During ice ages, the most characteristic change to the planet has been the formation and spread large ice sheets and glaciers across much the Northern Hemisphere.The sheer weight of the ice at the height of the last ice age depressed Earth’s crust to such an extent that many areas are still slowly but noticeably rebounding to this day, 18,000 years after the retreat of the glaciers.[ MORE ]The formation of the ice also removed so much water from the global ocean that sea levels during ice ages were notably lower than interglacial periods such as the present day—as much as 400 feet lower during some periods.Some of the isotopes used for this purpose are uranium-238, uranium-235 and potassium-40, each of which has a half-life of more than a million years.
These were most likely driven by regular changes in Earth’s orbit and rotation known as the Milankovich Cycles that govern the seasonal timing and intensity of solar energy entering the atmosphere.
Other factors that may have contributed to the formation and cessation of ice ages are the amount of greenhouse gases (mainly carbon dioxide, methane, and water vapor) in Earth’s atmosphere, the extent of sea and land-based ice across the northern hemisphere, and shifts in patterns of wind and ocean currents.
The most widely known form of radiometric dating is carbon-14 dating.
This is what archaeologists use to determine the age of human-made artifacts. The half-life of carbon-14 is only 5,730 years, so carbon-14 dating is only effective on samples that are less than 50,000 years old.
Each of them typically exists in igneous rock, or rock made from cooled magma.
Fossils, however, form in sedimentary rock -- sediment quickly covers a dinosaur's body, and the sediment and the bones gradually turn into rock.
Earth’s climate has undergone many changes over the course of geologic history, but the past one million years or so have been among the most dynamic.
During that time, the planet has experienced repeated cycles of glacial (cold) and interglacial (warm) periods lasting about 80,000 years on average.
For example, over time, uranium atoms lose alpha particles (each made up of two protons and two neutrons) and decay, via a chain of unstable daughters, into stable lead.