Earth's History

2. Eons are broken into these smaller units of geologic time
4. a gap in the layers of rock that shows where rock was eroded away before new layers formed on top. It happens when there is a period of time where no new rock was made, or older rock was worn away, creating a break in the geological record.
7. the time it takes for half of a radioactive parent material to change into a non-radioactive daughter material.
9. the idea that the natural processes shaping the Earth today, like erosion, volcanoes, and weathering, have been happening in the same way for a very long time. This helps scientists understand Earth's history by studying how these processes work now. Superposition
11. is a fossil from a specific time period that helps scientists figure out how old rock layers are. These fossils come from organisms that lived for a short time but were found in many places.
13. is a system that scientists use to describe Earth’s long history. It divides Earth’s 4.6-billion-year past into different time periods based on major events, like the appearance of life, mass extinctions, and changes in the planet’s surface.
16. the original, unstable atom in a radioactive substance that breaks down over time into a different atom, called a daughter isotope.

1. a rule in geology that says if a rock or fossil is found inside another rock, the one inside (the inclusion) must be older than the rock around it.
3. the actual age of a rock, fossil, or other object in years.
5. a rule in geology that says if a rock or feature, like a fault or an igneous intrusion, cuts through another layer of rock, it must be younger than the rock layers it cuts across.
6. the age of a rock, fossil, or event compared to other rocks or events around it. It doesn’t tell the exact age in years but shows whether something is older or younger than something else.
8. Eras are broken into these smaller units of geologic time
10. a rule in geology that says in layers of rock, the oldest rocks are at the bottom, and the youngest rocks are at the top—unless something has disturbed them.
12. the new substance that forms when a radioactive atom (called the parent isotope) breaks down or decays.
14. the longest unit of geologic time, lasting for hundreds of millions to billions of years.
15. a long, tube-shaped sample of ice drilled from glaciers or ice sheets. Scientists study ice cores to learn about past climates because the ice traps air bubbles, dust, and other materials that give clues about Earth's history. Isotope