Fossils & Formation
100: What is the name for the preserved remains or traces of once-living organisms?
100: Fossils.
100: What are the tiny bits of material that make up sedimentary rocks called?
100: Sediment.
100: Which dating method tells whether something is older or younger than something else without giving an exact age?
100: Relative dating.
100: What term describes all of Earth’s known rocks and the information they contain?
100: Rock record.
100: Name one type of environment where quick burial by sediment commonly happens, increasing fossil preservation chances.
100: Rivers, lakes, oceans (or floodplains, deltas) — any water environment.
200: Name two common types of fossilized hard parts found in sedimentary rock.
200: Bones, teeth, shells (also acceptable: footprints/traces).
200: When sediments settle in layers and become compressed under the weight of layers above, what process forms rock?
200: Compaction and burial leading to lithification (compression into rock).
200: Which method gives an actual age in years?
200: Absolute dating.
200: What is an index fossil? Give the two main features that make a fossil useful as an index fossil.
200: An index fossil is a fossil of an organism that was widespread and abundant but lived during a relatively short time span.
200: In a place with no exposed rock layers (everything is underground), propose one tool or technology scientists could use to explore the rock layers and find fossils.
200: Ground-penetrating radar, drilling core samples, seismic surveys, or remote sensing; also controlled excavations and test pits.
300: Why are aquatic organisms more likely to become fossils than most land animals? (Give two reasons from the text.)
300: Because rapid burial by sediment is more common in rivers, lakes, and oceans; aquatic organisms are quickly buried and protected from decay/predation.
300: What happens when water containing dissolved substances seeps through sediment layers and the substances harden?
300: The dissolved substances precipitate and act as cement, hardening the sediment into rock.
300: What is a half-life? Give a short definition
300: A half-life is the time it takes for half of an unstable substance to decay into a more stable form (e.g., potassium to argon).
300: What is an unconformity?
300: An unconformity is a gap in the rock record where layers were not deposited or were eroded away.
300: Describe how the law of superposition helps scientists determine relative ages of layers when building the rock record.
300: The law of superposition states that in undisturbed layers, lower layers are older; this allows ordering layers and fossils from oldest to youngest.
400: Describe one way soft parts can sometimes be preserved in fossils.
400: Soft parts can be preserved when conditions are ideal (rapid burial and low oxygen) allowing impressions, mineral replacement, or preservation of proteins/genetic material
400: Describe how sediment can be transported to a new location before becoming sedimentary rock. Name at least two agents.
400: Transport by water, wind, ice (glaciers), or gravity.
400: Why can absolute dating be used on igneous rocks but not usually on sedimentary rocks to find the rock's formation age?
400: Absolute dating uses decay of unstable particles in igneous rocks (starts when rock forms). Sedimentary rocks are made of older particles and therefore usually not datable directly by radiometric methods.
400: Use the football-field analogy: If Earth is 4.6 billion years old and the field is 100 yards, how many years does each yard represent?
4,600,000,000/100 = 46,000,000
400: What can finding the same index fossil in different rock layers tell scientists about the age of those layers?
400: It suggests those layers are about the same age (the organism lived during a narrow time range), so the layers are likely the same age where the index fossil occurs.
500: Explain why sedimentary rocks commonly contain fossils — include the role of burial and sediment.
500: Sedimentary rocks form from sediment that buries remains; quick burial and compression preserve remains, so fossils are most commonly found in sedimentary rock.
500: If sedimentary layers remain undisturbed, how are the ages of the layers arranged from oldest to youngest? Use the correct name for this idea.
500: Law of superposition — oldest on bottom, youngest on top (if undisturbed).
500: Explain how scientists use both relative and absolute dating together to learn about Earth’s history (one clear example is enough).
500: Example: Use relative dating (superposition, index fossils) to order layers, then use absolute dates from igneous intrusions or meteorites to assign actual ages; together they give a timeline.
500: The Morrison Formation is rich in fossils from about 150 million years ago. Does a high concentration of fossils in one area necessarily mean that more organisms lived there at that time than elsewhere?
500: No — a high concentration can reflect conditions favorable for fossil preservation (like a floodplain with rapid burial) rather than higher original population. The Morrison Formation’s environment was good for quick burial, increasing fossil concentration.
500: A meteorite was dated to about 4.6 billion years ago and helps understand Earth's age. Explain why meteorites, rather than most Earth rocks, are used to estimate Earth’s absolute age.
500: Because the oldest Earth rocks were altered, melted, or erased; meteorites formed at the same time as the solar system and can preserve the original age of formation, so their radiometric ages represent the age of the solar system/Earth.