Radiometric dating earth science
But for humans whose life span rarely reaches more than 100 years, how can we be so sure of that ancient date? Even the Greeks and Romans realized that layers of sediment in rock signified old age.
But it wasn't until the late 1700s -- when Scottish geologist James Hutton, who observed sediments building up on the landscape, set out to show that rocks were time clocks -- that serious scientific interest in geological age began.
Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4.6 billion years.
Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free.
Droughts and other variations in the climate make the tree grow slower or faster than normal, which shows up in the widths of the tree rings.
There are over forty such techniques, each using a different radioactive element or a different way of measuring them.
Each dark band represents a winter; by counting rings it is possible to find the age of the tree (Figure 11.22).
The width of a series of growth rings can give clues to past climates and various disruptions such as forest fires.
A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved.
However, by itself a fossil has little meaning unless it is placed within some context.