Rock Layer Detectives
In undisturbed sedimentary rock layers, where are the oldest rocks?
At the bottom.
What is the fossil record?
All known fossils and information about where they occur.
Which method can estimate a rock’s numerical age: superposition or radioactive dating?
Radioactive dating.
Put these eras in order from oldest to youngest: Cenozoic, Paleozoic, Mesozoic.
Paleozoic, Mesozoic, Cenozoic.
A scientist records fossils in a cliff without changing any conditions. Is this an experiment or an observational investigation?
An observational investigation.
Layers are arranged A on top, B in the middle, and C on the bottom. Put them in order from oldest to youngest.
C, B, A.
Scientists find fossils of ocean organisms in rock far inland. What does this suggest?
The area may once have been covered by a sea.
What does an isotope’s half-life tell scientists?
How long it takes for half of the radioactive parent atoms to decay.
Put these divisions in order from largest to smallest: period, eon, epoch, era.
Eon, era, period, epoch.
Students change the slope of a stream table, keep other conditions the same, and measure sediment movement. What makes this an experiment?
They deliberately change a variable and control other conditions to test its effect.
A fossil is found in layer B. Another fossil is found in layer D, below B. Assuming the layers are undisturbed, which fossil is older? Explain.
The fossil in D, because lower layers formed first.
The same index fossil appears in rock layers in two different locations. What can scientists infer about the layers?
They formed during approximately the same time interval.
After one half-life, what percentage of the original radioactive parent atoms remains?
50%.
Why do divisions of geologic time have different lengths?
Major changes in Earth’s history happened at different intervals.
Must scientists perform an experiment to learn about Earth’s ancient past? Explain.
No. They can study and compare evidence from rocks, fossils, and measurements.
A student says, “The bottom layer must be exactly 10 million years old because it is the oldest.” What is wrong with this explanation?
Superposition shows relative age (older or younger), not a numerical age.
Many fossil species disappear above the same rock boundary at locations around the world. What event could this evidence suggest?
A mass extinction.
A mineral began with radioactive parent atoms and now has 25% remaining. If the half-life is 1 million years, how much time has passed?
Two half-lives, or 2 million years.
What is erosion?
The movement of weathered rock or sediment from one place to another.
A geologist compares fossils in rock layers. A biologist tests how light affects plant growth. Must both scientists follow identical steps for their work to be scientific? Explain.
No. Methods depend on the question, but both must collect and analyze evidence to support their explanations.
An exposed rock layer erodes away before a new layer forms above it. How does this affect the rock record?
It creates a gap because some of the evidence of Earth’s history is missing.
How can changes in fossils help scientists divide geologic time?
Major changes in fossil groups show changes in life and can help identify boundaries between time divisions.
FINAL CHALLENGE — save this clue for last! Undisturbed rock layers contain an ocean fossil below a volcanic ash layer. Radioactive dating shows the ash formed 8 million years ago. 1. Is the fossil older or younger than 8 million years? 2. What does the fossil suggest about the past environment? 3. Is comparing these rocks and fossils an experiment or an observational investigation?
1. Older than 8 million years: its layer formed before the ash above it. 2. The area may once have been covered by a sea. 3. An observational investigation: scientists examine existing evidence.
1. A river carries sand downstream. Is this weathering, erosion, or deposition? 2. Water slows down and drops the sand it was carrying. What process is happening?
1. Erosion: sediment moves. 2. Deposition: sediment is dropped. Weathering breaks material down.
How could a geologist investigate erosion along a river without changing the river’s conditions?
Measure and record changes in the riverbank over time, compare photographs or maps, and analyze the observations. This is an observational investigation.