Relative Dating

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

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, depositional sequence from a series of sedimentary beds or lava flows, the material on which any layer is deposited is older than the layer itself. This means that the layers are successively younger, going from bottom to top.

Law of Original Horizontality Most sediments, when originally formed, were laid down horizontally. However, many layered rocks are no longer horizontal. Because of the Law of Original Horizontality, we know that sedimentary rocks that are not horizontal either were formed in special ways or, more often, were moved from their horizontal position by later events, such as tilting or folding during episodes of mountain building. 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.

Intrusions Piles of sedimentary rock layers and lava flows may be intruded by sheets of magma that crystallize to form igneous rock layers (sills) parallel to the rock layers they intrude. The Sill from the magma chamber “intruded” between the rock layers 29 and 30 after they were laid down Evidence would be contact metamorphism of both layers 29 and 30 from the heat of the magma touching them The question would be whether Lava Flow B and Layer 31 were laid after the Sill intruded or before. 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 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 that was fossilized and knowing the approximated 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: Where 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 (or 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