How old is the Earth?: Crash Course Geology #17
CrashCourse
A strange discovery in Michigan 0:00
In 1844 government surveyors walking through Michigan's Upper Peninsula toward Lake Superior found their compasses going haywire. What they had stumbled on was a large iron deposit, and that find would eventually help change what people understood about the history of the world.
Breaking down geologic time 0:32
Geologic time, also called deep time, is the history of planet Earth and it began about 4.6 billion years ago when a cloud of dust and gas collapsed into a planet. That scale is divided into eons lasting hundreds of millions to billions of years, starting with the Hadean Eon when Earth was covered in molten rock, up through today's Phanerozoic Eon. Eons break into eras, such as the Mesozoic, the age of reptiles and dinosaurs, and eras break into periods lasting tens of millions of years, each of which often ends with a major event like the extinction of the dinosaurs at the close of the Cretaceous period. Periods break further into epochs of several million years, and we currently live in the Holocene epoch, marked by human civilization and climate change. Humans themselves only appeared about 300,000 years ago, a tiny sliver of that whole history.
What the iron formation revealed 3:31
The Michigan discovery turned out to be a banded iron formation, or BIF, which threw off the surveyors compasses. Billions of years ago, dissolved iron floated through early oceans with little oxygen present, until around two and a half billion years ago oxygen suddenly surged, likely from photosynthesizing bacteria, in an event called the Great Oxidation. That oxygen bonded with the iron, turning it into a solid that sank to the ocean floor in the alternating iron rich and silica rich stripes that give BIFs their zebra look, though the exact cause of those stripes remains unclear. BIFs stopped forming around 1.85 billion years ago, and today most of the iron used to make steel comes from them.
From catastrophism to uniformitarianism 5:51
Before the 1950s most people believed Earth was only a few thousand years old, shaped suddenly by one dramatic event, a view called catastrophism. In the 1700s Scottish geologist James Hutton proposed uniformitarianism instead, the idea that the same slow processes happening today shaped Earth throughout its history, a view later popularized by Charles Lyell. This thinking supported the principle of faunal succession, developed through English surveyor William Smith's fossil observations, which holds that fossils appear in a consistent order through rock layers, deeper ones being older. Eventually scientists recognized that catastrophic events, like the floods that carved the channeled scablands in Washington State, also shaped the planet.
Dating rocks precisely 7:52
Faunal succession is a form of relative dating, showing which rocks are older or younger without giving exact ages. Absolute dating, part of the field of geochronology, solves that using radiometric or isotopic dating, which tracks how radioactive parent isotopes decay into daughter isotopes at a constant, known rate. In 1955 American geochemist Clair Patterson and colleagues used this method, comparing uranium decay in Earth rocks and meteorites, to calculate for the first time that Earth is about 4.6 billion years old.
Living with deep time 9:51
That number is almost impossible to grasp, and against it human existence looks like a tiny blip. Still, some scientists suggest that reflecting on deep time, such as imagining what your own neighborhood looked like millions of years ago or will look like millions of years from now, can offer a sense of awe and useful perspective on life.
AI-generated summary. It can be wrong or incomplete - check anything that matters against the original.
