So you know about the different kinds of volcanoes and eruptions. Now you ask the question that makes the world turn 'round: why?
A: One word: Magma. There are tons of different kinds of magma around the world, almost as many kinds as there are volcanoes. They are divided up into 4 main groups:

What are the differences among all these lavas? It all comes down to one thing again: composition.
Silica, also known as silicon dioxide (SiO2), is responsible for the distinction between lava types, viscosity, and the explosivity of the eruption (though not solely responsible for the latter two). All magma contains a great deal of silica-it is one of the mantle's lighter elements and that is where the magma is coming from (directly or indirectly). I have listed the magmas from lowest silica content to highest below.

Lowest among our listed classifications in silica content is Basalt. Basalt is the most common rock found on the Earth's crust and has a silica content of less than 53%. This causes the basalt to be extremely fluid (low viscosity) and is almost the sole product of shield volcanoes. Due to its low viscosity, basalts spread out to form large lava flows and are most often produced in hawaiian, strombolian, or submarine eruptions (where the main product is fluid lava). Basalts are usually ejected at temperatures between 1100 and 1250 degrees celsius.

Andesites are next on the list with a silica content between 53 and 63% and are typically extruded at temperatures between 900 and 1100 degrees celsius. Andesites are more explosive because they have a higher viscosity than basalts, i.e. they can't flow as quickly. Thus you find them most commonly at stratovolcanoes.

Next on the list comes Dacites. Dacites have a silica content of 63 to 68% and are ejected at temperatures of 800 to 1000 degrees celsius. Dacites are usually responsible for plinian eruption such as Mt. St. Helens. Dacites are usually the most explosive form of lava there is.

Well, as I mentioned, the next and last lava is the Rhyolites. Rhyolites have a silica content of more than 68% and are extruded at temperatures of 700 to 850 degrees celsius. Rhyolites, interestingly enough, are typically too viscous to erupt explosively which is why dacites are listed as being the most explosive.

Now you know the different types of lava and basically all you need to know to ace a report for your high school geo class, but do you want to know more? This is about to get interesting. Next we'll be discussing physical lava formations-basically what it does after it leaves the volcano. There'll be no list for this- we're just gonna go through and discuss them one by one.

Volcanology can be fun to learn because of all of the unusual names you get to say. Some basalt vents produce a'a (ah-ah) lavas. A'a lavas are lavas of higher viscosity (thicker), thus they are usually slow moving and massive. The appearance of a'a flows can be impressive, though not because of their speed; it is becuase they both appear and act like a bulldozer. As the flow moves, lava on the top begins to cool, congealing (much like grease on a frying pan) to form lava blocks known as clinkers. Now, as the lava moves, the bottom layer comes in contact with the ground causing friction. The result: the bottom slows down while the top maintains the same speed. This causes the flow to "roll", much like a bull dozer and can be observed by watching the clinkers move across the surface of the flow. Like the tread of a tank, the blocks move to the edge, fall over and get buried by the rest. Because a'a flows are so heavy and move in such a fashion, they have been known to bury entire houses and can be some of the most impressive to see. When the a'a flow hardens, it becomes covered with tiny spines which may be painful to the hand or feet. When clinkers on a flow get "out of control" they form accretionary lava balls. These occur when a clinker is rolled about in the lava for some time. As it rolls, lava sticks to the ball and it grows. Forming much like a snow ball, accretionary lava balls can range from several centimeters in diameter to several meters.

Because of its high viscosity, a'a flows are rather uncreative when it comes to cool lava formations. The other type of lava flow is pahoehoe . Almost all flows start out as pahoehoe before becoming a'a. Typically, pahoehoe flows of high viscosity or of low viscosity and high rate of shear become a'a. Pahoehoe flows differ from a'a in that they are simply less viscous. Pahoehoe (pronounced "pa-hoy-hoy") flows often appear ropy and smooth. Ropy pahoehoe is most common and gives the appearance of a bunch of intertwined ropes. It is formed when the flow is slowed or halted; the cooling surface becomes rippled and twisted to make some odd-looking formations. Entrail pahoehoe looks like it sounds and forms when it flows down a steep slope. Pahoehoe toes are some of the funnest formations. They occur as the pahoehoe flow advances. The toes advance ahead of the flow and merge together like little peninsulas of pahoehoe. Lava coils are just rippled lava that appears in a spiral. This happens in shear zones or in areas where lava velocities differ. Finally, a pahoehoe blister is just a glass bubble. It is a spot where a gas bubble tried to exit, but the lava shell around it hardened before it could burst. Basaltic lavas of all kinds can form kipukas. A kipuka is a lava island. It is piece of land that the lava managed to flow around, but not through, leaving a little vegetated sanctuary. Kipukas are often short lived because subsequent flows usually cover it. Pele's hair and Pele's tears are two formations that are rather unusual. Pele's hair looks like tiny pieces of straw scattered about. It forms when basaltic glass is stretched. It occurs at the edge of a'a flows or at very vigorous or fast moving portions of pahoehoe flows. Most often it is a result of lava fountaining. Pele's tears look as the name suggests. They are formed when fluid lava is flung from a vent. The lava cools in mid-air, warped by the wind. The result is the tear-shaped formation. Pahoehoes travelling over a plain or gentle slope often contain tumuli. A tumulus is formed when there is a swell that pushes up the crust and breaks it. It hardens this way and usually has a lateral fracture indicating the direction of the swell. Lava deltas are what cause islands to grow. When the lava reaches the water, it spreads outward in a fan shape to form a delta-of lava. Lava deltas can be very dangerous because they may be eroded from below or unstable due to faults causing them to unpredictably crash into the sea. If there is a vent near the delta, it may be accompanied by an eruption. Moving on, lava domes are created by rhyolites or dacites that are too viscous to flow far from the vent. They then proceed to form a dome around or near the vent as it piles up on itself (these are discussed in detail elsewhere). Lava lakes are exactly what the name suggests. These occur when a flow is stopped and lava is backed up. As the lava continues to flow in, a lake comes as a result. Lava channels occur when the outsides of a new flow harden and the interior continues to move. This forms a river of lava. Sometimes the top of a channel may harden while the inside continues to flow. This forms a lava tube. When the lava source quiets, the lava drains out leaving a hollow tube underground. On occasion, the top of a tube may collapse in a spot revealing the interior. This is called a skylight. Limu is formed when lava flows into the ocean and explodes. A small wafer of volcanic glass may result called limu. Reticulite is a glass honeycomb of hardened glass bubbles that is formed during lava fountaining. Hornitos form when lava comes up through a vent in a viscous or pre-hardened state. Finally we reach tree mold, the last formation I've decided to mention. A tree mold is where lava flows around a tree. As the tree burns the lava hardens. After the tree is burned away, you are left with a hollow circle of where the tree used to be. There may also be a standing cast of the tree where it once stood And that is a tree mold. And that's it for the lava formations. Are you tired of reading? Go grab a Coke and do something else for a half hour. Then come back, because we're about to discuss volcano formations.

Welcome back. Lets get right down to volcanic formations. These are formations very much related to volcanoes, but not formed by flowing lava, giving them their own little category. Let's start off with the most confusing formation: the cinder cone. The cinder cone can be both a volcano or a volcanic formation. We've already discussed cinder cones as volcanoes; so, now let's talk about them as formations. A cinder cone is a form of a vent. Vents are merely parts on the side of a volcano that eject lava besides the main cone. A cinder cone vent is exactly the same as a cinder cone volcano except that it is usually much smaller and on the side of a much larger volcano. It is still a conical hill of fragments ejected from the cone and may be oddly shaped and/or pointing in a certain direction if it is located in a place where there are strong winds to blow the ejecta.

A very simple formation is a lava dike. It forms when lava cuts through a pre-existing rock fracture. This can be a volcano formation or even a seperate type of volcano depending on the definition (dikes are discussed in greater detail elsewhere). A fissure can be a form of volcano or a volcanic formation just like the cinder cone. It can be a volcanic formation when it occurs on the face of a pre-existing volcano such as the case at Kilauea. Fumaroles and geysers are also volcanic formations which we previously discussed. A littoral cone can be found on the edge of a lava flow entering the sea. They may form when water enters a lava tube and reacts explosively, punching a hole through the surface. This becomes a permanent exit for the explosions and so a cone is formed from the ejected debris. These are usually short lived since the cones frequently break into the sea. A maar is the result of subsurface eruptions that I briefly explained in the phreatic eruption part on the previous page. A maar is basicly a crater in the ground that is left from the subsurface phreatic explosion. Often filled with lakes, they can exhibit recurring phreatic eruptions. Maars are not technically volcanoes because they describe only the surface crater. Mud volcanoes are not really volcanoes, but more like small geysers. They are usually 1 to 2 meters high and eject burning hot and slightly acidic mud and clay. Plugs can be found when lava rises in a volcano and solidifies. When the shell of the volcano crumbles away, you get a tower of solid lava rising out of the ground, much like a giant hornito. These can rise hundreds of feet into the air. The most famous is Devil's Tower which made an appearance in Close Encounters of the Third Kind. Spatter cones are basically the same as cinder cones, except that ejected material is much more fluid. This causes the cone to be composed of millions of hardened lava fragments. Spatter ramparts are very similar to spatter cones, but occur along fissures. The defining characteristic of spatter cones and ramparts is the presence of spatter which is just lava that congeals when it hits the ground.

Well, now you know the very basics of volcanology. It might be enough for a high school science report, but are you really satisfied with that?

See the world's worst volcanic disasters in recorded history