Lab 2 – Geologic Time and Rock & Mineral Identification

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Geological Time & Rock Mineral Identification

Lab 2

GEO101: Earth Science with Lab


  1. What unstable isotope would be best to refine the date of bones found in a cave hearth built by humans between 30,000 and 50,000 years ago?

The best isotope to use for refining the date of bones would be Carbon-14 (C-14).

  1. We find samples of an igneous rock demonstrate it has been through 4 half-lives. The test element has a half-life of 150 million years. How old is the rock?

The rock is approximately 600 Million years old. 4 x 150 Million = 600 Million

  1. Argue for or against the following: A stone tool fashioned from a chunk of obsidian yields a date of 5,000,000 years old, therefore, the tool was made by a human 5,000,000 years ago.

The obsidian itself may have been formed over 5 Million years ago, but human ancestors did not begin crafting tools until the stone age which began 2.6 million years ago. Therefore, the tool could not have been made my humans 5 million years ago.

  1. You are trying to figure out the age of what is thought to be a very old fossil with a volcanic ash layer immediately above the fossil. We know the fossil is at least more than 250 million years old. Should we use carbon 14 to date the fossil, or uranium 238 to date the volcanic ash layer, and why?

We would want to use uranium 238 to date the volcanic ash layer because the fossil is more than 250 million years old. Carbon-14 have a half life of 5,730 years, whereas uranium 238 has a half life of 4.5 billion years.

  1. If the parent isotope starts with 100 grams, but your samples yield only 12.5 grams of the parent isotope, how many half-lives have passed?

8 half-lives have passed. 100grams / 12.5 grams = 8 half-lives.

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