Geologic Time

What is relative dating?

The ordering of rocks, fossils, or geological events determined by whether they are older or younger than one another (do not know the exact age)

What is Absolute Dating?

Determining the numerical age of a rock, fossil, or geologic event, often using radioactive isotope decay

What is the main difference between Relative and Absolute Dating?

Relative tells you the order of events, while absolute tells you the actual numerical age

What is uniformitarianism?

The idea that the processes that act today must act at the same speed as they did in the past. (“the present is the key to the past”)

Who articulated the idea of uniformitarianism?

Hutton

What is Stratigraphy?

A branch of geology focused on the study of rock layers

What are the 5 Principles of Stratigraphy?

1) Law of Superposition

2) Original Horizontality

3) Lateral Continuity

4) Inclusion

5) Unconformities

What is the law of superposition?

The idea that the oldest rocks are at the bottom and the youngest rocks are at the top (assuming the rocks are deposited horizontally and undisturbed)

Who did the law of superposition come from?

Nicholas Steno/Nield Stenson

What is original horizontality?

The idea that sediments naturally deposit in flat layers

What is lateral continuity?

The idea that layers extend until they thin out encounter barriers, so materials forming any strata layer were once continuous (if they have the same rock type, thickness, age, fossils, and position)

What are Cross-cutting relationships?

The ideas that the features that cut rock (Fault, Intrusion) are younger than the layers being cut

What is inclusion?

the idea that the fragment inside a rock are older than the rock containing them

What are unconformities?

Gaps in the rock records

What are the three types of unconformities?

1) Angular unconformity

2) Disconformity

3) Nonconformity

What is an angular unconformity?

When layers are tilted and then overlain by flat layers

What is a disconformity?

When there is a missing time between parallel laters (rock layers above are much younger than rock layers below)

What do disconformities often create?

Noticeable divots in the rock layers

What are nonconformities?

Sedimentary rocks that are deposited over much older igneous or metamorphic basements

What was the Great Unconformity?

A major gap in Earth’s geologic record where billions to hundreds of millions of years of rock layers are mission

What is the importance of the Great Unconformity? Where is it seen?

It represents a vast period of missing geologic time and is famously visible in the Grand Canyon (much younger sedimentary rocks rest directly on much older igneous/metamorphic rocks)

Who created the first geologic map using fossil correlation and superposition?

William Smith

What is Biostratigraphy?

The use of fossils to determine the relative ages of rock layers and to correlate rock layers from different locations

What is faunal succession?

The principle that fossil organisms appear and disappear in a predictable order through geologic time, so fossil assemblages can be used to determine the relative ages of rock layers.

What are index fossils?

Fossils or organisms that were widespread geographically but lived for a relatively short period of geologic tine, making them useful for dating and correlating rock layers (any rock containing the same index fossils must have formed during the same time)

What are isotopes?

Atoms of the same element with different masses/number of protons

What is radioactive decay?

The process by which an unstable radioactive isotope (parent isotope) spontaneously breaks down into a more stable isotope (daughter isotope), releasing energy

Why can radioactive decay help with absolute dating?

Radioactive decay occurs at predictable rates, so geologist can use it to determine the absolute ages of rocks and geologic events through radiometric dating.

What is a Half-life

The time it takes for half of the parent atoms to decay in an unstable isotope

What are zircons? Why are they important?

Robust, chemically stable minerals that serve as a key tool in geology for dating rocks, tracing sediment sources, and understanding early earth conditions. Their widespread occurrence in igneous, metamorphic, and sedimentary rocks, along with their ability to retain isotopic info over billions of years, makes it important for geoscientific research.

What is the best rock for radiometric dating? Why

Igneous rocks because minerals crystallize and “lock in” isotopes. (Metamorphic rocks sometimes work; sedimentary rocks generally do not work)

What are the three types of decay?

Alpha Decay, Beta Decay, and Electron Capture

What are the most common elements used for Absolute Dating through radioactive decay?

Carbon-14, Uranium-238, Potassium-40, and Rubidium-87

List the 3 types of rocks in order of the ability to use in absolute dating from easiest to most difficult

1) Igneous Rocks

2) Metamorphic Rocks

3) Sedimentary Rocks

Why are sedimentary rocks so hard to use for absolute dating?

They are made from fragments of older rocks, so radiometric dates often reflect the age of the minerals, not the actually sedimentary rock

Why can dating Metamorphic rocks using radioactive decay be difficult?

Metamorphism can reset the radioactive clock, so dates usually indicate when metamorphism occurred (not when the original rock formed)