Relative Dating

Geologists can easily compare sedimentary rock layers in one canyon, across a state, or across a continent and decide which layers are older, and which are younger.

It's taken hundreds of years to make enough observations to interpret the relative age of rocks on every continent, and there is still so much work to do!

Geologists make many interpretations by following a few principals for how sedimentary rocks form. Paleontologists refine relative age estimates for rocks by tracking fossils in the rock layers.

Fundamental principles
Unlike Absolute Dating, where we can simply measure something and assign a number, for relative dating we rely on a series of principles and assumptions that help us place events and samples into sequence.

Uniformitarianism
Uniformitarianism is the principle that the physics, chemistry, and basic rules that play out on earth today had the same behavior in the past. Some geologists say, "The present is the key to the past." If we research ancient limestone rocks, we need to visit modern depositional environments that form calcite and aragonite minerals. If we research ancient sandstone rocks, we need to visit modern rivers and beaches.

Uniformitarianism is not a law. Certain animals, plants, and chemical settings that existed long ago appear to be quite different than what we have today, and can cause odd rocks to appear. But in general, we think the physics and chemistry details remain the same.

Superposition
Generally, old sedimentary rocks are on the bottom and young sedimentary rocks are on the top.

If you have a huge stack of mail, or notebooks, or books near your desk, you've probably read the one on top more recently.

This happens in normal sediment deposition and layered rock formation.


 * Consider a stack of rocks that has:
 * sandstone at the bottom,
 * a set of layered volcanic ash beds in the middle,
 * and some coal on top.


 * We could interpret that this depositional environment changed over time:
 * from a sandy area,
 * to an area pestered by frequent volcanic eruptions,
 * to an area filled with dense forest and plants.

Superposition is not a law, it's just logic. Some sedimentary features build differently. Consider a coral reef, which piles calcite made by animals on top of each other in a big growing pile. This will be more like a pile of laundry in your room: that shirt you wore last month is probably at the bottom. A sock from yesterday might be on the floor. In this case, the superposition refers not just to the HEIGHT of the material, but its position away from a core of deposition.

NOTE: Igneous rocks that flow across a landscape, or layers of ash that gently fall down over a landscape, can form nice layers that DO follow superposition. But not all igneous rocks behave like this!

Original Horizontality & Lateral Continuity
Sediments usually spread far and flat before becoming rock.


 * Consider the mud at the bottom of the Great Salt Lake, or the ancient Lake Bonneville.
 * One layer of this mud might be only a few mm thick,
 * but it would spread a long distance,
 * with very little elevation change.
 * Consider the sand that spreads out during floods of the Mississippi river, or sand from a river during the Jurassic Period.
 * One layer of sand from a huge flood might only be a few cm thick,
 * but it would spread over a huge distance,
 * with very little elevation change.

The area that we call a "flood plain" around a river is the area that has a better chance of collecting sediment that can become rock, because this is an area of deposition. The river itself is a tiny area, by comparison, and it is frequently eroding material.

We can make two practical interpretations using these two rules.


 * First, if we see sedimentary rock units that are tipped at a big angle, we can stand back and say, "I bet when these ORIGINALLY formed sediment layers, that each layer was basically flat."
 * Second, we can see, or we can guess, where a layer of rock should be visible far away, even if it's covered by some plants in between.

What do we mean by a "layer" of sedimentary rock?


 * It depends on what we're studying!
 * In the Fredrick Albert Sutton Building on campus, you can see displays of fossil fish and fossil leaves. The displays are framed in slabs of rock that show little mud layers. Each thin mud layer had lateral continuity, and had original horizontality.
 * We could also be mapping the shores of Lake Bonneville by tracing outcrops of ancient limestone clusters on a map. This might be a layer of rock on meter thick, that is extended for dozens of miles in each direction.

Scientists who look for oil and natural resources need the most sophisticated techniques to consider layers of sedimentary rock. Here's a good page explaining sedimentary rock layers that are not exactly flat, but still spread very far.

Again, igneous rocks that flow across a landscape, or layers of ash that gently fall down over a landscape, can form nice layers that DO follow original horizontality and lateral continuity. But igneous rocks are sneaky and have their own rules!

Faunal Succession
This rule is like superposition, but for life.

If you're looking at a rock cliff of sedimentary layers:


 * Fossils in the bottom layers should represent animals that died a long time ago.
 * Fossils in the top layers should represent animals that lived less long ago!

If an animal lived all around the world, and then it went extinct, we can use its fossils to decide which rocks are RELATIVELY older or younger.

If we use fossils to reconstruct a sequence of animals (A, then B, then C), we can use any of these to compare rocks (maybe you find only animals A and C, but you know their relative age now).

Cross-Cutting Relationships
Ancient sedimentary rocks are usually pretty messed up by the time we can see them!

Events, features, or even other rocks can break up, disrupt, or cut across the sedimentary layers. In order for the cutting features to be present, the disrupted layer(s) needed to already be there first! This can help establish sequence, or identify areas of missing time, in some records.

Inclusion
If I find a chunk of rock A inside a layer of sedimentary rock B, then rock A must be older. Rock A must have already existed before Rock B could include a chunk of it!



The principal of inclusion has helped geoscientists advance Absolute Dating dramatically in the past decade.


 * The best methods for absolute dating on rocks that are millions of years old requires mineral grains that form in igneous processes.
 * Those minerals are very durable, though, so a sandstone from a beach setting might have a bunch of those grains.
 * Zircon is a super-durable mineral that we can measure atoms in really well.
 * By the principal of inclusion, zircon grains found in a sandstone are OLDER than the day that sand actually stopped moving and started its journey to becoming rock.
 * If we zap one zircon and get an age of 200 million years, we know the sandstone layer cannot be OLDER than 200 million years. But how much younger is it? Dunno!
 * If we zap twenty zircon grains and get a range of ages, we can make a better guess of the sedimentary layer's formation time.

Combining observations
NOTE!!


 * Extrusive igneous rocks can don't have to follow the rules for sedimentary rocks, but sometimes they CAN.


 * Intrusive igneous and metamorphic rocks don't have to follow any rules of sedimentary rocks.

If geologists discover fossils of a new dinosaur, they will need to decide how old the fossils are.

Consider all the observations that are necessary to even make a good guess!



Consider three people exploring some rocks near a river.
 * On the left bank, who is standing on the oldest rocks? Can we use superposition?
 * We can apply original horizontality to interpret that these rocks were originally more flat, and have only become tipped at this angle due to Earth motions.
 * Lateral continuity tells us that the rocks on the right side of the river should connect to the rocks on the left side. BUT! If there is a fault causing offset in these rocks, we might not see it right here because the river might cover it up.

Index Fossils
Some fossils are very easy to correlate between layers of rock far away. Criteria include:


 * The animal (or plant, or spore, etc.) lived in very distant places at the same time.
 * The animal fossilizes well.
 * The shape of the fossilized animal makes it easy to distinguish from animals that lived at different times.
 * Specific varieties of the animal are widespread but go extinct pretty fast, then evolve new, differently-shaped varieties that also spread out and go extinct pretty fast.

Some of the best index fossils are ammonites, extinct squid-like animals that had a coiled shell divvied into air chambers.

Ammonites rapidly evolved distinctly-shaped shells that made abundant, distinct fossils.

Ammonites died really easily, and then added diversity really easily, and lived all around the world.

The drag is that ammonites only lived in the ocean, so they aren't helpful for relative dating of rocks from land habitats.