<P> Uplift of the region started about 75 million years ago during the Laramide orogeny; a mountain - building event that is largely responsible for creating the Rocky Mountains to the east . In total, the Colorado Plateau was uplifted an estimated 2 miles (3.2 km). The adjacent Basin and Range Province to the west started to form about 18 million years ago as the result of crustal stretching . A drainage system that flowed through what is today the eastern Grand Canyon emptied into the now lower Basin and Range province . Opening of the Gulf of California around 6 million years ago enabled a large river to cut its way northeast from the gulf . The new river captured the older drainage to form the ancestral Colorado River, which in turn started to form the Grand Canyon . </P> <P> Wetter climates brought upon by ice ages starting 2 million years ago greatly increased excavation of the Grand Canyon, which was nearly as deep as it is now, 1.2 million years ago . Volcanic activity deposited lava over the area 1.8 million to 500,000 years ago . At least 13 lava dams blocked the Colorado River, forming lakes that were up to 2,000 feet (610 m) deep . The end of the last ice age and subsequent human activity has greatly reduced the ability of the Colorado River to excavate the canyon . Dams in particular have upset patterns of sediment transport and deposition . Controlled floods from Glen Canyon Dam upstream have been conducted to see if they have a restorative effect . Earthquakes and mass wasting erosive events still affect the region . </P> <P> At about 2.5 and 1.8 billion years ago in Precambrian time, sand, mud, silt, and ash were laid down in a marine basin adjacent to an orogenic belt . From 1.8 to 1.6 billion years ago at least two island arcs collided with the proto - North American continent . This process of plate tectonics compressed and grafted the marine sediments in the basin onto the mainland and uplifted them out of the sea . Later, these rocks were buried 12 miles (19 km) under the surface and pressure - cooked into metamorphic rock . The resulting Granite Gorge Metamorphic Suite, which is part of the Vishnu Basement Rocks, consists of the metasedimentary Vishnu Schist and the metavolcanic Brahma and Rama Schists that were formed 1.75 billion to 1.73 billion years ago . This is the resistant rock now exposed at the bottom of the canyon in the Inner Gorge . </P> <P> As the volcanic islands collided with the mainland around 1.7 billion years ago, blobs of magma rose from the subduction zone and intruded the Granite Gorge Metamorphic Suite . These plutons slowly cooled to form the Zoroaster Granite; part of which would later be metamorphosed into gneiss . This rock unit can be seen as light - colored bands in the darker garnet - studded Vishnu Schist (see 1b in figure 1). The intrusion of the granite occurred in three phases: two during the initial Vishnu metamorphism period, and a third around 1.4 billion years ago . The third phase was accompanied by large - scale faulting, particularly along north--south faults, leading to a partial rifting of the continent . The collision expanded the continent from the Wyoming--Colorado border into Mexico and almost doubled the crust's thickness in the Grand Canyon region . Part of this thickening created the 5 - to - 6 - mile (8 to 10 km) high ancestral Mazatzal Mountains . </P>

What type of rock is at the bottom of the grand canyon
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