Rocks

Rocks: Geologic Books on a Shelf

Geologic cross-section

Geologic cross-section

Sedimentary Rock

Weathering and erosion of the Earth's surface, primarily by flowing water but also caused by other agents such as wind and chemical dissolution, causes rocks and soil to wear away. Streams, wind, landslides, and glaciers transport the sediment downhill to a region of deposition, usually but not always underwater. Deposition of mineral and organic particles can take place on land or underwater, causing successive strata of sediment to build up in layers. As more layers of sediment accumulate above, the lower layers are transformed into rock by heat and pressure.

Depending on the type and size of particles that are deposited, as well as the environment, different types of sedimentary rock are created. Although the technical definitions are beyond our needs here, sandstone forms when small grains of hard minerals such as quartz and feldspar are cemented together. In contrast, shale forms when the grains are silt-sized, or much finer than sand.

Limestone is usually formed underwater from skeletons of small sea creatures such as coral and mollusks. Coal is a sedimentary rock formed from accumulations of plants. Chemical precipitation of minerals in solution, as when a large lake dries up, creates sedimentary rocks from evaporite minerals such as halite (common table salt) and gypsum.

Less commonly, sedimentary rocks form from landslides, volcanic flows of hot ash, and impact debris from a meteorite strike.

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Igneous Rock

Major rock layers of the eastern Grand Canyon

Major rock layers of the eastern Grand Canyon

Igneous Rock

Igneous rock is formed from the cooling of molten rock, or magma. Intrusive molten rock forms and solidifies underground, while extrusive rock reaches the surface in a liquid state and creates volcanic eruptions and lava flows. Magma is created from older, solid rock by heat and pressure, sometimes aided by a change in chemical composition.

Metamorphic Rock

Either sedimentary or igneous rock, as well as metamorphic rock, can be changed by the heat and pressure found deep underground into an entirely new type of rock. Although much of the original rock's character is lost during the transformation, often clues remain. Sandstone, for example, is often metamorphosed into quartzite, and shale and siltstone often become schist.

A basic principle of geology is that younger rocks are always deposited on top of older rocks. Study of the sequence of rock layers is our main source of knowledge about the Earth's history, including the evolution of life and changes in climate.

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Vishnu Schist

Dark gray Vishnu Schist with intrusions of lighter-colored granite form the V-shaped Granite Gorge

Dark gray Vishnu Schist with intrusions of lighter-colored granite form the V-shaped Granite Gorge

The oldest rock formations in the canyon are found in the three deep, V-shaped, inner gorges exposed along the river bottom. Granite Gorge, the first of the three, is visible from many of the South Rim overlooks, while Middle Granite Gorge starts below the west side of Powell Plateau in the central canyon, and Lower Granite Gorge is in the Diamond Creek area in the western Grand Canyon. The Vishnu Schist is dark gray to black, and gives the three Granite Gorges their somber tone. The rocks look ancient, and they are. Originally deposited as sediments and volcanic rocks 1.9 billion years ago, the rocks were later metamorphosed into schist by the intense heat and pressures of mountain-building forces. Schist is characterized by thin layers or plates, created by extreme pressure. Liquid magma from deep in the Earth subsequently intruded into the dark schist and formed pinkish bands of granite, which can be seen zigzagging up the walls of the inner gorge like solidified lightning bolts. Erosion then wore the ancient mountains down until they formed a nearly flat plain.

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Grand Canyon Supergroup

Tilted layers of the Grand Canyon Supergroup form the floor of the eastern Grand Canyon

Tilted layers of the Grand Canyon Supergroup form the floor of the eastern Grand Canyon

About 1.2 billion years ago, over a period of millions of years, more than 13,000 feet of sedimentary and volcanic rocks were deposited on top of the plain, which lay near a coastline and under shallow seas. The major formations in the supergroup are the Dox Limestone, Shinumo Quartzite, Hakatai Shale, and the Bass Limestone. About 725 million years ago, another round of mountain building forces lifted and tilted these layers of rock into mountain ranges. Erosion wore away at these mountains until they too were reduced to a nearly level plain with only a few remnants remaining. Today, the Grand Canyon Supergroup is exposed mainly in the far eastern Grand Canyon, where it forms the floor of the canyon as seen from Lipan Point and Desert View, and along Granite Gorge west of Bright Angel Creek. There are also outcrops of the supergroup in the Shinumo and Hakatai Canyon areas.

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Tapeats Sandstone

Erosion continued for a long period when no new rocks were deposited, leaving a gap in the geologic record known as "the great unconformity." Then, about 525 million years ago, a new sea invaded the area of the Grand Canyon. Along the margins of this sea, a new layer of sediment was deposited, which eventually became the Tapeats Sandstone. It forms a distinctive, brown cliff about 200 feet high capping the Vishnu schist along the rims of Granite Gorge. Up close, you can clearly see the cross-stratified layers of coarse sand that reveal the Tapeats Sandstone's shoreline origins.

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Bright Angel Shale

Thick layers of Bright Angel Shale erode into the Tonto Platform

Thick layers of Bright Angel Shale erode into the Tonto Platform

As the ocean covering what is now the Grand Canyon region grew deeper, the sediments that were deposited became finer. Over time, thick layers of these fine sediments hardened into the Bright Angel Shale, a soft, greenish rock. Because the Bright Angel Shale is thickest in the eastern Grand Canyon, it erodes out into a plateau known as the Tonto Platform. The Tonto Platform is prominent from South Rim viewpoints from Moran Point to Pima Point. The Tonto Trail takes advantage of the relatively easy terrain to wind along the Tonto Platform for more than 70 miles.

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Muav Limestone

Toward the top of the Bright Angel Shale, gray layers of Muav Limestone begin to interfinger with the shale, showing that the ancient ocean was continuing to deepen around 505 million years ago. The sea receded and advanced several times during this period, resulting in the small cliffs of gray Muav Limestone now exposed toward the top of the Bright Angel Shale slopes.

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Redwall Limestone

The hikers are camping on ledges of Tapeats Sandstone. Above, the broad slope is formed on the Bright Angel Shale. A small cliff of greenish Muav Limestone is visible on the left, and above it, the massive Redwall Limestone cliff

The hikers are camping on ledges of Tapeats Sandstone. Above, the broad slope is formed on the Bright Angel Shale. A small cliff of greenish Muav Limestone is visible on the left, and above it, the massive Redwall Limestone cliff

By 340 million years ago, deep ocean covered the entire Grand Canyon region. As trillions of microscopic sea creatures lived and died, their tiny shells rained down on the ocean floor, forming layers of sediment that eventually formed the Redwall Limestone. Today, the Redwall Limestone forms a 550-foot cliff that persists throughout the length of the Grand Canyon, snaking in and out of side canyons to form hundreds of miles of Redwall rim. Fresh exposures of Redwall Limestone are pearly gray, but the massive cliffs are stained deep red by runoff from the overlying rocks.

All the cliffs in the Grand Canyon present challenges to would-be explorers, but the Redwall is the most formidable. Within the Grand Canyon, there are only about two hundred known natural routes though the Redwall. Most current Grand Canyon trails, including the Bright Angel Trail, take advantage of such natural breaks, but the Kaibab Trail is an engineered route that required plenty of explosives to create. It was probably the Redwall Limestone that stopped the Spanish conquistadors from reaching the Colorado River in 1542.

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Supai Group

The South Kaibab Trail descending through the upper Supai Group

The South Kaibab Trail descending through the upper Supai Group

After such a long period under the Redwall sea, the region was once again exposed as dry land, and a period of erosion left stream channels filled with river sediments on the top of the Redwall Limestone. Around 315 million years ago, a sea began encroaching on the area once again. This resulted in the deposition of the red rocks of the Supai Group, alternating layers of sandstone, shale, and limestone. The harder layers of limestone and sandstone form cliffs, with some, like the uppermost layer known as the Esplanade Sandstone, up to 200 feet high. The layers of soft shale form slopes between the cliffs.

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Hermit Shale

About 280 million years ago, a long period of tidal flat deposition resulted in the Hermit Shale, a bright red formation that forms slopes at the top of the Supai Group. In the eastern canyon, along the Desert Facade, the Hermit Shale disappears and the Coconino Sandstone above forms sheer cliffs overlying the Supai Group below. In the area visible from Moran Point to Hermits Rest, the Hermit Shale forms the deep red slopes at the base of the Coconino Sandstone buttes and temples. In the western canyon, the Hermit Shale increases to more than 1,500 feet thick. Here, the Esplanade, a broad terrace, forms in the Hermit Shale, replacing the Tonto Platform as the dominant mid-level terrace in the Grand Canyon.

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Coconino Sandstone

The Coconino Sandstone is the lowest white cliff near the top of the photo

The Coconino Sandstone is the lowest white cliff near the top of the photo

More erosion planed off the upper surface of much of the Hermit Shale then a vast, Sahara-like desert of giant sand dunes covered the region by around 275 million years ago. This resulted in the formation of the 350-foot thick Coconino Sandstone. The sloping faces of the ancient dunes are clearly visible in the cross-stratified layers. If examined with a good hand lens, the sand grains that make up the Coconino Sandstone are clearly sandblasted, an effect created by wind tumbling the sand grains together. From the rim viewpoints the Coconino Sandstone is the prominent buff-colored to white cliff about 1,000 feet below the rim.

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Toroweap Formation

Around 273 million years ago, changing near-shore ocean environments caused the deposition of the shale, limestone, and sandstone layers of the Toroweap Formation. These pale yellow and gray layers form steep slopes and small cliffs directly below the rim cliffs.

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Kaibab Limestone

The Kaibab Limestone forms the rim cap rock throughout most of the Grand Canyon. The slope at the lower left is the top of the Toroweap Formation

The Kaibab Limestone forms the rim cap rock throughout most of the Grand Canyon. The slope at the lower left is the top of the Toroweap Formation

An off-white to cream-colored cliff about 250 feet high forms nearly all of the rim of the Grand Canyon and much of the surface of the plateaus on either side of the canyon. This layer, the Kaibab Limestone, was deposited in an ocean about 270 million years ago. Fossil seashells are found throughout the Kaibab Limestone and can be seen in the rocks along the Rim Trail.

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The Missing Rocks

Cedar Mountain, near Desert View

Cedar Mountain, near Desert View

As much as 20,000 feet of younger rocks once covered the Grand Canyon region, but have been entirely eroded away except for isolated remnants at Cedar Mountain and Red Butte near the South Rim. These younger rocks still cover large areas of northeastern Arizona, east of the Grand Canyon.

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Volcanoes

On the western portion of the North Rim, volcanic activity began about six million years ago and continued until several thousand years ago, resulting in lava flows on the Shivwits Plateau and cinder cones in the Uinkaret Mountains and at Vulcans Throne near Toroweap. Lava flows dammed the Colorado River at least four times, creating dams as much as 1,200 feet high and 50 miles thick along the course of the river. The lakes behind these lava dams would have backed up all the way to Lees Ferry. As a perfect illustration of the power of the Colorado River, each of these massive rock dams have been eroded away, leaving only a few patches of lava rock on the lower walls of the Canyon to prove they existed.

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