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Relative Time and Fossils Help

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By — McGraw-Hill Professional
Updated on Aug 30, 2011

Relative Time

Geological time is studied by two different methods. The first is a hands-on inspection of the positioning of the different layers of the Earth. This is known as relative or chronostatic time measurements. Relative time measurements are used to find the age relationships between layers and samples. Using relative time measurements, the age of earth layers is found by comparing them to neighboring layers above and below.

Even when the exact date of rock or materials is not known, it is possible to figure out the sequence of events that led to the current position of a sample. This ordering of samples and events is known as relative dating .

Placing a sample in an approximate time period compared to other samples with known ages is called relative dating .

The earliest attempt to order geologic events was done by Nicolaus Steno in 1669 when he described the following three laws that placed samples in time:

1. Law of Superposition,

2. Law of Original Horizontality, and

3. Law of Lateral Continuity.

The first law is the simple description of layers as they were piled on top of each other over time. Figure 2-2 shows the simple layering in the Law of Superposition that occurs when layers are left undisturbed.

Geological Time

Fig. 2-2. The oldest rock layers are found below younger layers in the Law of Superposition.

This is the foundation of all geological time measurements. For example, when archeologists study layers of ancient settlements and cities, they record the most recent top layers first, followed by older layers that are uncovered the deeper they dig.

Steno’s second idea, the Law of Original Horizontality, was also a new idea at the time. He believed that sediments were geologic layers found mostly in a flat, horizontal direction. Figure 2-3 illustrates how uneven layers are still horizontal even after base-layer bending and folding has taken place.

Geological Time

Fig. 2-3. The Law of Original Horizontality describes the overall flat-layered deposition of sediments.

Any solid material (rock or organic) that settles out from a liquid is known as sediment.

Driving along highways that have been cut into hills and mountainsides, you will see horizontal rock layers shifted at steep angles. These sediment layers were shifted after the original sedimentation took place.

The third of Steno’s laws describes the Law of Lateral Continuity . This law describes the observation that water-layered sediments thin out to nothing when they reach the shore or edge of the area where they were first deposited. This happens even though they were originally layered equally in all directions.

Sometimes scientists find in studying sediments that layers of different sections are missing. These layers have been split far apart through geological movements or by timeless erosion. If a sample is taken from a section with a missing or eroded layer, the true picture of its sedimentation can’t be seen.

An unconformity is a surface within several layers of sediment where there is a missing sedimentary layer. This is usually found between younger and older rock layers. If this unconformity happens in a wide area of erosion, maybe over an entire mountain range, the time period under study may be misunderstood or completely lost. We will learn more about different kinds of unconformities found in sedimentary rock when we take a closer look at strata and land eras.

A disconformity contains an area where the sedimentary rock layers, located above and below, are aligned in parallel, but have an area in-between that is different. This can happen when layers of water-covered sediment are uncovered for a time in one era, allowing the environment to erode away or add a different layer, then recovered with water. The layer that was exposed to the environment will be different from its neighbor. However, when they are both covered with water again, the sediment layer will be deposited on both the same way. So unless scientists look closely or the exposed layer is especially thick, the differences might not be noticed.

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