Unit 3 · Segment 1: Organisms and Nonliving Things Are Made of Atoms · printed p. 88 (PDF p. 124)
Earth Processes Through Geologic Time
Despite the lack of volcanoes and flowing water, igneous rocks and sedimentary rocks are found throughout the Black Hills.
Anchoring phenomenon
Chapters
- 08 Investigating Rock Strata
How can rock layers tell us the order of events in Earth's past, and when can they tell us how long ago those events happened?
1 presentation10 worksheets3 labs - 09 Reconstructing Earth's History
How do scientists use rocks, fossils and dated materials to reconstruct and organize a history that no one was there to see?
1 presentation10 worksheets3 labs
Standards in this unit
Unit vocabulary
relative datingWorking out the order of rocks and events, which came first, next and last, without saying how many years ago they happened.Chapter 8
absolute datingPutting a number of years on when a rock formed or an event happened, usually by measuring radioactive atoms in igneous minerals. The number always comes with an uncertainty.Chapter 8
strataLayers of rock, especially sedimentary rock. One layer is a stratum; several are strata.Chapter 8
law of superpositionIn a stack of layers that has not been overturned or disturbed, each layer is younger than the one below it and older than the one above it.Chapter 8
principle of original horizontalitySediment settles under gravity into flat or nearly flat layers, so layers that are now steeply tilted or folded were moved after they formed.Chapter 8
principle of lateral continuityA layer of sediment spreads sideways in every direction until it thins out or meets a barrier, so matching layers on two sides of a gap were once one connected layer.Chapter 8
law of cross-cutting relationshipsAny feature that cuts through rock, such as an igneous intrusion, a fault or an erosion surface, must be younger than whatever it cuts through, because the rock had to exist first.Chapter 8
intrusionA body of igneous rock that formed when magma pushed into cracks or weak zones in existing rock and cooled there. A sheet-like intrusion that cuts across layers is called a dike.Chapter 8
law of fossil successionFossil species appear in rock layers in the same order everywhere on Earth, because life changed over time in one sequence that was never repeated.Chapter 8
index fossilA fossil of a species that lived for a short span of geologic time, spread over a wide area and is easy to recognize, which makes it a good marker for layers of about the same age.Chapter 8
radiometric datingMeasuring how many radioactive parent atoms in a mineral have decayed into daughter atoms, and using the known decay rate to calculate when the mineral formed.Chapter 8
half-lifeThe time needed for half of a sample's radioactive atoms to decay. Each half-life halves whatever is left; heat and pressure do not change it.Chapter 8
geologic time scaleA model that divides Earth's whole history into named spans of time, built from the order of rock layers and the fossils they hold and pinned to years by dated materials such as volcanic ash.Chapter 9
eon, era, period and epochThe nested units of the geologic time scale, from longest to shortest. Eons contain eras, eras contain periods, and periods contain epochs. Today falls in the Phanerozoic Eon, Cenozoic Era, Quaternary Period and Holocene Epoch.Chapter 9
Ga, Ma and kaShorthand for billions, millions and thousands of years, from the Latin annum, year. On a timeline a label such as 66 Ma usually means 66 million years ago.Chapter 9
relative datingPutting rocks, fossils and events in order from older to younger without measuring an age in years, using rules such as lower layers being older in an undisturbed stack.Chapter 9
absolute datingMeasuring a rock's or mineral's age in years, usually from the steady decay of certain atoms inside it. It works best on volcanic rock and ash, meteorites and moon rocks, not on most sediment.Chapter 9
principle of lateral continuityThe idea that a sediment layer is laid down as a wide sheet that keeps going sideways until it thins out or meets an edge, so a layer seen in one cliff can be the same layer seen in another cliff far away.Chapter 9
key bedA thin, distinctive layer, often volcanic ash, that is easy to recognize and formed quickly over a wide area, so it can be used to match sequences in different places and sometimes to date them.Chapter 9
mass extinctionAn event in which a large share of the kinds of living things on Earth die out within a geologically short time, which shows up in rock layers as many fossil types disappearing at about the same level.Chapter 9
stromatoliteA layered mound of rock built up over time by mats of microbes, including cyanobacteria, that trap sediment. Fossil stromatolites are among the oldest evidence of life on Earth.Chapter 9
banded iron formationAncient seafloor rock made of thin alternating layers rich and poor in iron, formed when oxygen in seawater reacted with dissolved iron and the rusty minerals settled out.Chapter 9
Cambrian explosionA burst of animal evolution beginning about 539 million years ago in which the early relatives of most major animal groups, many with hard shells, appear in the fossil record within a few tens of millions of years.Chapter 9
ice age, glacial and interglacial periodsAn ice age is a long stretch of cooler global climate with lasting ice at the poles. Within it, glacial periods are colder spans when ice sheets spread toward the equator, and interglacial periods are warmer spans when they shrink back.Chapter 9