Volcanoes and Geysers

1. How many volcanoes are on the earth?  What is the biggest one ever?

There are about 1500 known volcanoes known from the past several thousand years. The vast majority are around the Pacific Ring of Fire. The largest volcano known which has its top in the air is Mauna Loa in Hawaii. It is not as tall as nearby Mauna Kea, but it is much larger.
Five volcanoes make up the big island of Hawaii in the state of Hawaii.  Here is how much room they each take up:

Below is Mauna Loa. It is a shield vocano, so it doesn't have the pointed top people usually think of when they think of volcanoes.

When a shield volcano erupts, it opens up along the sides and spills out lava. Here is a Mauna Loa eruption:

Below is Mauna Kea, near Mauna Loa.

Mauna Kea is so high and clear (and quiet) that there are a number of observatories on its peak

All of that aside, the largest known volcano is underground: under Yellowstone National Park

The outer red line is the volcano’s caldera – that’s just the top of the thing. Crater Lake is the caldera of a volcano:

Crater Lake is 5 miles across one way and six miles the other.

The Yellowstone Caldera is about 34 miles across one way and 45 the other. In the picture below, the smaller circles show that it has two domes, not just one.

The mantle plume (magma reservoir beneath it) is larger than any other measured:

If Yellowstone were to erupt again, here is the approximate area that would be affected:

2. How many times does a volcano explode?

There is no set number of times any single volcano can or will explode. It depends on the pressure underneath it, where it is on the earth’s crust, and how rapidly the pressure under it is building.

3. What happened to Pompei?

Mt. Vesuvius exploded and buried it.

4. Why did Mt. St. Helens explode? If Mt. St. Helens was built in a day, was that why it exploded?

You have misunderstood something here.  Mt. St. Helens was not built in a day.  It was there for a long time.  But when it exploded, it formed a lot of features in one day, like a canyon.

The earth is still heating in its core, but not nearly as fast as several thousand years ago. Nevertheless, pressure is still building and this pressure is often released as volcanic activity at weak points in the earth’s crust. Mt. St. Helens is part of the “Pacific Ring of Fire” which produces most of the world’s volcanic activity.

5. Did anyone die at Mt. St. Helens?

Yes.  Here is the Memorial Plaque and a list of the names who died at that time due to the eruption:

6. Was the mini Grand Canyon inside Mt. St. Helens? Was it formed by lava or explosion?

Engineers’ Canyon was formed as the result of a massive mud flow which tore out the earth.  The mud flow was a result of the eruption of Mt. St. Helens. Engineers Canyon is not inside Mt. St. Helens, but below it.

Here is Mt. St. Helens showing how exploding material carved out large drainage patterns on its side:

The Mt. St. Helens caldera looks like this:

The canyons are carved along the sides and nearby.  Engineer’s Canyon was formed along the north fork of the Toutle River. Here is the mudflow that carved it:

and here is the canyon itself:

7. How can you measure 600 feet from the Mt. St. Helens eruption?  What do you use to measure?

It’s a geometry thing. You stand a little way from the base of the cliff and measure the angle to the top of the cliff with an instrument. That is the inside angle of a triangle you are making. One side of the triangle is the cliff itself. One side is the distance from you to the base of the cliff. The angle where you are standing is the angle from the base line to the top of the ‘triangle’ where the top of the cliff is. When you have those three measurements, you will learn in geometry how you can figure the opposite side, or how tall that cliff is.

Here is the idea, using an illustration with a tree:

8. There are some volcanoes that are nowhere near tectonic plates.  How did those form?

Although the majority of volcanoes are at tectonic plate boundaries, there are other weak spots in the earth’s crust in other places.
Here is a good explanation from the Science Daily website:

“Volcanoes can also form where there is stretching of the Earth's crust and where the crust grows thin (called "non-hotspot intraplate volcanism"), such as in the African Rift Valley or the European Rhine Graben with its Eifel volcanoes).
Finally, volcanoes can be caused by "mantle plumes," so-called "hotspots;" these hotspots can occur far from plate boundaries, such as the Hawaiian Islands.”

9. How can geologists know water makes pillow lava?

We see it happening, primarily in Hawaii. Here is pillow lava being formed at Kiluea, Hawaii:

Pillow lava is also formed when magma simply oozes out of the ocean floor, such as along the Atlantic Rift:

Here is what it looks like on land in an area that used to be under water when a volcano erupted:

10. How do geysers work? Do they burst from heat?

Geysers are areas of weakened crust or, in the case of Yellowstone, areas on top of an ancient volcano that is now underground. It is heat coming from deep under the crust which puts pressure on the water and then heats it up enough to force it up through the weakened area of the crust. You can get a picture in your mind of this when you think of a pot of water on the stove with a lid on it and you heat up the water until the lid is bouncing on top of the pot.

The Yellowstone Park service has a very good webpage explaining how geysers work.