Volcanoes are built by successive eruptions over many decades, centuries, or thousands of years. Volcanoes come in different sizes and shapes.
The overall size of a volcano is determined by the total volume of lava that has erupted.
The shape of a volcano is largely determined by the type of lava that has erupted, and importantly, its viscosity. Viscosity is a fluid's resistance to flow. Water, for example, is a low viscosity fluid. It is thin and runny. Cold syrup, on the other hand, has a higher viscosity. It is thick and goopy. But when syrup is heated, its viscosity goes down. Hot syrup becomes thinner, runnier, more like water.
Temperature, composition, and volatile (gas) content largely determine the viscosity of lava.
Temperature: The hotter the lava, the lower the viscosity (the thinner it is). The cooler the lava, the higher the viscosity (the thicker it is).
Composition: he more felsic the lava (the more silica in the lava), the higher the viscosity because silica forms chains in the cooling lava even before it crystallizes. The more mafic the lava (the less silica in it), the lower the viscosity. It turns out that mafic lava is high temperature lava because high temperatures are required to melt mafic minerals in the first place. Felsic lavas are low temperature lavas because lower temperatures are required to keep felsic minerals molten (and if it was hotter it would have incorporated more iron and magnesium in comparison to silica).
Volatile content refers to gases dissolved in the lava, like carbon dioxide in soft drinks. Lavas may contain up to 6% or more of their mass as gases. The two most abundant gases in lava are water vapor and carbon dioxide. There is commonly also nitrogen, sulfur dioxide, and small amounts of chlorine, hydrogen, argon, and a few other gases. When lava approaches the surface, the pressure on it is greatly reduced and the dissolved gases come out of solution; they form bubbles and rise. The escape of gases may produces tremendous force in a volcano, producing explosive eruptions. In general, the more felsic the magma the greater the volatile content. So mafic/basaltic volcanoes are fairly quiescent, intermediate/andesitic volcanic eruptions are moderately explosive, and felsic/rhyolitic volcanoes may be extremely explosive.
So, mafic lavas are hot , low in silica and volatiles, and have relatively low viscosity. They flow easily outward from the vent (where it comes out of the ground), and may travel great distances before completely solidifying.
Felsic lavas are not as hot, high in silica and volatiles, and have a high viscosity. They are thick and gooey and resist flowing. Their high volatile content makes them potentially highly explosive.
Because mafic lava is low viscosity, when it erupts from a volcano it flows downslope away from the vent, gradually cooling and crystallizing. Because of the relative ease of flow, basaltic volcanoes are broad, with gentle slopes. They don't have the stereotypical steep volcanic cone shape. Rather, they are shaped more like a shield laid on the ground.
Even though their slopes are gentle (typically 6° - 12°) they may be quite large. The largest mountain on the Earth is the island of Hawaii, which rises up 30,000 ft from the seafloor. In comparison, the top of Mt Everest is 29,035 ft above sea level, but the base of Everest is well above sea level. Hawaii has grown to its great size by continual eruptions of basaltic lava for about 700,000 years. Most shield volcanoes are much smaller. In fact, the island of Hawaii itself is composed of several volcanoes, not one, though each is itself very large.
Basaltic lava flows on Hawaii and elsewhere produce may have two kinds of surface. One type has a smooth surface with ridges that look like coiled ropes. This type of volcanic flow is called by the Hawaiian name pahoehoe. Thick lava flows may remain molten in the interior and continue to move even while the surface solidifies. But continued movement breaks up the surface into jagged rock fragments. These broken, jagged volcanic flows are called aa.
Sometimes basaltic lava erupts from a series of fissures, or cracks in the Earth and spreads widely over the landscape, rather than erupting from a singe volcanic vent. These kinds of eruptions produce flood basalts.
Sometimes, in the late stage of basaltic volcanic activity, small cinder cones become active. When its supply of magma from deep in the Earth slows or stops, the magma chamber beneath a volcano will cool and begin crystallizing. The first-formed minerals will be high-temperature, mafic minerals like olivine which are rich in iron and magnesium and poor in silica. The result is that the remaining magma becomes depleted in iron and magnesium and enriched in silica. Consequently, the viscosity increases in the remaining magma and it does not flow out of the ground as easily. Rather, it builds up enough back-pressure to eject a spray of lava into the air. The lava droplets cool and crystallize rapidly (or at least the surface of large blobs of lava crystallizes rapidly) and then they fall as volcanic cinders, or pyroclasts, on the flanks of the volcano and tumble down its side. This process forms small volcanic cones with slopes at the angle of repose (the maximum slope angle that loose, unconsolidated materials can lie on), typically 30° or more (the angle of repose). Most are less than 1,000 ft high. They usually have a large crater where the pyroclastics were ejected. Sunset Crater in Arizona is an example of a cinder cone.
ash = dust-size
lapilli = gravel-size
volcanic bombs = from larger blobs of lava
angle of repose:
Composite Cones - Stratovolcanoes
Intermediate composition (andesitic) lavas are common where two plates converge to produce a subduction zone, especially along continental margins like western South America. Intermediate lavas are more viscous than the mafic lava which forms shield volcanoes (see above) and normally contain more volatiles. Sometimes the lava flows out as a thick, slowly-flowing fluid which is not able to flow too far before crystallizing. Oftentimes, the pressure builds up enough to produce large pyroclastic eruptions. Alternating eruptions of lava and pyroclasts produce large, layered volcanoes with steep slopes. The well-known volcanoes of the world are composite cone volcanoes. Mt. St. Helens, Mt. Ranier, Mt. Hood, and Mt. Shasta in the U.S. Pacific Northwest, Mt. Fuji in Japan, Mt. Pinatubo in the Philipines, and Mt. Vesuvius in Italy are examples.
Relatively cool, viscous felsic (rhyolitic) magma has great difficulty flowing out of a volcano. If charged with gases, the result may be a highly explosive eruption. As magma slowly rises, the ground will be bowed upward. As this upbowing continues, eventually cracks may form which allow the pressure to be released. The gases immediatly bubble out initiating a great explosive eruption which rapidly extrudes huge volumes of lava mostly as ash. After the eruption the rock that was above the magma chamber settles into the now half-empty magma chamber. The remaining depression in the ground is a caldera.
Lava flows may destroy property and reshape the landscape but are normally advance too slowly to endanger human lives. There is typically plenty of warning before a volcano begins to emit lava flows.
Ashfalls can bury nearby land and buildings with several feet of ash and choke animals.
Nuée ardentes or glowing avalanches are dense clouds of ash and superheated gas that may flow down volcanic slopes at 60 miles per hour cooking every living thing in their way.
Lahars or volcanic mudflows are produced when thick deposits of volcanic ash become saturated from rainfall or snowmelt.
Tsunamis or so-called "tidal waves" may result if the flanks of island volcanoes collapse into the sea during major eruptions.