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Course: Middle school Earth and space science - NGSS > Unit 4
Lesson 2: Fossils and rock layersFossils and rock layers
Review your understanding of fossils and rock layers with this free article aligned to NGSS standards.
Key points:
- Sedimentary rocks typically occur in horizontal layers called strata.
- In undisturbed strata, younger layers sit on top of older ones. This is known as the law of superposition.
- Strata can be cut by other geologic features, such as faults or intrusions. A fault is a crack in Earth’s crust. An intrusion is a body of igneous rock formed within Earth’s crust.
- When two features intersect, the one that cuts through the other is younger. This is known as the law of crosscutting relationships.
- Some strata contain index fossils, or fossils associated with a specific time in Earth’s history. If the same index fossil is found in different strata far apart, the layers likely formed around the same time.
- By studying strata, geologists have determined the relative order of major events in Earth’s history. The timeline of these events is called the geologic time scale.
Want to join the conversation?
Are the different rock layers actually different colors? If not, how do geologists figure out the different rocks from different ages?(20 votes)
Rock layers are sometimes different colors and sometimes they are the same. When they are different colors, the layers are easy to distinguish. When they are the same color, then geologists have to take a closer look to tell them apart.
There is a lot that can be looked at to tell rocks apart. What is the rock made out of? Does it have big grains or small grains? Which minerals are present? What fossils can we find?
After examining the layers for those characteristics, a geologist might find that one layer is sandstone and the other is limestone. The geologist would then know that the layers are different rocks deposited at different times.(33 votes)
- can you tell the rocks apart(5 votes)
- yea by shape and color(5 votes)
- how do plants grow in rocks(3 votes)
They don't grow inside of them, the rock forms around the plant. When sediment is compressed around the plant it turns into sedimentary rock.(8 votes)
what are all those colors ? is that the layers of the rock ?(6 votes)- they are different layers of sedemantating rocks(1 vote)
do rocks have salt on them?(1 vote)- I think salt is a rock.(8 votes)
- wow it looks beuatifull amazi(4 votes)
where can i find a rock(4 votes)- outside in your yard(1 vote)
Why in the photo are the rocks red?(3 votes)
because different rocks have different pigments depending on the type of rock and what kind they are(2 votes)
does it heat up?(0 votes)
The further down you go in the earth's crust, the hotter it generally gets. This is because these deeper rocks are closer to earth's hot core and are under more pressure.
Does that answer your question?(8 votes)
