tree rings ice cores and varves

tree ring dating

In locations where summers are warm and winters are cool, trees have a distinctive growth pattern. Tree trunks display alternating bands of light-colored, low density summer growth and dark, high density winter growth. Each light-dark band represents one year. By counting tree rings it is possible to find the number of years the tree lived (Figure 1.1). The width of these growth rings varies with the conditions present that year. A summer drought may make the tree grow more slowly than normal and so its light band will be relatively small. These tree-ring variations appear in all trees in a region. The same distinctive pattern can be found in all the trees in an area for the same time period. Scientists have created continuous records of tree rings going back over the past 2,000 years. Wood fragments from old buildings and ancient ruins can be age dated by matching up the pattern of tree rings in the wood fragment in Cross-section showing growth rings. question and the scale created by scientists. The outermost ring indicates when the tree stopped growing; that is, when it died. The tree-ring record is extremely useful for finding the age of ancient structures.

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ice cores

Besides tree rings, other processes create distinct yearly layers that can be used for dating. On a glacier, snow falls in winter but in summer dust accumulates. This leads to a snow-dust annual pattern that goes down into the ice (Figure gather allows them to determine how the environment has changed as the glacier has stayed in its position. Analyses of the ice tell how concentrations of atmospheric gases changed, which can yield clues about climate. The longest cores allow scientists to create a record of polar climate stretching back hundreds of thousands of years.

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varves

Lake sediments, especially in lakes that are located at the end of glaciers, also have an annual pattern. In the summer, the glacier melts rapidly, producing a thick deposit of sediment. These alternate with thin, clay-rich layers deposited in the winter. The resulting layers, called varves, give scientists clues about past climate conditions (Figure 1.3). A warm summer might result in a very thick sediment layer while a cooler summer might yield a thinner layer.

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instructional diagrams

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questions

tree trunks alternating bands of light-colored, high density summer growth and low-density winter growth.

a) true

-->  b) false

if there is a summer drought ____.

a) the tree will grow faster and light bands will be small

-->  b) the tree will grow slower and the light bands will be small

c) the tree will not grow and the bands will not change

d) none of the above

which of these statements is true?

a) the outmost ring in a tree cross-section indicates when the tree stopped growing

b) the tree-ring record is useful for finding the age of ancient structures

c) distinctive patterns can be found in all trees in an area for the same time period

-->  d) all of the above are true

ice cores show that snow falls in winter but in summer, dust accumulates.

-->  a) true

b) false

what is true about ice cores?

a) ice cores can determine how the environment has changed

b) ice cores can tell how concentrations of atmospheric gases have changed

c) ice cores can help scientists create a record of the local climate for thousands of years

-->  d) all of the above are true

in a varve, summer is represented by

-->  a) sediment from melting glaciers.

b) clay from wind-blown dust.

c) ash from nearby volcanoes.

d) none of these.

a cold year is indicated in a varve by a thick deposit of sediment.

a) true

-->  b) false

varves form in ____ .

a) the deep ocean

b) coastal regions

-->  c) lakes

d) shallow ponds

a piece of wood from an archeological site can be dated by tree rings if

a) it is no more than around 200 years old.

-->  b) it has a pattern that matches the scale created by scientists.

c) it was not taken from a tree that was cut a long distance from the site.

d) all of these.

an ice core can reveal the climate in the region for hundreds of thousands of years.

-->  a) true

b) false

diagram questions

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