Radioisotopes provide a way to date rocks
Half of the remaining radioactive material of the radioisotope decays
Radioisotope dating have been expanded and refined
The decay of potassium-40 to argon-40 has been used to date most of the ancient events in the evolution of life.
Fossils in the adjacent sedimentary rock that are similar to those in other rocks of known ages provide additional clues.
Radioisotopes dating of rocks, combined with fossil analysis, are the most powerful method of determining geological
How Have Earth’s Continents and Climates Changed over Time?
Earth’s crust consists of a number of solid plates approximately 40km thick, collectively make up the lithosphere. It
floats on a fluid layer of molten rock (magma)
Magma circulates b/c heat produced by radioactive decay deep in Earth’s core sets up convection currents in the fluid.
The plates move b/c magma rises and exerts tremendous pressure.
Where plates are pushed together, either they move sideways past each other, or one plate slides under the other,
pushing up mountain ranges and carving deep rift valleys (underwater known as trenches).
Where plates are pushed apart, ocean basins may form between them.
The movement of the lithospheric plates and the continents they contain is known as continental drift.
Oxygen has steadily increased in Earth’s atmosphere
Increase in atmospheric oxygen has been largely unidirectional.
The atmosphere of early Earth probably contained little or no free oxygen gas.
The increase in atmospheric oxygen came in two big steps; the first step occurred 2.4 billion years ago, when certain
bacteria evolved the ability to use water as the source of hydrogen ions for photosynthesis.
One group of oxygen-generating bacteria, cyanobacteria, formed rocklike structures called stromatolites.
Cyanobacteria liberated enough O t2 open the way for the evolution of oxidation reactions as the energy source for the
synthesis of ATP.
When it first appeared, oxygen was poisonous to anaerobic prokaryotes that inhabited Earth.
Those prokaryotes evolved the ability to metabolize O 2urvived and gained numerous advantages: aerobic respiration
proceeds at more rapid rates and harvests energy more efficiently.
An atmosphere rich in O 2lso made possible larger cells and more complex organisms.
In contrast to this largely unidirectional change in atmospheric2O concentration, most physical conditions on Earth have
oscillated in response to the planet’s internal processes.
Earth’s climate has shifted between hot/humid and cold/dry conditions
Earth’s climate was considerably warmer than it is today, and temperatures decreased more gradually towards the
At other times, Earth was colder than it is today; large areas were covered with glaciers during the end of the
Precambrian and during parts of the Carboniferous and Permian periods.