2Historical notes Catastrophism Modern geology Landscapes developed by catastrophesJames Ussher, mid-1600s, concluded Earth was only a few thousand years old (created in 4004 B.C.)Modern geologyFundamental principle of geologyUniformitarianism aka “The present is the key to the past”James HuttonTheory of the EarthPublished in the late 1700s
3“Hutton’s Unconformity” on Siccar Point, Scotland, is a common destination for geologists.
4Geologic Time There are two ways of dating geological materials. Relative ages – Based upon order of formation.Qualitative method developed 100s of years ago.Permit determination of older vs. younger relationships.Numerical ages – Actual number of years since an event.Quantitative method developed recently.Age is given a number.
5Relative vs. Absolute Relative ages assign order to events. Numerical agesassign exactdates to events.
6Relative dating Law of superposition Developed by Nicolaus Steno in 1669In an undeformed sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom
7Superposition is well-illustrated by the strata in the Grand Canyon
8Principle of original horizontality Layers of sediment are generally deposited in a horizontal positionRock layers that are flat have not been disturbedPrinciple of cross-cutting relationshipsYounger features cut across older features
10Inclusions Unconformity An inclusion is a piece of rock that is enclosed within another rockRock containing the inclusion is youngerUnconformityAn unconformity is a break in the rock record produced by erosion and/or nondeposition of rock units
11Unconformity Types of unconformities Angular unconformity—Tilted rocks are overlain by flat-lying rocksDisconformity—Strata on either side of the unconformity are parallelNonconformity—Metamorphic or igneous rocks in contact with sedimentary strata
16Angular Unconformity“Hutton’s Unconformity” on Siccar Point, Scotland, is a common destination for geologists.
17Fossils: Evidence of past life Fossil—traces or remains of prehistoric life now preserved in rockFossils are generally found in sediment or sedimentary rock (rarely in metamorphic and never in igneous rock)Paleontology—study of fossils
18Geologically fossils are important because they Aid in interpretation of the geologic pastServe as important time indicatorsAllow for correlation of rocks from different placesConditions favoring preservationRapid burialPossession of hard parts (skeleton, shell, etc.)
19Types of fossils Petrified - turned into stone Replacement – cell walls and and solid material replaced with mineralsMold and castImpressionsAmberCoproliteTracksBurrows
22Fossils and correlation Matching of rocks of similar ages in different regions is known as correlationCorrelation often relies upon fossilsWilliam Smith (late 1700s) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content
23Principle of fossil succession—Fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil contentIndex fossil—Geographically widespread fossil that is limited to a short span of geologic time
25Dating with radioactivity Reviewing basic atomic structureNucleusProtons—positively charged particles with massNeutrons—neutral particles with massElectrons—negatively charged particles that orbit the nucleusAtomic numberElement’s identifying numberEqual to the number of protons
26Reviewing basic atomic structure Mass numberSum of the number of protons and neutronsIsotopeVariant of the same parent atomDiffers in the number of neutronsResults in a different mass number than the parent atom
27Types of radioactive decay RadioactivitySpontaneous changes (decay) in the structure of atomic nucleiTypes of radioactive decayAlpha emissionEmission of 2 protons and 2 neutrons (an alpha particle)Mass number is reduced by 4 and the atomic number is lowered by 2
28Types of radioactive decay Beta emissionAn electron (beta particle) is ejected from the nucleusMass number remains unchanged and the atomic number increases by 1Electron captureAn electron is captured by the nucleus and combines with a proton to form a neutronMass number remains unchanged and the atomic number decreases by 1
30Parent—an unstable radioactive isotope Daughter product—the isotopes resulting from the decay of a parentHalf-life—the time required for one-half of the radioactive nuclei in a sample to decay
31Radiometric dating Principle of radioactive dating The percentage of radioactive atoms that decay during one half-life is always the same (50 percent)However, the actual number of atoms that decay continually decreasesComparing the ratio of parent to daughter yields the age of the sample
34Dating with Carbon-14 (Radiocarbon dating) Used to date very recent events (half-life of 5,730 years) and historic eventsContinually produced in the upper atmosphere from cosmic-ray bombardmentAll living things have carbon-14 in themAnthropologists, archeologists, and historians use carbon-14 dating
35Importance of radiometric dating Sources of errorA closed system is requiredTo avoid potential problems, only fresh, unweathered rock samples should be usedImportance of radiometric datingRocks from several localities have been dated at more than 3 billion yearsConfirms the idea that geologic time is immense
36The geologic time scale The geologic time scale—A “calendar” of Earth historySubdivides geologic history into unitsOriginally created using relative datesStructure of the geologic time scaleEon—The greatest expanse of timePhanerozoic (“visible life”)—The most recent eon, began about 540 million years agoProterozoicArcheanHadean—The oldest eon
37Structure of the geologic time scale Era—Subdivision of an eonEras of the Phanerozoic eonCenozoic (“recent life”)Mesozoic (“middle life”)Paleozoic (“ancient life”)Eras are subdivided into periodsPeriods are subdivided into epochs
40Precambrian Nearly 4 billion years prior to the Cambrian period Not divided into smaller time units because the events of Precambrian history are not known in great enough detailFirst abundant fossil evidence does not appear until the beginning of the Cambrian
41Difficulties in dating the geologic time scale Not all rocks can be dated by radiometric methodsGrains comprising detrital sedimentary rocks are not the same age as the rock in which they formedThe age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formedDatable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at ages