Chapter 4: A Trip Through Geologic Time

I. FossilsA fossil is the preserved remains or traces of an organism. The fossil record shows that life on Earth has changed

over time (example: organisms used to be much simpler)Different types of fossils:

Mold and castPetrified fossils – when minerals replace the organism(and it

turns into rock)Carbon filmTrace fossils – provide evidence of ancient organisms’

activitiesPreserved remains - tar, amber, or ice

Species of organisms that are no longer living are extinct.Fossils usually occur in sedimentary rock.Only the hard parts of organisms can become fossilized

because soft parts decay too quickly.

II. The Relative Age of RocksRelative age of a rock is its age compared to the ages

of other rocks.Law of Superposition - in horizontal layers of

sedimentary rock, each layer is older than the layer above it and younger than the layer below it (this helps geologists determine relative ages)

Finding relative age:Igneous intrusions – always younger than the rock layers

around and beneath itIgneous extrusions – always younger than the rock layers

below itFaults –always younger than the rock layers it cuts

throughIndex fossils – fossils of organisms that were only

alive for a short time period but lived in many placesGeologists use index fossils for finding the relative ages

of rocks

III. Radioactive DatingRadioactive decay happens when the

atoms of an unstable element break down into atoms of a new element

A half-life is how long it takes for half of a radioactive element’s atoms to decay into a different element.

Geologists use radioactive dating to determine the absolute ages of rocks.

Radioactive dating works best with igneous rocks.

IV. The Geologic Time ScaleThe geologic time scale is Earth’s timeline – it is a

record of life forms and geologic events in Earth’s history. Geologists use it because the time span of Earth’s past is

so great, and it splits Earth’s long history into smaller units.The geologic time scale is split into divisions called

eras:Paleozoic EraMesozoic EraCenozoic Era

Eras are split into periods (just like years are split into months).Example: The Mesozoic Era is subdivided into three

periods: the Triassic, the Jurassic, and the Cretaceous.

V. Early EarthThe planet forms (Precambrian Time) – scientists

believe that Earth formed approximately 4.6 billion years ago

Earth’s atmosphere - mostly carbon dioxide from volcanic eruptions

Earth’s oceans - when Earth cooled down and water vapor turned to liquid water

Earth’s continents - rock formed from the magma that cooled down

Early lifeVery simple, single-celled organismsSimilar to bacteria

VI. Eras of Earth’s History Paleozoic Era (544 million-245 million years ago)

Cambrian Period – “explosion” of life forms called the Cambrian Explosion

Ordovician Period – first vertebratesSilurian Period – insects and spidersDevonian Period – Age of Fishes, but the first amphibians show

up hereCarboniferous Period – Appalachian Mountains begin to formPermian Period – mass extinction, perhaps because of a climate

change after continental drift Mesozoic Era (245-66 million years ago)

Triassic Period – Age of Reptiles & Pangaea still mostly together Jurassic Period – dinosaurs rule the Earth; Pangaea breaks apartCretaceous Period – mass extinction and end of dinosaurs,

perhaps because of a giant object hitting Earth Cenozoic Era (66 million years ago to today)

Tertiary Period – Age of MammalsQuaternary Period – humans!

Chapter ReviewReview Questions

Pg. 149-150, #1-25

Test FormatM.C. and T/FShort Answer (diagrams)

“Rock Layers”“Radioactive Decay of Strontium-90”

**Be able to determine the relative ages of the rock layers and explain how you know.**

**Be able to read the graph and find the half-life of strontium-90.**