The Relative Age Of Rocks Chapter 8 Section 2

Absolute Age Absolute Age of a rock is the number of years since that rock formed, its EXACT age. Ex. The rock layer is 45 million years old. Ex. This rock is 1.4 million years old. ***Uses actual numbers, example would be 450 million years ago (or 450 mya)

Relative Age Relative Age is the age of the rock COMPARED to other rocks around it. Ex. That rock layer is OLDER than that fossil. Ex. This rock is YOUNGER than that rock. ***Not exact, used more frequently in science

The Law of Superposition When layers of sedimentary rock are undisturbed the oldest rock layer is on the bottom. As you move from the bottom up to the top, the layers get younger. This is the Law Of Superposition—oldest rock layers are on the bottom, newer ones are on top.

Disturbances to Rock Layers When trying to determine the relative ages of things—what’s older or younger, scientists don’t just look at where the layers are in the “stack”, they also look at disturbances in the rock layers. Intrusions, Extrusions, Faults and Unconformities disturb the rock layers, and are all YOUNGER than the rock layers…the rocks had to be there FIRST in order to be “disturbed.”

Cross-cutting Relationships Faults are a break in the Earth’s crust, a place where rock moves on either side of it. They are always YOUNGER than the rock it breaks through. Unconformities are where an old rock surface meets a new rock surface, rock layers are missing (gaps). This is often because of erosion. They are always YOUNGER than the rocks below it.

Extrusions and Intrusions Extrusions are lava flows that covered the rock surface and hardened into igneous rock. They are always YOUNGER than the rock below it. Intrusions are pockets of magma that push through layers of rock, stops, then hardens into rock. They are always YOUNGER than the rock it cuts through.

Index Fossils Index Fossils are fossils that were widely distributed and only lived on Earth for a short period of time. They are helpful in determining how old rock layers are because when an index fossil is found, the rock layer its found in is the same age.

Try this one!

How about this one?

3-2-1 Name 3 ways that rock layers can be “disturbed.” Compare and contrast extrusions and intrusions. What one process causes rock layers to “go missing” over time and create unconformities?