Geological Timelines
Scientific consensus informs us that the Big Bang occurred about 14 billion years ago. Out of the swirling cosmic dust, the process of accretion formed our solar system about 4.5 billion years ago.
As you can see from the Timeline chart of the earth, time is measured in eons, eras, periods, and epochs. “Ma” in the header column indicates millions of years ago (sometimes written as Mya). Throughout history, the earth has alternated between glacial periods in which enormous masses of ice covered much of the earth’s land mass, and interglacial periods when the glaciers melted, sea levels rose, and vast areas of land were revealed beneath the melted ice. These alternating phases of glaciation happen over thousands of years. The last ice age ended about 9.5 million years ago and marks the beginning of the current Holocene Epoch.
How do we know this geological overview of time is correct? As Charles Darwin wrote in the Origin of Species, “The crust of the earth is a vast museum, but the natural collections have been made only at intervals of time immensely remote.” (Darwin 1859) The natural collections he was referring to are fossils.
Fossils are the petrified casts of plants and animals preserved in sedimentary rocks. Early geologists discovered that the newer strata of rocks lie atop the older layers of rocks. Paleontologists dig into the rock strata, often in remote and dangerous regions of the world, to discover the fossilized evidence of long-extinct species and ancient civilizations. And the order of stratum in which fossils, pottery shards, shells, skeletons, etc. are found allowed these early geologists to develop a system of determining the relative ages of their finds based on their position in the strata. Relative ages, however, aren’t precise.
“Since about 1945,” wrote Jerry A. Coyne in his bestseller Why Evolution is True “we have been able to measure the actual ages of some rocks—using radioactivity. Certain radioactive elements (radioisotopes) are incorporated into igneous rocks when they crystallize out of molten rock from beneath the earth’s surface. Radioisotopes gradually decay into other elements at a constant rate, usually expressed as the ‘half-life’—the time required for half of the isotope to disappear… Different isotopes decay at different rates. Old rocks are dated using uranium-235, found in the common mineral zircon. U-235 has a half-life of around 700 million years. Carbon-14, with a half-life of 5,730 years is used for much younger materials like wood, bone, or human artifacts such as the Dead Sea Scrolls. Several radioisotopes usually occur together, so the dates can be cross-checked, and the ages invariably agree. The rocks that bear fossils, however, are not igneous but sedimentary, and can’t be dated directly. But we can obtain the ages of fossils by bracketing the sedimentary layers with the dates of adjacent igneous layers that contain radioisotopes.” (Coyne 2009)
Coyne goes on to say that young earth creationists, staunch opponents of evolution, believe that Genesis reveals that the true age of the earth is only six to ten thousand years old. That’s 6,000 to 10,000 years old as opposed to the scientific consensus based on radioisotope dating that the earth is 4.5 billion years old. That’s 6,000 years old versus 4,500,000,000 years old. Quite a shocking difference in perspective! Scores of books have been written by Christian fundamentalists who have espoused this Biblical point of view despite a mountain of scientific evidence to the contrary.
This anti-intellectual attitude in modern America has long been championed by Christian fundamentalists. And the Great Divide between those who believe in science and those who don’t is one of the underlying reasons the United States is so profoundly polarized.
Another example in Coyne’s excellent book further illustrates the point by clarifying the meaning of scientific theory. (to be continued in next post)
