Grand Canyon Geology


地質学的基盤

The Grand Canyon: a monumental earthen fissure stretching 277 miles (466 km) long and over a mile deep, streaked with colorful bands from top to bottom. These stripes are actually several different rock layers and have provided a near-perfect timeline for geologists to discover and date the canyon's history. Through comprehensive analysis, geologists have determined that the initial foundation was laid over 1.2 billion years ago, long before even the oldest species of dinosaurs. At this time, the face of the Earth looked much different than we see on maps today. All of the continents were connected as one landform called "Pangaea".

Pangaea was separated into the continents we know today because of what geologists call continental drift. The Earth's crust is composed of about twenty "plates" that constantly shift. 550 million years ago, Pangaea was connected to several different plates, and their shifting is what eventually caused the massive landform to "break apart" and spread across the Earth. This is evidenced not only by the apparent puzzle piece fit of the modern continents, but also by fossils and rock types found on opposite sides of the world. The fossil of an ancient animal found in both South America and in Africa suggests that at one point these two continents were connected and the animal roamed free across the entire expanse.

Plates not only shift apart from one another, they collide as well. When collisions happen, plates are driven together with so much force that it pushes its land upward, forming mountains. This is what formed the Rocky Mountains, the Sierra Nevada, and the coastal mountains of California. Millions of years ago, another mountain range existed that was much higher than the Rocky Mountains and possibly even the Himalayas. It is the sediment from these long-gone mountains that forms the base of the Grand Canyon.

Volatile climate change in these early days of Earth often raised the sea level immensely. When this happened, these mountains would be surrounded - and often submerged- in water. The water high sea level, the rock face of the mountains would be eroded by the water. When the sea level was high the water would erode the rock face of the mountains, sometimes for hundreds of years. When the water level lowered, the sediment that was loosened would drop with it, settling in a layer on the ground. Over millions of years, the entire mountain range had been eroded and deposited layer by layer onto the ground in a flat plain. These built-up layers of sediment formed what is called the Colorado Plateau. Geologists can easily verify that the ocean caused this major landform alteration for two reasons. The rock comprising the Colorado Plateau is the same rock found at the bottom of the ocean, which indicates that it was pushed upward during a plate collision to form these ancient mountains. Additionally, fossils of prehistoric oceanic animals have been discovered within the layers of rock in the Colorado Plateau. While geologists can estimate the length of time required for each layer to form, large gaps in the rock strata make it difficult to determine exactly how much time passed in between layers.

But how did the Grand Canyon appear within the Colorado Plateau?

The answer is again erosion.

60-70 million years ago the Rocky Mountains formed, and the western drainage of these mountains became the Colorado River. Over these millions of years, the Colorado River twisted its way through the plateau - changing courses several times as the terrain below it was altered. Another shift of the Earth's plates caused the plateau to tilt westward. This slanted the ground, allowing gravity to pull the westbound river water faster. The increased speed of the water flow is how the Grand Canyon began to erode.

Over the next 5-6 million years, the rushing Colorado River cut into the earth, exposing each stacked sediment layer of the plateau little by little. These are the stripes stretching across the canyon walls - in fact, nearly 40 different rock layers have been identified. The deepest section is called the Vishnu Schist - Early Proterozoic crystalline rocks dating back 1.7 billion years. Visitors to the canyon are not only witnessing the glorious vision of the banded walls, they are also standing atop a natural masterpiece almost 2 billion years in the making.

豆知識

Rock strata located within the Grand Canyon ranges in age from 1,700 to 245 million years old, the oldest occurring within the Precambrian period.

The Chinle and Moenave Formations are ripe with Mesozoic fossils (245-66 million years ago). Petrified forests, small and large dinosaurs, lungfish, brachiopods, coral, mollusks, sea lilies, fish teeth and crocodilians have been uncovered so far.

Fossilized reptile tracks from the Permian Age (286-245 million years ago) are imbedded in Coconino Sandstone.

Vulcan’s Anvil (River Mile 178) is the core of an extinct volcano within the Colorado River. Not far from Vulcan’s Throne, it overlooks Lava Falls on the north side of the river. This cinder cone is the largest of many volcanic cores found within the Canyon.

Marble Canyon does not contain any marble. Instead, it gets its name from its polished limestone and Hermit shale walls.