Eras & Periods
What are the three main eras of the Phanerozoic Eon?
Paleozoic, Mesozoic, Cenozoic.
What type of dating gives the age of rocks in years using radioactive decay?
Radiometric (radiometric dating / radiometric methods)
About how many years ago did the Earth form (order of magnitude, in billions)?
About 4.6 billion years ago.
What is an eon?
The largest time unit in the geologic time scale (billions of years).
What is the name of the supercontinent that existed during the late Paleozoic and early Mesozoic?
Pangaea
Which period is known for abundant dinosaurs and ended with a major extinction about 66 million years ago?
Cretaceous
What is the principle of superposition?
In an undisturbed sequence, older layers are below younger layers.
What mass extinction ended the Paleozoic Era and paved the way for the Mesozoic?
Permian mass extinction (end-Permian).
What does "epoch" refer to compared with a period? (shorter or longer?)
An epoch is shorter than a period.
Which era is often called the "Age of Reptiles"?
Mesozoic
In which era did mammals first become the dominant large land animals?
Cenozoic
Which fossil type is used to identify and correlate the relative ages of rock layers across large areas (short-lived, widespread)?
Index fossils.
During which period did the first large forests appear, helping change Earth's atmosphere?
Devonian (first large forests)
Define "absolute age" in geology.
The age in years (e.g., millions of years) determined by methods like radiometric dating.
Name one major plant or animal group that first appears in the fossil record during the Paleozoic.
Fish diversification (and later amphibians, insects, etc.); trilobites in earlier Paleozoic.
Name the geologic period when extensive coal-forming swamps existed and many amphibians thrived.
Hint: Think "age of amphibians".
Carboniferous (Pennsylvanian/Westphalian coal-swamp intervals).
How does radiocarbon dating differ from other radiometric methods in terms of the materials and ages it can measure?
Radiocarbon (C-14) dates organic material up to ~50,000 years; other methods (like U-Pb) date much older rocks/minerals.
Which event marks the boundary between the Mesozoic and Cenozoic eras?
The asteroid impact and mass extinction at the end of the Cretaceous (~66 Ma).
What does the term "half-life" mean in radioactive decay?
The time required for half of a radioactive isotope to decay to its daughter product.
Why are sedimentary rocks especially important for understanding geologic time?
Sedimentary rocks commonly contain fossils and layered records of past environments.
Place these units from LARGEST to SMALLEST:
period, era, epoch, eon.
Eon > Era > Period > Epoch.
Explain how index fossils and relative dating together help build the geologic time scale.
Index fossils give relative ages; combined with radiometric ages from volcanic layers, they place relative sequences into absolute time.
Describe one major change in life or environment at the start of the Paleozoic Era.
Rapid increase in diversity of marine life and the first appearance of many animal phyla (Cambrian Explosion)
Explain the difference between relative dating and absolute dating, with one method example for each.
Relative dating places events in order (e.g., superposition); absolute dating gives numerical ages (e.g., U-Pb radiometric dating).
Give one reason why the geologic time scale is divided into named units (like Cambrian, Jurassic) rather than just numbers.
Named units reflect distinct changes in life, climate, or geology and help scientists communicate specific intervals.