how is geological time measured?

Answer 1

GEOLOGIC TIME

The Earth is very old -- 4.5 billion years or more -- according to recent estimates. This vast span of time, called geologic time by earth scientists, is difficult to comprehend in the familiar time units of months and years, or even centuries. How then do scientists reckon geologic time, and why do they believe the Earth is so old? A great part of the secret of the Earth's age is locked up in its rocks, and our centuries-old search for the key led to the beginning and nourished the growth of geologic science.

Mankind's speculations about the nature of the Earth inspired much of the lore and legend of early civilizations, but at times there were flashes of insight. The ancient historian Herodotus, in the 5th century B.C., made one of the earliest recorded geological observations. After finding fossil shells far inland in what are now parts of Egypt and Libya, he correctly inferred that the Mediterranean Sea had once extended much farther to the south. Few believed him, however, nor did the idea catch on. In the 3rd century B.C., Eratosthenes depicted a spherical Earth and even calculated its diameter and circumference, but the concept of a spherical Earth was beyond the imagination of most men. Only 500 years ago, sailors aboard the Santa Maria begged Columbus to turn back lest they sail off the Earth's "edge." Similar opinions and prejudices about the nature and age of the Earth have waxed and waned through the centuries. Most people, however, appear to have traditionally believed the Earth to be quite young--that its age might be measured in terms of thousands of years, but certainly not in millions.

The evidence for an ancient Earth is concealed in the rocks that form the Earth's crust and surface. The rocks are not all the same age -- or even nearly so -- but, like the pages in a long and complicated history, they record the Earthshaping events and life of the past. The record, however, is incomplete. Many pages, especially in the early parts, are missing and many others are tattered, torn, and difficult to decipher. But enough of the pages are preserved to reward the reader with accounts of astounding episodes which certify that the Earth is billions of years old.

Two scales are used to date these episodes and to measure the age of the Earth: a relative time scale, based on the sequence of layering of the rocks and the evolution of life, and the radiometric time scale, based on the natural radioactivity of chemical elements in some of the rocks. An explanation of the relative scale highlights events in the growth of geologic science itself; the radiometric scale is a more recent development borrowed from the physical sciences and applied to geologic problems.

Answer 2

I know it sounds simple, but - geologist dig holes and determine how old the rocks are. The deeper the rocks the longer ago they appeared on the earth. Now this isn't an absolute measurement, but the stratification of rock and the type of rock will tell you how much time had passed since that rock was deposited. This can now be confirmed with carbon dating studies, which look for the decay of radioactive isotopes of carbon (naturally occuring) in the stone (or anything else that is found in those layers).

Answer 3

Verrrrrrrry slowwwwwwllly.

Serioiusly, because geological process take to long, things are dated going back millions and multiples of millions, even billions of years.

You may see terms used such as eons and ages, but there is no specific figure for how long an eon is. Just accept that it is a very long time and "eons and eons" is a much longer time.

Answer 4

Geological time is broken up into eons, eras, periods, and epochs. The are four eons, and many of each of the smaller divisions.

http://en.wikipedia.org/wiki/Geologic_time

Scientists use radiometric dating to determine the scales used in geologic time, if that's what you're looking for.

http://en.wikipedia.org/wiki/Radiometric_dating

Answer 5

Potassium Argon Dating.

Answer 6

in years

Answer 7

Several means, depending on how long ago we're talking about. For recent past, tree rings and radiocarbon (C-14) dating. For farther back, other radioactive species. For really far back, uranium radioactive dating.