![]() 800+ volcanoes that erupted over the past few million years dot the landscape - some are a few hundred feet tall, and some tower many thousands of feet above the surrounding countryside. ![]() ![]() Today, running water is cutting into and wearing down this southern flank of the Colorado Plateau.Perfect for bikepackers looking to get off the beaten path while following dirt roads and two-tracks, the San Francisco Volcanic Field Loop meanders through northern Arizona’s San Francisco Volcanic Field. As eruptions continued during the period 3 million to 1000 years ago lava of the San Francisco volcanic field poured onto, exploded through, or was injected into Paleozoic and Mesozoic sedimentary layers of the plateau.įinally, San Francisco Mountain, the high stratovolcano that towers over the volcanic field, was scoured by glacial ice several times during the last 1.8 million years. About 6 million years ago, molten rock (called magma inside the earth and lava when it erupts) migrated upward along some of these fractures and flowed onto the land surface as lava flows. Movement occurred again along the old faults of the Flagstaff area. The uplift also caused formerly sluggish rivers to cut deep canyons into the younger sedimentary layers.īeginning about 25 million years ago, the crustal rocks of western North America were stretched, thinned, and broken along steep faults. In the Flagstaff area movement along faults deformed once-horizontal layers into long folds, such as the Black Point monocline north of Wupatki National Monument. The exact timing and causes of the uplift are still debated by geologists. Vertical movement along these faults elevated the Precambrian basement rocks and the thick sequence of younger sedimentary layers thousands of feet, eventually forming the Colorado Plateau. This stress reactivated old faults and created new faults and folds. The Rocky Mountains, for example, were formed during this period. The total thickness of sedimentary rock deposited during the Paleozoic and Mesozoic Eras may have reached 10,000 ft (3050 m), but much of this was stripped off by erosion.īeginning about 65 to 75 million years ago, western North America was subjected to intense horizontal compression during an episode of mountain building called the Laramide Orogeny. Younger layers of sediment accumulated, but were later eroded away. The Moenkopi Formation is the only Mesozoic rock that covers large parts of the Flagstaff area. More rock layers were laid down during the Mesozoic Era (248 to 65 million years ago). ![]() The Coconino Sandstone and the Toroweap and Kaibab Formations are the only Paleozoic rocks exposed in the area covered by this guidebook. These younger units, named in ascending order, the Tapeats Sandstone, Bright Angel Shale and Muav Limestone, Martin Formation, Redwall Limestone, Supai Group, Coconino Sandstone, and the Toroweap and Kaibab Formations, were deposited when this part of the continent was a shallow sea floor, a muddy tidal zone, a coastal plain crossed by silt-laden rivers, or a vast desert covered by sand dunes. Horizontal layers of sandstones, limestones, shales, and siltstones of the Paleozoic Era (544 million to 248 million years ago) were deposited on the ancient Precambrian rocks. These rocks, which make up the original crust of North America, were beveled by erosion and offset by faults that moved again during younger geologic periods. The oldest known rocks underlying this part of the Plateau are 1.7-1.8 billion-year-old (Precambrian) granite and schist. The landscape of this southern Plateau margin is dominated by the young San Francisco volcanic field and the underlying limestone-capped plateau. The Flagstaff area is on the southern margin of the Colorado Plateau, a 130,000-square-rnile geologic province of vast plains, high mesas and buttes, deep canyons, volcanic fields and isolated mountain clusters. General geology as described in the booklet: According to Michael Conway, Chief of the AZGS, Geologic Extension Service, “This 53-page, Down-to-Earth booklet includes pictures, illustrations and jargon-free text to open the geology of northern Arizona to those who otherwise lack a geology background.”
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