Students who demonstrate understanding can: Construct a scientific explanation based on evidence from rock strata for how the geologic time scale is used to organize Earth's 4.6-billion-year-old history.
Clarification statement: Emphasis is on how analyses of rock formations and the fossils they contain are used to establish relative ages of major events in Earth's history. Examples of Earth's major events could range from being very recent (such as the last Ice Age or the earliest fossils of homo sapiens) to very old (such as the formation of Earth or the earliest evidence of life). Examples can include the formation of mountain chains and ocean basins, the evolution or extinction of particular living organisms, or significant volcanic eruptions.
Assessment boundary: Assessment does not include recalling the names of specific periods or epochs and events within them.
Official wording from the Next Generation Science Standards (NGSS Lead States, 2013). NGSS is a registered trademark of WestEd. Neither WestEd nor the lead states and partners that developed the NGSS were involved in the production of this page, and they do not endorse it. View on nextgenscience.org
Layers of rock work like the pages of a very old book. Where sediments settle in water or wind, each new layer is laid down on top of the last, so in undisturbed strata the bottom layer is the oldest. Students use this idea, along with clues such as fossils, faults and intrusions of igneous rock that cut across layers, to put events in order and explain how scientists organized Earth's 4.6-billion-year history into the geologic time scale.
The emphasis is on relative ages: deciding what happened before or after something else. A fault that breaks through three layers must be younger than all three. Fossils of the same species found in layers on different continents suggest those layers formed at about the same time. Major events, from the formation of oceans and mountain chains to mass extinctions and the most recent ice age, are placed on the time scale using this kind of evidence.
Students are not expected to recall the names of eras, periods or epochs. They should be able to read a rock column and argue for an order of events.
Folding and faulting can tilt or overturn layers. Students should check for clues such as cross-cutting features before assuming the bottom layer is oldest.
Non-avian dinosaurs died out about 66 million years ago, long before the earliest humans. Laying out a scale timeline on a long rope shows how huge the gap is.
A thick layer might form quickly in a flood, and a thin layer might represent a long, slow period. Thickness alone does not give age.
Relative dating gives the order of events, not a number of years. Other methods supply ages in years; this expectation focuses on ordering.
A cliff shows, from bottom to top: sandstone (A), shale with trilobite fossils (B), limestone (C). A fault cuts through A, B and C. A layer of basalt (D) lies on top of C and is not cut by the fault. Put the events in order from oldest to youngest.
Answer: Oldest to youngest: sandstone A, shale B, limestone C, the fault, then basalt D.
Have students build their own strata with colored sand or modeling clay, then cut a fault or push a pencil intrusion through it, so the rules come from something they made. Follow with photos of real outcrops and drawn rock columns to interpret. A timeline of Earth's history drawn to scale on adding-machine tape helps students feel how recent humans are.
Typical assessment items show a cross-section and ask for the order of events with justification, or ask which fossil would be most useful for matching layers in two locations.
Original questions written for this standard. Choose an option or type your answer, then press Check. Every question has a worked explanation.
Answer: C) The bottom layer
Sediments are laid down on top of earlier layers, so the bottom layer formed first.
Answer: B) The fault is younger than all three layers
Something that cuts across rock must be younger than the rock it cuts.
Answer: A) It lived for a short time but was widespread
Fossils of organisms that were widespread but existed only briefly point to a narrow time window, so they are good markers.
Answer: 4.6 (also accepted: 4.6 billion, 4.6 billion years)
Evidence from rocks and meteorites gives Earth an age of about 4.6 billion years.
Answer: C) Deposition rates vary, so a thick layer can form quickly
A flood can drop a thick layer in days, while a thin layer may build up slowly over a long time.
Answer: relative (also accepted: relative dating, relative age)
Relative dating orders events; it does not give a number of years.
Apply scientific reasoning and evidence from ancient Earth materials, meteorites, and other planetary surfaces to construct an account of Earth's formation and early history.
A full lesson with slides, activities and an exit ticket on rock strata and the geologic time scale, pitched to grades 6-8 and editable in PowerPoint or Google Slides.
Make a lesson βA printable, differentiated worksheet on MS-ESS1-4 with an answer key, ready in about a minute.
Make a worksheet βTurn rock strata and the geologic time scale into a quiz students answer online that marks itself, with a class summary for you.
Build a test βNo. The assessment boundary excludes recalling the names of periods, epochs and the events in them; the focus is on using rock evidence to order events.
The order of rock layers, the fossils they contain, faults, igneous intrusions and erosion surfaces are all used to establish relative ages.