5.3.1 Studying the Past

Learners can:

• explain the principle of dendrochronology and how tree rings can be used to determine the Earth's past

• describe the process of collecting and analyzing ice cores and how they provide information about conditions in the past

• identify and explain the concept of varves and how they can be used to determine the age of rocks and past conditions

• glacier

• rock record

• sedimentary rock

Prerequisite Vocab

• dendrochronology

• ice cores

• varves

New Vocab

the study of tree rings to determine the Earth's past

Dendrochronology

a core of ice that is drilled from a glacier or ice sheet

Ice Cores

​a type of sedimentary rock that is deposited in annual layers

Varves

The Geologic Time Scale can be thought of as the assembled known pieces of the puzzle.

The principles of relative dating provide essential insights into how to organize rocks and fossils.

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Puzzle of Time

Dendrochronology is the study of tree rings to determine the Earth's past. Trees form annual growth rings in their wood. The rings are associated with a specific part of the wood that transports water and nutrients from the roots to the leaves. Each ring represents one year of growth, and climate conditions determine its width during that year.

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Dendrochronology

The difference in the growth rates of the tree during the spring and summer versus the fall and winter is due to the layers of cells that produce new wood. These cells are active during the spring and summer but dormant during the fall and winter.

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Ice cores are long ice cylinders drilled from glaciers and ice sheets. They provide information about past temperatures, atmospheric composition, and other environmental factors.

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Ice Cores

Glaciers and Ice Sheets

Like rock layers, the oldest ice is at the bottom of the glacier or ice sheet. When snow and ice form, they trap gases, including greenhouse gases such as carbon dioxide, from the atmosphere. Scientists can also determine air temperature at the time of ice formation by studying the oxygen contained in the ice.

• can be difficult and expensive to drill

• conditions can be harsh

• specialized equipment is needed

• can be damaged by melting or contamination

Challenges

• provide a long-term record of climate change

• help scientists understand the causes of climate change and how it may affect Earth

Benefits

A varve is a type of sedimentary rock that is deposited in annual layers. These layers form when sediment from a river or stream is deposited in a lake or pond. Each year, a new layer of sediment is deposited. The thickness of the layer depends on the amount of sediment carried by the river or stream during that year.

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Varves

A varve's formation begins with sediment from a stream or a river.

First

When a stream or river enters a larger body of water, sediment settles to the bottom. The thickness of the deposition depends on how much sediment was carried.

Then

The thickness of the varve layer will reflect the annual conditions that provided the sediment that formed the rock layer.

Finally

What Varves Tell Us

Scientists can use varves to study climate change because they provide a record of the conditions during the year in which they were deposited. By analyzing the thickness of the varves, scientists can determine the average climates of different areas, including temperature and precipitation for each year. Geologists can use this information to create a climate record that spans thousands of years. Unfortunately, erosional processes make it challenging to locate and study varves worldwide.

Scientists can use dendrochronology to measure a timeline that generally extends back several thousands of years.

Dendrochronology

​The timeline that can be measured using ice cores extends back hundreds of thousands of years.

Ice Cores

The timeline that can be measured using varves extends back tens of thousands of years.

Varves