(Geologic) Timing Is Everything!

Part A: The History of Earth

Introduction

How can scientists determine when geologic events occurred in the 4.6 billion year history of our planet? One way to figure out relative age is by a process called superposition. This principle says that lower rock layers (and fossils in them) are older than higher ones, unless there is evidence that the layers have been overturned. This makes perfect sense since the lower sediments would have been deposited first.

Rock layers can be compared, or correlated, with each other based on their mineral composition, fossil content, and other features. When a rock unit or series matches those of another area, they are said to be correlated and considered to be of similar age. Ages of sections of sediment cores can be correlated by comparing their fossil assemblage. Index fossilsa fossil of an organism, known to have existed for a relatively short period of time, used to date the rock in which it is found. are particularly useful. They are fossils of organisms that lived for a relatively short period of time. Good index fossils are easily recognized and widely distributed around the world, so samples of rocks and sediments from all over the world can be compared and dated.

Scientists determine exact ages of rocks and sediments in labs on shore by measuring the amounts of different radioactive isotopes one of two (or more) atoms of the same element which only differ in the number of neutrons found within its nucleus. e.g. H¹ (no neutrons) and H² (1 neutron), H³ (2 neutrons) contained within certain minerals in the sediment cores. What is an isotope? Isotopes are elements, meaning they have a specific chemical identity due the number of protons they contain, but their masses vary because they have a different number of neutrons. For example, all isotopes of oxygen contain 8 protons, but they can have 8, 9, or 10 neutrons in the nucleus (¹⁶O, ¹⁷O, and ¹⁸O). Oxygen-16 is a stable isotope and is the oxygen we breathe. The other two isotopes are also stable but have additional neutrons.

Many isotopes are unstable and are radioactive. These isotopes decay at different rates, making them useful for determining the ages of different materials. Expedition 341 scientists will refine the ages of sediment cores by using the radioactive isotope Potassium-40 (⁴⁰K) to date minerals within layers of volcanic ash. Over time, Potassium-40 (⁴⁰K) slowly decays into stable Argon-40. By measuring the ratio of ⁴⁰K /⁴⁰Ar in a mineral, the age of the sediment can be determined. It takes approximately 1.3 billion years for half of the K⁴⁰ in a sample to decay ⁴⁰Ar, so K-Ar dating is useful for determining the age of rocks that are more than 100,000 years old. The amount of time for half of a radioactive isotope to decay is called the half life the time it takes for half the atoms in a radioactive substance to decay. of the isotope.

For younger sediments, up to 50,000 years old, scientists use the change in the amount of carbon 14 contained in in the sediments to date them. Organic matter like pollen a fine powder produced by flowers that fertilizes other plants for reproduction. and foraminfera a type of zooplankton, a single-celled marine organism. (and humans!) absorb ¹⁴C from the atmosphere. When a sediment is deposited, it stops absorbing ¹⁴C since it is no longer exposed to the atmosphere. Like Potassium-40, ¹⁴C is a radioactive isotope, so it slowly decays into stable carbon (¹²C) with a half life of 5,500 years.

Lab Procedure

1. Obtain one set of Earth History Timeline Cards for your group or download them here. (Microsoft Word 2007 (.docx) 21kB Nov2 21) Do your best to put them in sequence from the oldest to the most recent events.

2. Once your group has put the cards in a sequence, obtain a Timeline Organizer sheet from your teacher and check your sequence. Give yourselves a point for every event that your group has put in the correct order.

3. If any events were out of sequence, reposition them so your cards are in the correct order.

4. Your teacher will gather all the Timeline Cards and will then redistribute one set, so that your group only has a few cards. On the back of each Timeline Card you are given, write how many million years ago (MYA) the event occurred. This information is on the Timeline Organizer sheet.

5. Read Understanding the Timeline Model, below.

6. With your class, examine the placement of each of the cards on the timeline, and discuss the event's role in Earth's history.

• Figure out where each of your cards belongs on the string.
• Place each of your cards on the timeline string when your teacher asks you to.
• Be prepared to discuss your cards and their significance.
• As appropriate, make comments and ask questions about the other cards and their relationship in the emerging timeline.

Reading – Understanding the Timeline Model

Our solar system has existed for approximately 4.6 billion years. That entire span of time can be captured in a timeline just 4.6 meters long! In such a timeline, 1 millimeter equals 1 million years.

1 mm = 1 million years

10 cm = 100 mm = 100 million years

1 m = 100 cm = 1000 mm = 1000 million years = 1 billion years

4.6 billion years (the age of our solar system) = 4600 million years = 4.6 meters

One end of the timeline represents today. That mark is the 0-meter mark. The other end of the timeline represents the formation of the solar system — 4.6 billion years ago. That mark is the 4.6 meter (4 meters and 60 centimeters) mark. By hanging the cards between these two end points, you can place any event in the history of Earth.

For example, about 2.1 billion years ago enough oxygen had accumulated in the atmosphere so that Earth formed an ozone layer. 2.1 billion years equals 2.1 meters on the string timeline. Thus, the Earth's ozone layer forms card belongs at the 2.1 meter (2 meters and 10 centimeters) mark.

As another example, 540 million years ago the Cambrian Explosion occurred "540 million years ago" equals 54 centimeters on the string timeline. There is no 54-centimeter mark on the string. However, there is a 50-centimeter mark and a 60-centimeter mark. The Cambrian Explosion card belongs a little less than halfway between those marks.

Of course, with 1 millimeter equaling 1 million years, there are limits to the timeline. Events that occurred within 1 million years of each other are less than 1 millimeter apart! 1 million years is a very short period of time in the history of a planet.