Methods of Dating the Earth Part 1: Relative Dating

Summary

This geology series delves into the methods of dating Earth's history, highlighting relative dating through stratigraphic principles like original horizontality, superposition, and cross-cutting relationships, as well as faunal succession and unconformities. It also introduces absolute dating via radiometric dating, a technique that revolutionized the field by providing precise age measurements for rocks, crucial before the early 1900s when only relative dates were assigned.

Takeaways

• 🌏 The Earth's history is divided into the geological timescale, which spans 4.5 billion years from the Hadean eon to the present Phanerozoic eon.
• 📅 Geologists determine the age of Earth and its rock formations using two methods: relative dating and absolute (radiometric) dating.
• ⏳ Relative dating is a qualitative method that compares the age of formations based on their stratigraphic sequence without providing exact ages.
• 🔬 Radiometric dating calculates a rock's precise age using the decay of radioisotopes, which became possible with the advent of radiometric dating in the early 1900s.
• 🏞 Most sedimentary rocks are dated using relative dating due to not meeting the criteria for radiometric dating.
• 🛌 The principle of original horizontality states that sediments accumulate in horizontal layers, with exceptions like crossbedding in sand layers inclined up to 35 degrees.
• 🔄 The principle of superposition indicates that, barring tectonic disturbances, the lower beds in a sequence are older than those above.
• ✂️ The principle of cross-cutting relationships asserts that any geological feature cutting across a rock must be younger than the rock it interrupts.
• 💼 The principle of inclusions suggests that rock fragments within a formation are older than the formation itself.
• 🦋 The principle of faunal succession uses the historical order of organisms' evolution and index fossils to determine a rock's age.
• 🕰️ Unconformities represent 'missing time' or gaps in the rock record, including nonconformities, angular unconformities, disconformities, and paraconformities.
• 🔍 Prior to radiometric dating, geologists relied solely on relative dating principles to assign dates to rock layers, which presented certain challenges.

Q & A

What are the two primary methods geologists use to date rocks?

The two primary methods are relative dating and absolute dating, also known as radiometric dating.

How does relative dating compare the age of rock formations?

Relative dating qualitatively compares the age of formations based on their stratigraphic sequence.

What is the principle of original horizontality in geology?

The principle of original horizontality states that sediments accumulate in horizontal layers, with some exceptions like crossbedding where sand layers can be inclined up to 35 degrees.

Can you explain the principle of superposition in the context of dating rock formations?

The principle of superposition states that, unless overturned by tectonic forces, the beds on the bottom of an outcrop are usually older than those on top.

What does the principle of cross-cutting relationships imply about the age of geological features?

The principle of cross-cutting relationships implies that any geological feature that cuts across a rock must be younger than the rock it disrupts.

According to the principle of inclusions, how can the age of rock fragments within a formation be determined?

The principle of inclusions states that any rock fragments that are part of a larger formation must be older than the formation itself.

What is faunal succession and how is it used in dating rocks?

Faunal succession is the historical order in which organisms evolved over time. Certain specific fossils, known as index fossils, can be used to determine a rock's age based on when the organisms existed.

What is an unconformity and why is it significant in the geological timescale?

An unconformity is a gap in the rock record, representing missing time. It signifies a period of erosion, nondeposition, or a change in the geological environment.

Describe the four types of unconformities mentioned in the script.

The four types of unconformities are a nonconformity (between an older non-sedimentary rock and younger sedimentary rock), an angular unconformity (between tilted sedimentary layers and overlying horizontal strata), a disconformity (an erosional boundary between two sedimentary beds), and a paraconformity (a non-erosional boundary representing a period of nondeposition).

Why were most sedimentary rocks dated using relative dating before radiometric dating was developed?

Most sedimentary rocks were dated using relative dating because they did not meet the criteria for radiometric dating, which requires the presence of certain radioisotopes.

How has radiometric dating changed the way geologists determine the age of rocks?

Radiometric dating has allowed geologists to calculate a rock's precise age by using the decay of radioisotopes, providing a more accurate method than the relative dating techniques that were used prior to its development.

Outlines

00:00

🌏 Geological Timescale and Dating Methods

This paragraph introduces the geological timescale, which is a record of Earth's history spanning 4.5 billion years. It explains the two primary methods geologists use to date rocks: relative dating, which compares rock formations based on their stratigraphic sequence, and absolute dating or radiometric dating, which uses the decay of radioisotopes to determine precise ages. Before radiometric dating, geologists relied on relative dating techniques, such as using marker beds to compare outcrops. The paragraph also outlines principles like the principle of original horizontality, superposition, cross-cutting relationships, inclusions, and faunal succession, which help geologists determine the relative ages of rock formations. Additionally, it discusses the concept of 'missing time' or unconformities, which represent gaps in the rock record due to periods of non-deposition or erosion.

05:03

📊 Types of Unconformities and the Evolution of Dating Techniques

The second paragraph delves into the different types of unconformities that indicate periods of missing time in the geological record. These include disconformities, which are erosional boundaries between sedimentary rock beds; paraconformities, which represent periods of non-deposition and are less indicative of missing time than disconformities; nonconformities, which are boundaries between non-sedimentary rocks and overlying sedimentary rocks; and angular unconformities, which occur at the junction of tilted sedimentary layers and younger, horizontal strata. The paragraph concludes by acknowledging the limitations of relative dating and hints at the advent of radiometric dating as a more advanced technique for determining the age of rocks.

Keywords

💡Geological Timescale

The Geological Timescale is a system of chronological measurement used by geologists, demarcating the history of Earth into vast periods of time, from the Hadean eon to the current Phanerozoic eon. It is central to the video's theme as it provides the framework for understanding the age of Earth and its various rock formations, as discussed in the context of dating methods.

💡Relative Dating

Relative dating is a method that determines the age of rock formations based on their sequence in the stratigraphic record without necessarily determining their absolute age. It is a key concept in the video, illustrating how geologists used to assign dates to rock layers by comparing their deposition sequence, as highlighted by the use of marker beds.

💡Radiometric Dating

Radiometric dating is a technique used to calculate the precise age of rocks and minerals based on the decay of radioisotopes. The video mentions it as a significant advancement over relative dating, allowing for more accurate age determinations in the study of Earth's history.

💡Stratigraphic Sequence

The stratigraphic sequence refers to the order in which rock layers, or strata, are deposited over time. It is a fundamental principle in the script that helps geologists understand the relative ages of rock formations, as it underpins the method of relative dating.

💡Marker Beds

Marker beds are distinctive rock layers that can be used to correlate the age of different rock outcrops. In the video, they are mentioned as a tool for geologists to compare different outcrops and establish relative dates before the advent of radiometric dating.

💡Principle of Original Horizontality

This principle states that sediments are deposited in horizontal layers, which is a basic assumption in reading the rock record. The video uses it to explain how geologists interpret the deposition of sedimentary rocks and the implications of finding inclined layers, such as crossbedding.

💡Crossbedding

Crossbedding is a structure within sedimentary rocks where layers are inclined at an angle, typically associated with wind-blown sand dunes. The script describes it as an exception to the principle of original horizontality, providing an example of how sedimentary environments can vary.

💡Principle of Superposition

The principle of superposition is a fundamental concept in geology stating that in an undisturbed sequence of rock layers, the layers on the bottom are older than those on top. The video explains this principle as a method for determining the relative ages of rock formations.

💡Principle of Cross-cutting Relationships

This principle states that any geological feature that cuts across another must be younger than the feature it cuts. The video uses this principle to illustrate how geologists determine the relative ages of rock formations, such as when an igneous intrusion cuts through sandstone.

💡Principle of Inclusions

The principle of inclusions states that rock fragments within a larger formation are older than the formation itself. The video script uses this principle to explain how the presence of older rock fragments, like a clast of schist in sandstone, helps determine the relative ages of rock layers.

💡Faunal Succession

Faunal succession refers to the historical order of organism evolution, with certain fossils, known as index fossils, indicating specific time periods. The video highlights this as a method for dating rocks, especially useful when finding fossils like trilobites that can pinpoint the age of a rock layer.

💡Unconformities

Unconformities are gaps in the rock record representing missing time due to erosion or nondeposition. The video script discusses different types of unconformities, such as disconformities and paraconformities, and explains their significance in the geological timescale and the challenges they pose for dating rocks.

💡Wilson Cycle

The Wilson Cycle is a concept describing the geological process of ocean opening and closing, which affects geological environments over time. The video uses this concept to explain how geological environments change, leading to periods of sedimentation and erosion, which in turn create unconformities.

Highlights

Introduction to the geological timescale spanning 4.5 billion years of Earth's history.

Two methods of dating rocks: relative dating and absolute dating or radiometric dating.

Relative dating compares the age of formations based on their stratigraphic sequence.

Absolute dating uses the decay of radioisotopes to calculate a rock's precise age.

Before radiometric dating, geologists assigned relative dates based on the inferred sequence of deposition.

Most sedimentary rocks are dated using relative dating as they do not meet the criteria for radiometric dating.

The principle of original horizontality states that sediments accumulate in horizontal layers.

Crossbedding is an exception to the principle of original horizontality, where sand layers may be inclined up to 35 degrees.

The principle of superposition states that beds on the bottom are usually older than those on top.

The principle of cross-cutting relationships indicates that a geological feature cutting across a rock must be younger than the rock.

The principle of inclusions states that rock fragments within a formation must be older than the formation itself.

The principle of faunal succession uses the historical order of organisms to determine a rock's age, with index fossils being particularly useful.

Unconformities represent gaps in the rock record or missing time, with four types discussed: nonconformity, angular unconformity, disconformity, and paraconformity.

A disconformity is an erosional boundary between two beds of sedimentary rock, indicating missing time.

A paraconformity is a non-erosional boundary between two beds of sedimentary rock, representing a period of nondeposition.

A nonconformity is a boundary between an older non-sedimentary rock and younger sedimentary rock.

An angular unconformity is a boundary between tilted layers of sedimentary rock and overlying horizontal strata.

Challenges of relative dating and the limitations before the development of radiometric dating.

Transcripts

All the way at the beginning of this geology  series, we talked about Earth’s history and  

the geological timescale spanning  4.5 billion years of Earthly events,  

from the Hadean eon to the Phanerozoic eon  we are still living in today. But how is it  

that geologists determine the age of the  Earth and its different rock formations? 

There are two methods of dating rocks. These  are relative dating, which qualitatively  

compares the age of formations based on their  stratigraphic sequence, and absolute dating,  

or radiometric dating, which uses the decay of  radioisotopes to calculate a rock’s precise age.  

Before the advent of radiometric dating in the  early 1900s, geologists assigned relative dates  

to different layers in an outcrop based on  the inferred sequence of their deposition,  

or emplacement, which they would then compare to  other outcrops using distinctive formations called  

marker beds. In fact, most sedimentary rocks  are dated using relative dating since they do  

not meet the criteria for radiometric dating,  which we will discuss in the next tutorial. 

Let’s now discuss some of the principles that  geologists use when determining relative ages. The  

principle of original horizontality states that  sediments accumulate in horizontal layers called  

beds, though some sand layers that are deposited  as dunes may be inclined as much as 35 degrees.  

This is called crossbedding. In  cross bedded sedimentary rocks,  

the sediment is deposited in sets at the angle  of repose on the lee side of dunes and ripples,  

which explains their inclination. In addition,  only sand-sized sediment can form cross beds.  

Say for example that you find an outcrop with  a horizontally layered siltstone on top of a  

vertically layered siltstone. When you apply the  principle of original horizontality, you realize  

that there was a large time gap between deposition  of the horizontal and vertical formations.  

The principle of superposition states that,  unless tectonic forces have overturned the  

outcrop, beds on the bottom are  usually older than beds on top.  

The principle of cross-cutting relationships  states that any geological feature which cuts  

across a rock must be younger than the feature  it disrupts. So, if an igneous intrusion is found  

cutting through a sandstone, the intrusion must  be younger than the sandstone it cuts through.  

The principle of inclusions states that any  rock fragments that are a part of a larger  

formation must be older than the formation they  are a part of. For example, lithic fragments,  

which are pieces of a preexisting rock, are common  types of grains in sedimentary rocks; so if you  

find a clast of schist in a sandstone, the schist  must be older than the sandstone it is a part of.  

The principle of faunal succession states that  there is a historical order in which organisms  

evolved over time, and that certain specific  fossils can be used to determine a rock’s age.  

Organisms that only existed for a short period  of Earth’s history are most useful for this,  

and their fossils are called index fossils.  For example, if you find a trilobite fossil  

in a bed, then it must have been deposited  between the Cambrian and Permian Periods. 

Another useful concept for qualitatively dating  rocks is that of “missing time”, or gaps in the  

rock record called unconformities. There are  four types of unconformities: a nonconformity,  

an angular unconformity, a disconformity, and a  paraconformity, all of which represent missing  

time. But what exactly is meant by missing time?  Recall the Wilson cycle from a previous tutorial,  

and the ways that geologic environments change  over time. For example, during one period an area  

may be a part of a sedimentary basin, but then  get uplifted 100 million years later during an  

orogeny, transforming the once sedimentary  environment into an erosional environment,  

and then, after another 100 million years, it  could once again become a sedimentary basin.  

Let’s consider what the rock record would look  like here. Sedimentary rocks would be deposited  

during the first period, which would later  get eroded during uplift, removing some amount  

of the sedimentary record, which would later  be capped by sediments from the last period.  

The rocks that were eroded during  uplift represent missing time,  

and the gap between the two sedimentary  layers is called an unconformity. 

Let’s rigorously define the types of  unconformities. A disconformity is an erosional  

boundary between two beds of sedimentary rock, as  in the example we just discussed. A paraconformity  

is also a boundary between two beds of sedimentary  rocks, but is not erosional and simply represents  

a period of nondeposition. They represent less  missing time than a disconformity. A nonconformity  

is a boundary between an older non-sedimentary  rock, like an igneous or metamorphic rock,  

and younger sedimentary rock layered on top. And  an angular unconformity is a boundary between  

tectonically tilted layers of sedimentary  rocks and overlying horizontal strata. 

So, that covers the principles that geologists use  to assign relative dates to Earth’s rock layers,  

and some of the challenges that  this method poses. As we mentioned,  

prior to the development of radiometric dating,  this was all that geologists were able to do.  

But radiometric dating is a powerful technique,  so let’s move forward and learn about that next.