The type of lava inside a volcano is what determines its shape and whether or not it explodes violently.
One of the most important properties of lava is how thick and sticky it is. This is called its 'viscosity'. Some volcanoes have lava that flows like honey. In others, the lava flows like thick paste. Viscosity depends on the amount of silica (a substance made of lots of tiny crystals) found in the lava. More silica means thicker, more viscous lava.
You can look at solidified lava and tell how viscous it was when it was hot by looking at its flow lines and folds. The higher the number of folds and signs of flow, the runnier it probably was. Crumbly, chunky solidified lava would have been less viscous when it was in its liquid form.
In areas where the Earth's crust is being formed the lava that erupts from volcanoes tends to be very fluid.
Most of the places in the world where crust is being formed are deep under the ocean and rarely seen, however, there are a few places where crust forms on dry land. Iceland is one such place.
In areas where the Earth's crust is being destroyed, the lava that erupts from volcanoes tends to be viscous.
This destruction of the crust happens most often in coastal areas, such as California in North American and Chile in South America. There, volcanoes form nearly perfect lines as they grow.