Pupils at Woodlands School, asks :-

How was the Grand Canyon formed?

Chuck Landis, a geologist at Otago University, responded.

The Grand Canyon formed as the result of millions of years of erosion by the Colorado River. The beautifully exposed sedimentary rocks seen along the eroded canyon walls were deposited on the sea floor between 500 and 250 million years ago. This was the Paleozoic Period of Earth history long before the canyon formed. Fossil trilobites and other primitive animals contained in these strata provide glimpses of the early stages of life. They also provide evidence that most of the rocks originated on the sea floor.

Change from a muddy and shelly seafloor to dry land occurred at end of the Paleozoic. As the Earth entered the Mesozoic Period, the age of dinosaurs, the seas in the region now occupied by the Grand Canyon retreated and the region became a low-lying landscape crossed by small rivers, then was buried under desert sand dunes, and still later inundated by coal swamps. During this time, between 250 and 65 million years, the Colorado River did not exist and there was no sign of the Grand Canyon. Similar low relief landscapes persisted into the Cenozoic Period, the age of mammals.

Late in the Cenozoic, about 5 million years ago, an enormous upwelling of hot rock from deep in the Earth's mantle rapidly lifted a huge slab of crust in western North America. This slab, larger than all of New Zealand, was uplifted from near sea level to one and a half kilometers high, forming the Colorado Plateau. A previously gently flowing river meandering across the plateau region was suddenly injected with new energy as it too was uplifted and began cutting down through the newly formed Colorado Plateau. Formation of the Canyon probably began as a series of waterfalls and rapids near the Pacific rim in western Arizona or southern California. The river began by cutting a small notch where it flowed over the edge of the new plateau. With time, river bed erosion caused the notch to work its way east across the Colorado Plateau into an area now underlain by northern Arizona. (This process, by which erosion gradually progresses from near the mouth of a river upstream towards its headwaters, "headward erosion", is also a characteristic of New Zealand rivers cutting headward into the Southern Alps. Over the next million years, through this process of headward erosion, the canyon gradually eroded its way eastward about 500 km across the monotonous plateau. At the same time, it was also eroding down its bed to produce one of the Earth's great wonders.