Methods of Dating the Earth Part 1: Relative Dating
Summary
TLDRThis 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
🌏 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.
📊 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.
Mindmap
Keywords
💡Geological Timescale
💡Relative Dating
💡Radiometric Dating
💡Stratigraphic Sequence
💡Marker Beds
💡Principle of Original Horizontality
💡Crossbedding
💡Principle of Superposition
💡Principle of Cross-cutting Relationships
💡Principle of Inclusions
💡Faunal Succession
💡Unconformities
💡Wilson Cycle
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.
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