The Mystery Behind Earth’s Missing Rocks: Crash Course Geology #16
CrashCourse
Grand Canyon's Missing Chapters 0:00
The Grand Canyon holds one of the most complete geological records on Earth, but even here entire chapters are missing, up to a billion years in some spots. The canyon's walls are made of layered rock called strata, which record what the planet looked like in the deep past. The Paiute described the canyon as a mountain lying down, and nineteenth century geologist John Wesley Powell called it a library of the gods. Its rock is grouped into three main sections, the Vishnu basement rocks, the Grand Canyon Supergroup, and the layered Paleozoic rocks, and together they can be read to trace floods, tectonic shifts, sea level changes, and volcanic eruptions.
How Rock Layers Form 1:31
Sediment is the raw material of the story, made of worn down rock fragments moved and deposited elsewhere. Weathering breaks rock down, erosion by water, wind, ice, or gravity carries it away, and it often settles in basins like the Colorado River Basin, where the Grand Canyon sits. Buried deep enough, sediment hardens into sedimentary rock, and beds stack into strata. The canyon's rocks let geologists look back at least 270 million years, even though the canyon itself was carved only in the last five to six million years by the Colorado River. Features like ripples, mud cracks, and angled crossbeds reveal wind direction, water flow, and dry spells. Moving down through the layers, marine fossils show when the area was under a shallow sea, single celled life appears in the Grand Canyon Supergroup, and the Vishnu basement rock at the bottom, over 1.3 billion years old, records volcanic islands crashing into an ancient continental edge.
Reading Rock Ages And Gaps 4:31
Relative dating uses principles like faunal succession, superposition, original horizontality, and cross cutting relations to order events without giving exact ages. Applying these to the canyon shows rocks ranging from about 1.75 billion years old at the bottom to 270 million years old near the top. Yet huge stretches of time are simply missing, called unconformities, including nonconformities, disconformities, and angular unconformities. The largest, the Great Unconformity, skips as much as 1.2 billion years, about 25 percent of Earth's history, and appears in rock layers worldwide, usually hidden. Possible explanations include frozen-earth glaciers scraping the surface, uplift and erosion from the breakup of the supercontinent Rodinia, or several separate periods of erosion mistaken for one event. Geologists have not yet solved the mystery.
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