relative rock age - superposition

FOSSILS and STRATIGRAPHY Relative Geologic Dating

FOSSILS READ / HIGHLIGHT / SUMMARY

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Some Questions to Consider

How old are you relative to your parents?

Younger than your parents

Relative to Mr. Winter?

Younger than Mr. Winter

Relative to Mr. Winter’s kids?

Older than Mr. Winter’s kids

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Relative Age

How old is the Earth?

How did we arrive at this number?

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4.5 Billion Years Old

Danish scientist Nicolaus Steno (17th Century) spent time observing the different layers of rock that had formed in Tuscany, Italy.

His observations helped to establish the basis for relative dating and started a new branch of geology called stratigraphy – the study of rock layers.

The concept of relative dating is that layers of rocks are ordered chronologically, so their ages can be compared.

Two Laws of Relative Dating

Law of Original Horizontality

All sedimentary rock layers initially form in horizontal layers.

Any change from this position is due to the rock being disturbed later.

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Two Laws of Relative Dating

Law of Superposition

In sedimentary rock, the older layers of rock are deposited first.

Newer layers are deposited after the older layers.

Therefore, newer layers are formed on top of older layers of rock.

Observing an undisturbed layer of rock, you can assume that the older rocks are at the bottom and the youngest are at the top.

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The Position of Rock Layers

According to the law of superposition, in horizontal sedimentary rock layers, the oldest layer is at the bottom.

Each higher layer is younger than the layers below it.

- The Relative Age of Rocks

Finding the age of Earth

Steno’s laws revolutionized the way people viewed the age of Earth.

In the 1800s, scientists measured the approximate thickness of all sedimentary rock layers at Earth’s surface.

These measurements ranged from about 25–112 m (15–70 mi).

They then observed and measured the rate at which sedimentary layers form, and calculated it to be around 0.3 m (1 ft) every 1,000 years.

Using these rates, scientists then calculated how long it would take for all of the sedimentary layers in the world to build up.

Based on these calculations, these scientists determined that Earth had to be perhaps a hundred million years old!

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Gaps in the rock layers

100 million years was still an underestimated age of Earth.

The revised age did not take into account processes, such as weathering, erosion, and underground geologic activities, that cause change at the surface.

These processes are gradual and sometimes are not noticeable for extremely long periods of time. It could take millions of years for weathering and erosion to unearth underground structures.

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Gaps in the rock layers

The folding and twisting of rock layers resulting from pressure below may also take millions of years.

For example, geologic forces can exert enough pressure to cause horizontal layers of rock to fold.

The forces that cause these features can often bring old layers of rock to the surface, where they are exposed to the agents of weathering and erosion.

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When the layers fold downwards and form a bowl-like shape, it is called a syncline.

When the layers of rock fold upward like an arch, it is called an anticline.

Anticline and Syncline

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Unconformity

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Over time, parts of the older layers in an anticline can be weathered and eroded away. New, horizontal layers can then form on top of this eroded surface.

These gaps in the geologic record, due to layers of rocks lost to weathering and erosion, are called unconformities.

Formation of an unconformity

An unconformity occurs where erosion wears away layers of sedimentary rock. Other rock layers then form on top.

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Formation of an unconformity

An unconformity occurs where erosion wears away layers of sedimentary rock. Other rock layers then form on top.

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Formation of an unconformity

An unconformity occurs where erosion wears away layers of sedimentary rock. Other rock layers then form on top.

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Formation of an unconformity

An unconformity occurs where erosion wears away layers of sedimentary rock. Other rock layers then form on top.

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Cross-Cutting Relationships

To determine relative age, extrusions and intrusions of igneous rock, faults, and gaps in the geologic record can be studied.

- The Relative Age of Rocks

Relative Intrusions

Analyzing the positions of rock layers is not the only method to find the relative age of rocks.

Clues from igneous rock formations can provide information as well.

Molten rock cools and solidifies, it forms igneous rock. Igneous rock usually forms as slabs of giant

rock.

However, hot molten rock can sometimes pierce through layers of other rock.

When this hot molten rock cools and solidifies within the pre-existing rock, it forms an intrusion.

An intrusion is always younger than the layers of rock that it pierces.

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Relative Extrusion

An extrusion is also an igneous formation.

However, an extrusion forms when lava cools and solidifies on top of older rock formations.

In undisturbed areas, extrusions are always younger than the rock layers below them.

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- Radioactive Dating

Relative Intrusion and Extrusion

The age of a sedimentary rock layer can be determined relative to the absolute age of an igneous intrusion or extrusion near the sedimentary rock.

Clues from Faults

A fault is a break in Earth’s crust usually caused by geologic forces within Earth.

These forces move or shift opposite sides of a fault and provide insight into the relative age of the rock layers.

Since the layer of rock had to be present in order to break, the fault is always younger than the youngest layer of rock cut by the fault.

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How a Fossil Forms

- Fossils

Most fossils form when living things die and are buried by sediment. The sediment slowly hardens into rock and preserves the shapes of the organisms.

How a Fossil Forms

- Fossils

Most fossils form when living things die and are buried by sediment. The sediment slowly hardens into rock and preserves the shapes of the organisms.

How a Fossil Forms

- Fossils

Most fossils form when living things die and are buried by sediment. The sediment slowly hardens into rock and preserves the shapes of the organisms.

How a Fossil Forms

- Fossils

Most fossils form when living things die and are buried by sediment. The sediment slowly hardens into rock and preserves the shapes of the organisms.

Using Fossils to Date Rock

Scientists can estimate a rock layers age by searching for certain organisms fossilized within the rock layers.

These fossils have been previously dated by other scientists and are called index fossils.

If an index fossil is found within a layer of rock, scientists will have a good estimate of when that layer of sediment was deposited to form the rock.

What makes a good index fossil?

Ideally, these fossils were ancient organisms that were fairly common and easily fossilized.

Most index fossils are organisms with a hard shell, because they do not decompose as quickly as soft tissue.

Index fossils often have distinctive features that allow them to be easily distinguished from closely-related organisms.

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Using Fossils to Date Rocks

Index fossils are useful because they tell the relative ages of the rock layers in which they occur.

- The Relative Age of Rocks

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Identify the relative age of each layer

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Identify the relative age of each layer

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Identify the relative age of each layer

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Identify the relative age of these areas

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