Yellowstone 黄石公园 - The Grand Canyon

黄石大峡谷（Yellowstone National Park Grand Canyon）位于美国黄石国家公园，在黄石国家公园90处观光景点中排名第1。

美国黄石公园里黄石湖流出的河水，流过一段平缓的丘陵地带后，进入一段长约38公里的险峻峡谷，被称为黄石大峡谷。这里是黄石公园最壮丽、最华美的景色之一。 在这里，湍急的河水浪花飞溅，形成两道壮丽的瀑布，轰鸣着泄人谷底。第一个瀑布有33米高，这是上瀑布（Upper Falls），第二个有92米高，称为下瀑布（Lower Falls）。

黄石大峡谷是由黄石河冲蚀被地热腐蚀的火山岩形成。大约在14000到18000年前，大峡谷又连接经历了三次冰川的侵蚀，逐渐形成这种典型的V型（river-eroded）峡谷。黄石大峡谷大约在10000年前才形成当今的模样，可以说它还是相当年轻。

The Grand Canyon of the Yellowstone is the first large canyon on the Yellowstone River downstream from Yellowstone Falls in Yellowstone National Park in Wyoming. The canyon is approximately 24 miles (39 km) long, between 800 and 1,200 ft (240 and 370 m) deep and from .25 to .75 mi (0.40 to 1.21 km) wide.

The specifics of the geology of the canyon are not well understood, except that it is an erosional feature rather than the result of glaciation. After the Yellowstone Caldera eruption of about 600,000 years ago, the area was covered by a series of lava flows. The area was also faulted by the doming action of the caldera before the eruption. The site of the present canyon, as well as any previous canyons, was probably the result of this uplift and related faulting, which allowed erosion to proceed at an accelerated rate. The area was also covered by the glaciers that formed during several ice ages. Glacial deposits probably filled the canyon at one time, but have since been eroded away, leaving little or no evidence of their presence.

The canyon below the Lower Yellowstone Falls was at one time the site of a geyser basin that was the result of rhyolite lava flows, extensive faulting, and heat beneath the surface (related to the hot spot). No one is sure exactly when the geyser basin was formed in the area, although it was probably present at the time of the last glaciation. The chemical and heat action of the geyser basin caused the rhyolite rock to become hydrothermally altered, making it very soft and brittle and more easily erodible (sometimes likened to baking a potato). Evidence of this thermal activity still exists in the canyon in the form of geysers and hot springs that are still active and visible. The Clear Lake area, which is fed by hot springs, south of the canyon is probably also a remnant of this activity. According to Ken Pierce, U.S. Geological Survey geologist, at the end of the last glacial period, about 14,000 to 18,000 years ago, ice dams formed at the mouth of Yellowstone Lake. When the ice dams melted, a great volume of water was released downstream causing massive flash floods and immediate and catastrophic erosion of the present-day canyon. These flash floods probably happened more than once. The canyon is a classic V-shaped valley, indicative of river-type erosion rather than glaciation. Today the canyon is still being eroded by the Yellowstone River.

he colors in the canyon are also a result of hydrothermal alteration. The rhyolite in the canyon contains a variety of different iron compounds. When the old geyser basin was active, the "cooking" of the rock caused chemical alterations in these iron compounds. Exposure to the elements caused the rocks to change colors. The rocks are oxidizing; in effect, the canyon is rusting. The colors indicate the presence or absence of water in the individual iron compounds. Most of the yellows in the canyon are the result of iron present in the rock rather than, as many people think, sulfur.