There are several varieties of volcanoes and each has a distinct shape. The most common volcanoes are shield volcanoes, stratovolcanoes, and pyroclastic cones. Shield volcanoes are fairly low volcanoes with rounded tops. When a shield volcanoe erupts, it's not very explosive and it tends to be mostly in the form of a liquid lava that flows down the side of the volcano and solidifies to form the next layer of the volcano's surface. The best examples of this sort of volcano are in Hawaii.
Stratovolcanoes are the sort of volcano you find in the Pacific Northwest such as Mt. St. Helens. Mt. Fuji in Japan is another famous example of this type of volcano. These volcanoes are usually fairly large and are made of a mixture of ash and lava flows. They tend to be very cone shaped and when they erupt can be very explosive, sending clouds of ash into the atmosphere and potentially causing lots of damage. Pyroclastic cones are usually smaller cone-shaped volcanoes that are explosive and eject mostly hot ash.
Other volcanic landforms include calderas which are very large basins that are left behind after an enormous volcaninc eruption (such as Long Valley near Mammoth, Calirfornia or the Yellowstone Basin of Yellowstone National Park or Crater Lake in Oregon which is a volcanic remnant, not an impact crater), and flood basalts which are very rare volcanic features where lava pours out of the ground over huge areas. The Snake River Plain is an example of this and these can also be found in the East African Rift Valley. The type of volcano depends on the variety of magma that feeds it.
Some magma are more liquid than others and have less gas. The liquid magmas tend to make lava flows and shield volcanoes like on Hawaii. Magmas that don't like to flow as much and have lots of gas tend to get bottled up under the Earth like gas in a Coke can when you shake it before opening.When this gas is released (opening the Coke can) it comes out very explosively and can cause a lot of damage because of the sudden release of energy. The hot energy of magma is what makes it want to rise to the surface and get out. This heat comes from the deep interior of the Earth where the mantle and core are very hot. This heat has trouble escaping because of the rigid crust of the Earth but it can come out through volcanoes among other features.

Why do you think the interior of the Earth might be hot? I encourage you to look at several good websites about volcanoes including the National Park Service (there are several wonderful National Parks that have unique landforms because of volcanoes) at http://www.aqd.nps.gov/grd/tour/volcano.htm and the volcano world website http://volcano.und.nodak.edu/vwdocs/. These sites also have links to other good volcano related websites. Have fun!

There are three different types of volcanoes, and each of these different types have different reasons for erupting, different ways of erupting, and different shapes (not all volcanoes are shaped like a triangle, although most are tapered that is, they are skinnier at the top and wider at the bottom). The basic reason why all volcanoes erupt is a process called plate tectonics. The earth is a sphere composed of three circular layers, like a peach. The core of the earth, the pit of the peach, is extremely hot and is "molten", that is, it is so hot that everything within it has melted. This core contains the heaviest material on the earth, although it never reaches the surface so we don't see it. The next layer is called the mantle, and would correspond to the juicy part of the peach. The mantle is not as dense or as hot as the core, but is hot enough that it is not completely solid but still soft, like thick pudding. The last layer of the earth is called the crust, and is like the skin of the peach because it is thin and tough. However, unlike peach skin, the earth's crust is broken up into several large pieces, called plates, and these plates are so light they float above the mantle and are in constant motion, moving away from each other and bumping into each other all the time, so slowly that we never feel it, except during rare instances like earth quakes.

Because the mantle is a boundary between an extremely hot surface (the core)and a cool one (the crust), it is always moving and circulating, like winds in the atmosphere. Usually when a volcano erupts, the mantle is what comes out. Depending on what kind of volcano it is, the mantle can ooze out as heavy, hot liquid or explode out as ash and rock debris. Probably the most common type of volcano is hidden way at the bottom of the ocean, on the sea floor. These volcanoes are not like the type you see on land. Called "mid-ocean ridges", they form very long lines wrapping around the earth, and are the shortest of the volcanoes, reaching only a few hundred feet high on average. Ask your teacher to show you a map of what the ocean bottom looks like, and you will see these ridges in all the major oceans, from the Pacific to the Antarctic. This chain of volcanoes forms when two plates of the earth's skin, or crust, pull apart. This leaves a gap, and the mantle will rush to the surface of the earth from below to fill the gap. This form of volcanism is fairly constant but not dramatic, as the magma just oozes out slowly. Sea water temperatures at these ridges can be hotter than boiling water, however.

The second type of volcano, the "hot spot" volcano, forms when a small part of the mantle gets very hot, so that it rises straight up from the depths of the earth as a plume of hot molten material, breaking through the crust and reaching the ocean or the atmosphere like steam escaping from a tea pot.
This is the kind of volcano that formed the Hawaiian islands, and if you go to the big island of Hawaii and visit the volcano, you can see the red-hot mantle (lava) flow down the island into the sea. This form of volcanism can be explosive or not, depending on what type of the earth's crust the hot spot breaks through. Generally, hot spots on land are explosive, whereas hot spots in the ocean (like Hawaii) are not. Hot spots can exist for millions
of years in the same place.

The last form of volcanism, called subduction volcanism, happens when an piece of the ocean bottom (an oceanic plate) bumps into a piece of land (a continental plate). Because the ocean bottom is heavier than land and a lot lower, it will slide under the continent it bumps into. As it slides under (or subducts), it reaches the mantle below and melts. As it melts, it rises(like hot air) and erupts from the crust of the earth as a volcano. These volcanoes are explosive, erupt once every few thousand or million years,generating large dust clouds and causing a lot of damage. Two examples are Mount Saint Helen's and Mount Pinotubo.

Volcanoes are pointed (thinner on top and thicker on bottom) because the rock in the crust gets warmed by the pool of hot lava sitting just below it and as the crust warms it expands. As the crust rises from the pressure of the magma below it and forms a volcano, the top cools and shrinks down,making it thinner on top. Also, in actively growing volcanoes, the lava flows down the volcano and collects at the bottom, making the base larger while the top stays thinner. Once explosive volcanoes erupt their top gets blown off and there is usually a big, bowl-like crater instead where the top used to be.

Ask your teachers and parents to point out some volcanoes on a map of California and guess what type of volcano it is, using information about its location and when and how it last exploded. Mid-ocean ridges have lots of marine life living o

Any encyclopedia will have the information you are looking for, or most library books on volcanoes. Briefly, though: the Earth is solid rock only for a few miles below the surface. This solid layer is called the crust. Deeper than this, it is all hot molten rock under enormous pressure from the weight of the rock above it. In some places, the hot molten rock from the interior of the earth bursts through the solid crust; this is a volcanic eruption. The hot molten rock comes streaming out, and then cools into solid rock. As the molten rock (called lava) piles up, it forms the "cone", or triangle, shape familiar to us. The tallest volcano is Mauna Kea, in Hawaii. To find out how tall it is, you can read about it in an encyclopedia or a library book.