must be younger than the material it is deposited on. This means that the layers are older as you go down in the rock layers

Question Where in your life have you had to understand the Law of Super position other than looking at rock layers? Example: When you create the sand in a glass characters, you know that you have to pour the colors in order that you want them from the bottom up.

Law of Original Horizontality Most sediments, when originally formed, were laid down horizontally because gravity lays them horizontally when they are in the fluid state (broken sediment/lava). Many layered rocks are no longer horizontal. Because of the Law of Original Horizontality, it can be determined that rocks that are not horizontal either were formed in a unique way or were moved from their original horizontal position by events, such as tilting or folding during mountain building at later times. This can sometimes be confusing for geologists if they only see a sample of the layers. Figure C and D would show the layers in an incorrect position to identify the oldest.

Unconformities Gaps in the sequence of layers at a particular location Can be caused for two reasons: Erosion of a layer, represented in a diagram by an irregular or jagged line or a U shaped stream bed. During a certain period of time, while layers of sediment were being deposited elsewhere, no layers were deposited at the location in question At location C, layers 1 through 5 were deposited and remained intact. The rock record is complete. At location A, layers 1 and 2 were deposited. However, during times 3 and 4, no layers were deposited. During time 5, deposition resumed, and layer 5 was deposited. At location B, layers 1 through 3 were deposited. During time 4, all of layer 3 plus the upper part of layer 2 were removed by erosion. During time 5, deposition resumed, with layer 5 being deposited on top of what remained of layer 2.

Warm up- Day Two Using the three vocabulary words, write a small summary in your own words or diagram each of the three vocab words Law of Superposition Law of Original Horizontality Unconformity

Intrusions Rock layers and lava flows may be intruded by sheets of magma to form rock layers (sills) parallel to the rock layers they intrude. A Sill is YOUNGER than the layers it lays in between. Evidence would be contact metamorphism of both layers above and below the sill (29 and 30) from the heat of the magma touching them In the field, it is likely that the connection between the sill and the magma chamber will not be exposed Lava flows and sills strongly resemble each other: both may be layers, both may have similar textures and mineralogy. If sills and lava flows are wrongly identified, age relationships will be wrongly interpreted.

The Law of Biotal Succession (Fossils) Sedimentary rocks frequently contain objects that have been interpreted as evidence that life existed at the time the sediment accumulated called fossils. Identifying the type of organism, and knowing the time frame that this organism existed can assist in determining a relative age of the rock it is found in.

Law of Cross Cutting Relationships Examples of the Law: If a rock is cut by a fracture or fault, the fracture is younger than the rock. After all, how could the rock be fractured if it wasn't already there? Where a body of igneous rock A intrudes some other rock B the intruding rock A must be younger than B. Where a rock is cut by an erosion surface, the erosion surface is younger than the rock it cuts.

Law of Inclusions Any rock, mineral or fossil that is entirely contained within another rock is older than the rock that contains it. The inclusion happens to get “stuck” in the early stages as the rock is forming Quartz Crystal