Interpreting Earth’s History Chapter 5: Interpreting Earth’s History

The Geologist as a Detective: Uniformitarianism: What happened in the past is happening the same way today. And visa-versa. Superposition: The oldest rocks are USUALLY on the bottom of the layers of sedimentary rocks and the youngest (newest) on top! There are some exceptions to superposition!

Original horizontality tells us that sedimentary rocks are first laid down in flat layers. If they are tipped and moved, the movement happened AFTER the rocks were down! Igneous intrusions, extrusions and inclusions can help you know when a rock was made. If there is an intrusion, then by definition it is between other layers, making that intrusion younger than any rock above and below it.

Extrusions are by definition exposed at the surface when they first form. These igneous rocks are OLDER than anything that comes on top of them later! These upper layers will NOT be affected by contact metamorphism. (See p. 116, fig 5-1 in the review book.) Inclusions are bits of OLDER rock that have been stuck inside of NEWER igneous rock.

Crosscutting Relationships: Like intrusions, folds, faults and tilts: The rock that cuts into or the movement that MOVES layers happened to come AFTER the rocks that were affected! (See p. 117 in the review book.) Folds and Faults – like mentioned above – came AFTER (are younger than) any rock they affect. If here is a rock layer NOT moved by the fold or fault, then that rock came LATER.

How can rock layers be Correlated? Matching rocks can be done by looking for obviously similar patterns of rocks in widely spread outcrops. Index Fossils help correlate a rock to a certain time period. Index fossils are found w i d e s p r e a d but only for a short period of time. Pgs 8 and 9 in the ESRTs includes info about important index fossils for NY!

Geologic Time Scale: Pgs 8 and 9 in the ESRTs has the relative and absolute times for our known Earth history – especially that of the rocks in NY! “Life on Earth” and “Important Geologic Events” are among the many things found on this time line. Time divisions are made based on significant changes in life on Earth – like extinctions or new organisms.

Evolution of Life: At first – very simple organisms. (See p. 8 in ESRTs!) Later, more complex life formed. During the changes in life, the Earth’s climate and atmosphere changed. For example, we didn’t even have much oxygen until over a billion years of the Earth’s existence. Humans represent less than 1/10th of 1% of the time that the Earth has been around. Humans = less than 4 million years ago versus Earth = over 4 BILLION years ago!

What is Radioactive Dating? Radioactive materials break down (decay) into other substances. The time it takes for the decay to occur is VERY STEADY! The “half-life” (time needed for exactly ½ of the original to change) will NEVER change for each radioactive material. See p. 1 in the ESRTs for examples.

As the amount of original radioactive material decays, the decay product (or daughter isotope) rises. See p. 132 and 133 in your review book for examples! The best radioactive substance to use for different dating problems depends on a few things: Is it a fossil containing not too old, once-living material? Use C-14! Is it a very old rock – no organic material – older than 50,000 years? DON’T use C14. Use Something else, like K-40 or U-238.