How do fossils form, and how do they help correlate rocks and divide geologic time?
Explain how fossils form and how index fossils correlate rock layers, and describe the divisions and major events of the geologic time scale, including in Virginia (Virginia 2018 Earth Science SOL ES.9).
A SOL-level answer on fossils and geologic time for the Virginia Earth Science EOC: how fossils form, what makes a good index fossil, using fossils to correlate distant rock layers and infer ancient environments, the eon-era-period divisions, major events like mass extinctions, and Virginia's fossil record, with worked exam questions.
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What this topic is asking
Virginia Earth Science SOL standard ES.9 asks you to use fossils to read Earth's history, to correlate rock layers, to infer ancient environments, and to know the structure and major events of the geologic time scale. The EOC tests this with index-fossil reasoning, environment-from-fossil items, and questions about the order of major events (such as mass extinctions). It links tightly to relative and absolute dating, because fossils help build both the order and the timeline.
How fossils form
Index fossils and correlation
The two-part rule is the whole point: widespread so it turns up in many places, and short-lived so it dates a layer precisely. A long-lived or local organism makes a poor index fossil.
Fossils and ancient environments
Fossils tell you about the environment where the rock formed. Marine fossils (corals, shells, trilobites) mean the area was once under the sea, even if it is now a mountain (uplift moved it). Coal beds, formed from lush plant remains, indicate a former warm, swampy environment. By reading the fossils and the rock together, geologists reconstruct past climates and geographies, including how plate tectonics has moved land over time.
The geologic time scale
A common EOC item asks you to place events in order (oldest first) or to read which era a fossil belongs to. Remember that Precambrian time is by far the longest, even though little complex life is recorded in it.
Try this
Q1. State the two features that make a fossil a good index fossil. [2]
- Cue. It was geographically widespread, and it lived for only a short span of geologic time.
Q2. Name the three eras of the Phanerozoic eon in order from oldest to youngest. [2]
- Cue. Paleozoic, then Mesozoic, then Cenozoic.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
VA Earth Science SOL 2023 (style)1 marksWhich feature makes a fossil a good index fossil? (A) it lived for a very long span of time. (B) it is found in only one small area. (C) it was widespread but existed for only a short span of geologic time. (D) it is very large.Show worked answer →
A 1-point multiple-choice item on index fossils.
The correct answer is C. A good index fossil is geographically widespread (so it appears in many places) but lived for only a short span of geologic time (so it pins down a narrow age). A long-lived organism (A) cannot date a layer precisely, one found in only one area (B) cannot correlate distant layers, and size (D) is irrelevant.
The test rewards the two-part rule: widespread plus short-lived makes a good index fossil.
VA Earth Science SOL 2024 (style)2 marksMarine shell fossils are found in a rock layer high in the mountains of western Virginia. (a) What does this tell you about the past environment of that area? (b) Explain how the same index fossil found in two distant locations is used to correlate the rock layers.Show worked answer →
A 2-point item on fossils and correlation.
(a) 1 point: the area was once covered by a sea (a marine environment), because the organisms that formed the shell fossils lived in the ocean; the rock has since been uplifted to form mountains.
(b) 1 point: because an index fossil existed only during a short, known span of time, finding it in both locations shows the two rock layers formed at the same time, so they can be matched (correlated) even though they are far apart.
Markers reward inferring a former marine environment in (a) and using the short time span of the index fossil to match layers in (b).
Related dot points
- Apply the principles of relative dating (superposition, original horizontality, cross-cutting relationships, inclusions and unconformities) to sequence events in a geologic cross section (Virginia 2018 Earth Science SOL ES.9).
A SOL-level answer on relative dating for the Virginia Earth Science EOC: the law of superposition, original horizontality, cross-cutting relationships, inclusions and unconformities, the difference between relative and absolute age, and how to sequence the events in a geologic cross section, with worked exam questions.
- Explain radioactive decay and half-life, and calculate the age of a sample or the fraction of parent remaining using the number of half-lives that have passed (Virginia 2018 Earth Science SOL ES.9).
A SOL-level answer on absolute dating for the Virginia Earth Science EOC: what radioactive decay and half-life mean, the parent-to-daughter ratio, how to count half-lives to find an age or the fraction remaining, why carbon-14 dates young organic material and uranium dates ancient rock, and how Earth's age (about 4.6 billion years) is known, with worked calculations.
- Classify igneous, sedimentary and metamorphic rocks by how they form and explain the rock cycle, including how cooling rate, lithification, and heat and pressure transform rock (Virginia 2018 Earth Science SOL ES.5).
A SOL-level answer on rocks for the Virginia Earth Science EOC: how igneous, sedimentary and metamorphic rocks form, the link between cooling rate and crystal size, clastic versus chemical sediment, foliated versus nonfoliated metamorphic rock, and how the rock cycle transforms one type into another, with worked exam questions.
- Distinguish mechanical and chemical weathering, identify the agents of erosion and deposition, and explain how particle size, sorting and water velocity control where sediment is deposited (Virginia 2018 Earth Science SOL ES.6 surface processes).
A SOL-level answer on surface processes for the Virginia Earth Science EOC: mechanical versus chemical weathering and what speeds each, the agents of erosion (water, wind, ice, gravity), how water velocity controls the size of sediment carried and deposited, sediment sorting and rounding, and landforms like deltas and moraines, with worked exam questions.
- Explain plate tectonic theory: the evidence for moving plates, mantle convection as the driving force, the features and motions at divergent, convergent and transform boundaries, and Virginia's geologic provinces (Virginia 2018 Earth Science SOL ES.7).
A SOL-level answer on plate tectonics for the Virginia Earth Science EOC: the evidence from continental fit, fossils and seafloor spreading, mantle convection as the driving force, the features at divergent, convergent and transform boundaries, hot spots, and Virginia's geologic provinces from the Coastal Plain to the Appalachian Plateau, with worked exam questions.
Sources & how we know this
- 2018 Science Standards of Learning (Earth Science) — Virginia Department of Education (2018)
- SOL Practice Items (All Subjects) — Virginia Department of Education (2024)