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This U.S.Geological Survey site shows how chemists and geologists use analytical chemistry to: determine the age of the Earth; show that an extraterrestrial body collided with the Earth; predict volcanic eruptions; observe atmospheric change over millions of years; and document damage by acid rain and pollution of the Earth\'s surface.
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INDEX
Foreword
I. Introduction
II. Understanding the Earth
IIa. History recorded in chemistry. How old is the Earth?
III. Mapping the chemistry of the Earth\'s surface
IIIa. Assessment of public lands
IV. Can we depend on chemical analyses?
IVa. Measuring quality
FOREWORD
This document describes the role of chemistry in issues vital to our economy, health, and well-being. When we are analyzing a sample of the Earth, we never ask if a specific element is present. Virtually every sample of the Earth contains every natural element at some amount. The more appropriate questions are: How much of it is present? Is there enough to be mined profitably? In the environment, is it dangerous at this level or in this form? And after we\'ve identified the issues that we need to solve about our planet, we then need to ask, What clues can we find that will give us the answer?
We will show you how many geologic problems are solved using routine analyses of the major components of rocks. We will also show you the complexity of analyzing trace amounts of common components in extremely small samples, such as rare samples of air from more than 100 million years ago, tiny samples of ore-forming fluids that were entombed in minerals 300 million years ago, or small amounts of naturally-occurring radioactive isotopes that are as old as the Earth. Because some elements in our environment are hazardous at trace levels, they must be analyzed down to those low levels. The impact of quality control on analyses will also be discussed, as well as the production of standard reference materials that are distributed internationally to Federal and private laboratories.
As the primary Federal Earth-Science Agency, the USGS studies and provides solutions to questions concerning our planet, assesses the mineral resources of Federal lands, and serves as a repository for geochemical data generated by numerous Federal programs. These data are being applied to new economic and environmental concerns and provide a cost effective method to solve geochemical problems, often with no impact on wilderness or fragile refuges.
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I. Introduction
Questions about geology -the science of the Earth- can be difficult to answer because many times we can\'t safely get close enough to the event. Even if we can, our senses are not sharp enough to detect everything that is happening. The Earth is relentless in its course of change, but the transformation occurs over a vast amount of time. Some geologic processes can take a million years or more to complete. We know that today\'s events have also occurred repeatedly throughout geologic time. To understand our planet Earth, we need to read and interpret the permanent records in the Earth\'s crust and interior. These records are the key to the future, and many of these clues are preserved in the chemistry of geologic samples.
Everything we touch in our daily lives is made up of elements. There are 92 elements that occur naturally, and in most cases, the human senses cannot recognize these elements when they are present in a compound. If, for example, we could always recognize what something is made of, there would be no such thing as \"fool\'s gold\" (a natural combination or iron and sulfur called pyrite). Because we have difficulty identifying these relatively pure compounds, it\'s not surprising that when rock or soil contains only a very small amount of an element we are incapable of recognizing the element\'s presence.
Using only our vision, pyrite is easily confused with gold, so much so that the common name for pyrite is \"fools gold.\"
Using analytical chemistry, we can even determine trace elements (elements present at very low levels) at the parts per million (ppm) or parts per billion (ppb) level. It\'s difficult to comprehend the concentration of a substance at this low a level. To get a mental picture, imagine an average 3-bedroom home. It would take about 1 million marbles to cover the floors of the home. One part per million would be represented by just one marble among all the other marbles. For that same marble to represent one part per billion, however, it would take 20 football fields covered with marbles.
Different elements have different physical properties. These properties determine what methods can be used to analyze each element (or group of elements). The methods described in the WWW document can be applied to many different geological problems, but no one method can solve every problem. The analytical methods described here are only a few that were selected to show the role of chemistry in geology.
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