Yellowstone National Park covers 2,221,766 acres, which is roughly the size of the state of Connecticut. Most of the park is  located in the northwestern corner of Wyoming, but a small portion overlaps that state's boundaries with Montana and Idaho. The park is comprised primarily of high, forested, volcanic plateaus that have been eroded over - the millennia by glaciation and stream flow and that are flanked on the north, east, and south by mountains. The Continental Divide traverses the park from its southeastern corner to its «-western boundary. The elevation of the park averages 8,000 feet, ranging from 5,282 feet in the north, where the Gardner River drains from the park, to 11,358 feet in the east, at the summit of Eagle Peak in the Absaroka Range.

"The Wonders of Yellowstone" - 98 Minutes - ~Telly Award Winner for Nature and Wildlife~ Two years in the making and just released, "The Wonders of Yellowstone" video has been highly requested, produced in DVD format and is now available. Take a complete tour of Yellowstone National Park as our Narrator Cathy Coan guides you to all the wonders of the park including all the geyser basins, wildlife, waterfalls and much more. We previously sold travel packets but these packets, maps and trail guides are all available at the park for free or minimal charge. More Info or Order Online

The Four Types of Thermal Features - Yellowstone Association

Geyser: A geyser is a hot spring with the intriguing habit of tossing underground water into the air. Water falling as rain or snow seeps through porous layers of rock. Eventually that water comes into contact with extremely hot rocks that have been heated by a large body of molten material, called magma, underneath the park. This hot water then rises through a series of cracks and fissures underneath the surface of the Earth. In a sense, these fissures are the "plumbing system" of a thermal feature. A geyser is the equivalent of a giant pressure cooker; even though the temperature of water deep down may be well above boiling, the weight and pressure of the water above prevents that boiling from happening. Eventually, though, the pressure builds enough to push the water in the upper reaches up and out, causing an overflow. That overflow, in turn, relieves the pressure on the super-heated water below, causing it to flash into steam. That flash, that explosion through a narrow, constricted place in the rocks, is what sends water shooting into the air,

Hot Spring: Hot springs let off enough heat by boiling or surface evaporation to avoid the kind of steam explosions common to geysers. Some of Yellowstone's hot springs take the form of quiet pools. Others are flowing. The waters of many of this latter type, such as those at Mammoth Hot Springs, become charged with carbon dioxide while underground, creating a mild carbonic acid. That acid dissolves underground limestone rocks and carries the mixture to the surface of the Earth. Once on the surface, the carbon dioxide gas escapes. Without carbon dioxide, the water is less able to carry the dissolved limestone. The dissolved limestone precipitates out, creating beautiful travertine terraces. In areas underlaid with volcanic rock, as opposed to more easily dissolved limestone, a modification of the plumbing system-perhaps through small earthquakes-can easily turn a hot spring into a geyser.

Fumarole (also called steam vent): In simplest terms, a fumarole is a vent in the Earth's crust. The supply of water around fumaroles is not as plentiful as in hot springs and geysers. Modest amounts of groundwater come into contact with hot rocks underground and are turned to steam. This steam rushes up through a series of cracks and fissures and out the vent, sometimes with enough force to create a loud hiss or roar.

Mudpot: In this feature, steam rises through groundwater that has dissolved surrounding rocks into clay; various minerals in the rocks make wide variations in the color of the mud. More often than not, such water is quite acidic, which help is the breaking down and dissolving process,

Geology - Grand Teton Historical Society

The Washburn Range in Yellowstone forms the skyline between canyon village and tower fall. From a parking area on Dunraven Pass, an altitude of 8,850' above sea level, an old road leads to the summit of Mt. Washburn at 10,243'. The 1,400' climb has much to recommend it, including geology. 

Seen along the road is a dark breccia consisting of anglular volcanic stones, embedded in a fine angular matrix. This breccia formed some 50 million years ago when watery mixtures of ash and rocks flowed down mountain slopes onto then tropical lowlands. There are countless volcanic mudflows that make up the Washburn Range and mountains to the east. All were deposited over a period of 10 million years.

Bedding planes separating breccia layers are instructive. In Mount Washburn and surrounding peaks, all slope northward. Shouldn't some slope southward? Didn't debris also flow down the south slopes of ancient volcanoes? Well, where is the evidence? 

We search to the south for these flows in vain. Far below us is Washburn Hot Springs, the Grand Canyon of the Yellowstone, Hayden Valley, and Yellowstone Lake. The nearest recognizable peak is Mount Sheridan, in the Red Mountains, 37 miles south of Mount Washburn. The summits of Washburn and Sheridan are within 65 feet of each other in elevation.

Volcanic breccias sloping only north combined with 
gently rolling plateaus extending south to the Red Mountains suggest that the Washburn Range is only a remnant; the northern remnant of a much larger and higher range that extended far to the south. This range is a part of the Absaroka volcanic field, which also forms the mountainous terrain east of Yellowstone Lake. But how to account for the missing southern part of the Washburn Range? The answer lies down on the plateaus forming the heart of Yellowstone. Road cuts between Dunraven Pass and Canyon Village glitter in the sun. The rock is rhyolite, the lava form of granite. It differs fundamentally in its composition, origin, and age from the volcanic rocks composing Mount Washburn. Shiny black volcanic glass (obsidian) causes the glitter. 

Tens of rhyolite lava flows were erupted one after another in central Yellowstone. Canyon Village is built on one. Elephant Back Mountain, west of Lake Hotel, is another. Several flows make up the plateau between Canyon Village and Norris, and several more bound the western margins of Yellowstone Lake. Flows enclose Lewis and Shoshone lakes; they form the wooded boundaries of the geyser basins. Many streams follow seams between flows of different ages. 

Lava flows can be readily dated. They contain various radioactive elements which decay to form daughter products. By measuring the relative amounts of parent material and daughter products and knowing the rate of change from parent to daughter, a geochronologist has a radioactive clock for dating the ages of flows. Analysis, though, is not simple and geologic dates are usually followed by a fudge factor such as +/- 6,000 years. Between the Washburn Range and the Red Mountains, lava flows range in age from about 500,000 years to 100,000 years. They are much younger than the 50 million-year-old Absaroka volcanics. 

To summarize, the Washburn Range is made of debris flows preserved in the north flank of an old dissected volcano. This volcano and the Red Mountains, about 37 miles to the south, are joined by an arc of Absaroka volcanic mountains east of Yellowstone Lake. They form, in aggregate, a sort of geologic horseshoe open to the southwest. Rocks forming the horseshoe are at least 50 million years old. Cradled within the horseshoe are half a million years old or younger. Both the large difference in age and fundamental, chemical composition show older and younger volcanic rocks are unrelated, though they to occupy common ground. 

Early students of Yellowstone geology failed to recognize the age break between the Absaroka volcanic breccias and the much younger lava flows of the Yellowstone Plateau. They believed that a continuum of volcanic activity linked the Absaroka voicanics and the lava flows. 

This comfortable scenario was shattered by a Harvard graduate student, Francis R. Boyd, who chose Yellowstone for his thesis project. Boyd did his field work in the 1950s. During his studies he saw that some of the so-called lava flows were something quite different-they were welded tuffs. Welded tuffs are products of explosive volcanism. Siliceous lavas charged with dissolved gas literally explode out of volcanoes as mobile froths flowing rapidly across surrounding landscapes. When such ash flows settle, they quickly begin to compact and if the ash retains enough heat to re-fuse, the rock becomes a welded tuff. Even after compaction, the individual shards are visible under a microscope or even to the naked eye, although they may be severely contorted by flowage and compaction. 

Before Boy&s time geologists were only beginning to recognize welded tuffs and their distinctive qualities. The significance of his work, published in 1961, was that a previously unrecognized volcanic event in Yellowstone had produced violent explosions and staggering volumes of volcanic ash, later consolidated into welded tuffs. He demonstrated that these tuffs covered thousands of square miles of Grand Teton and Yellowstone and that they rimmed a large tectonic basin in Yellowstone that contain even younger lava flows. 

The explosive volcanic events that produced these tuffs were unbelievably large and violent-many times greater than the 1981 eruption of Mount St. Helens. They destroyed the southern half of the Washburn volcano and whatever mountains existed between Mt. Washburn and the Red Mountains. Geologists have identified streaks and thin layers of Yellowstone volcanic ash from as far away as California, Saskatchewan, Iowa, and the Gulf of Mexico. Volumes of ash blasted into the stratosphere circulated around the globe and must have altered the weather worldwide.