Climate Clues from Sand and Mud
Someone who lives on the edge of the Appalachian Mountains can look out the window today see rolling green hills that average 3,000 feet in height.
But, if they had lived in the same place 300 million years ago, a completely different scene would be framed in your window—mountains as high as the Himalayas, over 25,000 feet in height.
It's hard for us to imagine that mountains originally tens of thousands of feet high can be reduced to piles of gravel and sand over millions of years, or that enormous boulders will end up as tiny grains of sand given enough time. The agents of weathering and erosion are diverse and work together to break down rocks into progressively smaller bits and pieces called sediments. These agents include wind, running water, and a physical process in which water is trapped in tiny cracks in rocks to freeze and expand repeatedly that dislodges bits of material. Gravity is another agent of erosion—landslides, avalanches, and rock slides all combine to bring materials down toward the basin floor. Also, the slow, grinding creep of glaciers and ice sheets wear away the surface rocks, pulverizing some to dust (called rock flour). Over millions of years, sediments of different types are produced. Where does all this sediment go?
The short answer is: downhill. Gravity moves the water and the water dissolves components of soil and carries sediments. The sediment is composed of both living and nonliving material— soil, sand, gravel, dirt, clay, and organic materials. Sediments from higher regions are carried by water and wind to fill valleys, lakes, and bays, while large amounts are carried by rivers and streams directly into bays, gulfs, and oceans.
Tucson, Arizona is located in a basin surrounded by mountains. Over the past 12 million years the mountains, once over 15,000 feet high, have been ground down to less than 9,000. The result: over a mile of sediment has been deposited in the basin. In regions where rivers empty into large bodies of water, silt, sand, and gravel pile up to impressive depths. Parts of the Mississippi delta are over 35 km thick. The mighty river can fill a bay with sediment 15 meters thick in just 150 years (Coleman, GSA Bulletin). In this Earthlab, we will focus on how sediments are deposited on ocean margins. Investigating the dynamics of how sediments are deposited at ocean margins helps us understand how sediment cores are interpreted and analyzed.
In Part A, you will learn how particle size affects the rate of sediment deposition and how sediment layers form. In Part B, you will build a physical model that shows how sediments are deposited at ocean margins during periods of glaciation.
After completing this Lab you should be able to:
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.