I am not going to post pictures of The Old Faithful Geyser as you can find those easily enough elsewhere, those pictures would likely be better than mine anyway. Yellowstone has some 300 geysers (as well as 10,000 fumaroles (steam vents from below) and hot springs. All of these are due to the very hot rocks immediately below the Yellowstone Caldera. Apparently the Earth's crust here is as thin as three miles, much thinner than almost anywhere else.
This sign shows Yellowstone with the rim of the caldera of the Supervolcano. The rim is the oval-shaped purple dashed-line near the center (as always, *click* to see a better view of any image). For orientation, the "You Are Here" annotation is some 20 miles north of Old Faithful. Almost all of the geothermal activity at Yellowstone (with the major exception of Mammoth Hot Spring in the far NW) is within the caldera rim.
From Breining's book Super Volcano: "An even bigger explosion 640,000 years ago spewed 240 cubic miles of lava and ash...
... The caldera created by this most recent super explosion measures 50 miles by 30 miles... Nearly a third of the park's 2.2 million acres either blew sky high or collapsed into the earth. The caldera is so filled with lava from subsequent smaller eruptions that most visitors never recognize they are standing on the remnants of an eruption so powerful it covered much of the continent with volcanic ash abd transformed the earth's climate for years, or perhaps decades."
A supervolcano is different from ordinary volcanoes in two ways: they are MUCH bigger and their origins are different. Almost all ordinary volcanoes, even the deadly ones (Vesuvius, Krakatoa, Mt. St. Helens, Pinatubo, etc.) are products of two tectonic plates grinding against each other, in particular when one plate pushes below the other it is colliding with (the Nazca plate grinding down under the west coast of the South American plate and the Italian (sub-)plate going under the Eurasian plate are well known examples). This is called "subduction", and the plate going underneath causes heat and magma to force its way up near the subduction zone.
Supervolcanoes have a different origin. The mantle of the Earth (the very thick layer of semi-molten rock under the crust) appears to have vertical currents, hot material from near the core pushing up and the slightly cooler material flowing back down. The process is not well understood (evidence comes from seismic studies of earthquake wave propagation through the Earth's deeper layers). In the areas where the hotter materials get close to the crust, the crust typically becomes thinner and more vulnerable to BIG volcanoes. These areas are called "hot spots", they believe there are about 30 hot spots around the world, and Breining writes that Yellowstone is the only hot spot found on a continent, lucky us... Once there is enough movement from the mantle just beneath the crust, the crust weakens, magma from the deep comes closer to the surface, and every now and then the pressure builds up (dissolved gases) and BANG!
Mt. St. Helens blew up in 1980 and killed some 50 people. It released about a cubic mile of debris and affected a fair area of the Northern Rockies and even the northern plains. Yellowstone was some 240 times larger.
Yellowstone erupted some 2.1 million years ago (biggest of the recent eruptions), about 1.3 million years ago (smallest of the three) and most recently about 640,000 years ago. Or about once every 700,000 years. Hmm. We are approximately due... However, scientists believe that the same kinds of phenomena seen before regular volcanic eruptions (earthquake swarms, ground uplift, etc.) would likely happen before Yellowstone next blows. And, yes, they are carefully studying Yellowstone...
This next photo is at the "Mud Volcano" stop in E. Yellowstone. The "mud pots" here are like hot springs, only with less water and here in Yellowstone often with sulfurous gas. Here, sulfurous aroma blend in with mud, water and steam bubbling up. (Yes, John R., it is H2S gas! Poisonous gas for the rest of you) Hard to see are bubbles (boiling water) in the middle of this small mud pot:
Here is a larger mud pot, mmm, I could almost feel brain cells dying from the gas...
Hot springs are similar to mud pots but are hot water that comes up typically at temperatures of 160 degrees or more. A hot spring at Midway Geyser Basin:
The yellow mat with black spots are all typical of these bacterial mat colonies.
Hot springs are not necessarily associated with active volcanoes (Yellowstone Supervolcano is, geologically speaking, active). Mud pots are rarely seen, but are essentially hot springs with less water. The next "step up" at Yellowstone in geothermal water activity are the "fumaroles", holes with steam coming out continuously. The below picture shows some fumaroles further up-stream (in the far distance) on the Firehole River, the steam in the foreground is coming from hot water flowing down from a nearby hot spring:
The below photo is, I believe, Great Fountain Geyser. This one runs all the time. Old Faithful erupts every 92 minutes or so, for some 4 minutes each time. The height of this geyser is about 3 - 4 feet. A smaller geyser (one foot or so) is at the right:
This next picture shows Castle Geyser, which erupts every 10 hours or so, you can see faint staem from fumaroles behind and to the right.
While not evidence per se of the Supervolcano, the below are the Upper Falls of the Yellowstone Grand Canyon, they fall some 100 feet. Yellowstone is mostly a caldera (ancient large crater) that was pushed up some 1500 feet or so higher than the surrounding lowlands by the Earth's mantle (the plastic-like thick layer of semi-molten rock below the crust). Here the Yellowstone River begins its descent towards Montana and the Missouri River. The Upper Falls and the Lower Falls alone account for some 400 feet of the 1500 feet descent from the plateau to the valley at Gardiner, MT.
Visible above (and in some of the other photos) are examples of the lodgepole pine, a tree that can grow in the poor soils found withing the caldera. Yellowstone National Park has the largest stands of lodge pole pine in the world.
The below picture shows what happened after Yellowstones last eruption. After the eruption blasted away part of the mountain chain that linked the Tetons with the Absaroka range, a lot of material NOT blasted away fell into the caldera as it was basically just a big unsupported hole in the Earth. That rock just tore right off the mountain...:
With such a clean break, those cliffs clearly show part of the caldera rim.
On a clear day it is possible to see all the way across the caldera from a vantage point near Dunraven Pass on Mt. Washburn (on the northern rim of the caldera). Unfortunately it was raining when we stopped there (those of you who have spent time in mountain country will know how changeable the weather is). Here is a picture taken from the Washburn Hot Springs Overlook just outside the northern rim of the caldera. You can make out a sign explaining what they believe happened. Note that you cannot see all the way across because of the clouds and rain (also note more lodgepole pine).
Just beyond the northern part of the park in Montana (between Gardiner and Livingstone), I took a photo of rock beds almost vertical... Rock beds are laid down horizontally... So, how did these beds get turned up? Geologists will appreciate this picture:
Because of the size and status of Yellowstone, there are many wild animals that tourists come to see (hey who wouldn't stop to watch a bear?). I now understand why wildlife photography is considered its own profession..., it is hard to take really excellent pictures of wild animals.
Bison ("American Buffalo"), there were over 100 in this herd:
We were lucky to see two wolves (one each on different days, WHAT is that second one (on the ridgeline) doing...?):
We briefly saw some elk in a village (Mammoth Hot Springs), but were unable to stop in time to get a decent picture, the below is a small herd some 200 - 300 meters away, if you look carefully you can make out five of them just to the left of the building there on the slope:
We were apparently lucky to get even a poor picture of a black bear through trees and brush. We saw cars stopped (a sign there is something worth looking at), and someone told us there was a black bear with her cubs. Some tourists left their cars to venture closer to the bear (not smart -- dangerous! -- and illegal, the Ranger came by and almost arrested them...). This was the best picture I could get, the mamma black bear is just above center (above the rotted jagged tree stump):