(Geologic) Timing is Everything!


The words slide off the tongue—Earth was formed from a cloud of hot gas and dust 4.6 billion years ago. But how long is that? One of the most difficult things for the human brain to comprehend is the scope of geologic time because it is impossible to visualize the magnitude of numbers like a million or billion.

Sediments are continually deposited by streams and rivers into both large and small bodies of water. The science of examining and analyzing layers of sediment and sedimentary rock is called stratigraphy a branch of geology which treats the formation, composition, sequence and correlation of the layered rocks as parts of the Earth's crust.. In almost all cases, the layer on the top of a series of strata is the youngest and the ones underneath are progressively older. So, geologists can establish the relative age of the layers by inspection. But how can we determine exactly how long ago each layer was deposited?

When scientists drill through sediment and bring up a core sample, each meter they pass through represents an opportunity to look further back in time. These sediments can be dated using a number of different methods, enabling scientists to determine relative (approximate) or absolute (exact) ages of the rocks and sediments. Determining the ages of the sediments is one of the most important jobs on board the JOIDES Resolution and is the primary responsibility of the shipboard paleontologists and paleomagnetists. These scientists determine the relative ages of the sediments by using proxies (indirect evidence), such as the magnetic properties of the sediments or by identifying the types of microfossils preserved in the sediments.

Watch the following video. Exp. 341 paleontologists a geologist who researches geological periods through the study of fossils. Dr. Maureen Davies and Dr. Christena Belanger explain how they use microfossils and paleomagnetism the fixed orientation of a rock's crystals, based on the Earth's magnetic field at the time of the rock's formation, that remains constant even when the magnetic field changes over time. of sediments to determine the ages of sediment cores.

Stop and Think

1. How do scientists use the paleomagnetic record to determine the ages of sediment cores?

2. What is an index fossil, and how are index fossils used to help determine the ages of sediments?

In Part A this activity, you will investigate the scope of geologic time by creating a timeline that marks major geologic and biological events over the 4.6 billion year existence of planet Earth. In Part B, you will make another timeline that encompasses the 23 million year span of time that the IODP expedition will focus on in terms of sedimentation and climate change.

After completing this Lab, you should be able to answer the following questions:

  • What were the major climate events during the past 23 million years?
  • How does the scope of human civilization compare to the entirely of geologic time?

Keeping Track of What You Learn

In these pages, you'll find two kinds of questions.

  • Checking In questions are intended to keep you focused on key concepts. They allow you to check to be sure the material is making sense. These questions are often accompanied by hints or answers to let you know if you are on the right track.
  • Stop and Think questions are intended to help your teacher assess your understanding of the key concepts and skills. These questions require you to pull some concepts together or apply your knowledge in a new situation.

Your teacher will let you know which questions you should answer and turn in.