Law of Superposition and Radiometric Dating

Law of Superposition & Radioactive Dating

SC.7.E.6.3: Identify current methods for measuring the age of Earth and its parts, including the law of superposition and radioactive dating

Why should we care?

The Earth is constantly changing, for example, the rocks on top of a mountain may once have been at the bottom of the sea. They were raised to the top of mountains because of plate tectonics. To understand how this all happens, we need to learn about the Earth’s history. Time as we know it is measured in seconds, minutes, hours, days, weeks, months, decades, centuries and millennia. Studying Geologic Time is a bit different.

Geologic Time

When scientists talk about the history of Earth, it is measured in millions and billions of years. When studying the past, historians use dates to put events in their proper order. However, geologists have developed the geological time scale, which divides the Earth’s history into eons that are subdivided into eras, which are further divided into periods and then into epochs.

 Fossils are used as markers when building up the geologic time scale. The names of most of the eons and eras end in “zoic” which means life, because these time periods were recognized by the animal life present at the time.

There are also fossils known as Index Fossils, which help paleontologists determine the relative age of a rock. Index fossils must meet two criteria, they are geographically widespread and lived for short periods of geologic time. As such, if an index fossil is found in a rock, it’s easier for scientists to determine the age of that rock.

Fossils

Sedimentary rocks are the most common rocks at the Earth’s surface. Sedimentary rocks are formed in layers called beds. The layers are piled one on top of the other, so sedimentary rocks are formed bed by bed. Thus, in any sequence of layered rocks the lower bed is older than any bed on top of it.

Sedimentary Rocks & Relative Dating

Draw

Circle the layer with the oldest rocks.

Relative Dating - The Law of Superposition

The fact that younger rock is deposited on top of older rock is called the Law of Superposition, which simply states that older rocks are found under younger rocks. This is critical to the interpretation of Earth’s history, because it indicates the relative ages of rock layers and the fossils in them. Relative means that you are comparing the age of that rock or fossil to another rock or fossil. You do not get an exact age of the rock or fossil. This is known as Relative dating. The Law of Superposition is one of the most important relative dating methods.

Open Ended

What is the Law of Superposition and why is it essential to the interpretation of Earth’s history?

Absolute Dating

The most accurate form of dating is absolute dating. Absolute refers to getting an exact age, not just a comparison. Absolute Dating is done using a method called Radioactive Decay. This is when one element turns into a different element.

Open Ended

Explain the difference between relative dating and absolute dating.

This method works because some unstable (radioactive) elements turn into, or decay, at a known rate into a stable element.
Ex: sometimes the element uranium (U) turns into the element lead (Pb). This known rate of decay is called a half-life.

Therefore, to find the exact age of something, scientists measure the ratio of the remaining radioactive atom to the amount of stable element and voila, they know how long the molecule has been hanging out and decaying.

Absolute Dating - Radioactive Decay

Open Ended

How do scientists use radioactive decay to determine the exact age of a rock?

Multiple Choice

A paleontologist was studying several different fossils in her collection. She recorded the depth at which each was collected and recorded it in the table. What conclusion can she make about the ages of the fossils?

• Fossil 1 is the most recent because it was found the deepest.

• Fossil 5 was deposited in its rock layer much earlier than Fossil 2.

• Fossil 3 could date back to the beginnings of Earth.

• Fossil 4 was deposited at the same time that Fossil 2 broke apart on the surface.

Multiple Choice

Suppose a fossil is found in the Redwall Limestone Layer. Which of the following statements is a correct comparison to fossils found in other layers?

• Any fossils found in the Esplanade Sandstone layer are much older.

• Any fossils found in the Temple Butte layer are younger.

• Any fossils found in the Manakacha Formation are younger.

• Any fossils from the Hermit Shale layer are approximately the same age

Multiple Choice

Which rock layer at Site 2 is the youngest according to the Law of Superposition?

• Layer 1

• Layer 2

• Layer 3

• Layer 4