Not all volcanic rock is "light" or has a low density—basalt is quite heavy and organic, whereas granite is lighter but still volcanic in nature. Both types of rock can be found near surface levels (within 10 miles or 16 km) of active volcanoes.
Volcanic rocks are very dense: 300 to 700 kg/m3 for basalt and 200 to 500 kg/m3 for granite. These densities are much higher than those of other sedimentary rocks (10-50 kg/m3) and igneous rocks (5-25 kg/m3).
Basalt is an extremely abundant volcanic rock that forms the majority of the ocean's outer continental shelves. It is hard and glassy and usually dark blue in color. Basalt can be large; one example is Ocean Island, which is nearly 5 by 3 miles (8 by 4 km) and rises more than 1000 feet (300 m) above the surrounding seafloor.
Granite is a relatively rare rock that makes up many of the Alps and other mountainous regions of Europe and North America. It is hard and shiny and usually light gray in color. Granite can be large; one example is Mount Rushmore, which is nearly 400 feet (120 m) high.
Basalt is a term that may be found in a dictionary. Volcanic rock (or lava) has a generally dark hue (gray to black), 45 to 53 percent silica content, and high iron and magnesium content. Basaltic lavas are more fluid than silica-rich andesites or dacites. Basalt is the most prevalent form of rock found in the Earth's crust (the outer 10 to 50 km).
Dacite is a term that may be found in a dictionary. Vulcanized volcanic rock (or lava) has a light gray to white color, usually less than 40 percent silicon dioxide content, and low iron and magnesium content. Dacitic lavas are more fluid than silica-rich andesites or basalts.
Basalt and dacite are two forms of volcanic rock. Both are composed of glassy droplets of molten rock ("lavas") mixed with water and chemicals dissolved from the surrounding rock. The size of the droplets within basalt varies from nanometers to hundreds of microns. Within dacite, the size of the droplets can be as small as 0.1 mm or larger.
Basalt and dacite are different types of volcanic rock formed at different times during volcanic activity. Basalt often forms the first part of a volcano to be released into the atmosphere, while dacite typically forms later after the volcano has cooled down. This means that basalt is usually more fluid than dacite.
Basaltic magma produces light-colored rocks with lower density. Granitic magma has a lot of silica. When granitic magma cools, it forms dense rocks like granite. But because basaltic magma is less dense than water, when it cools it leaves behind large crystals of silicon dioxide (essentially sand).
These are just some of the many different types of rock that form due to differences in the minerals and elements they contain. As you explore our planet, you will find many more varieties of rock than these few examples. In fact, there are so many different types of rock on Earth that we cannot possibly identify every single one of them!
Rocks are formed under pressure and heat over time. As molten material deep within the earth's crust solidifies into rock, the remaining materials are trapped inside the solid matrix of the rock. The size of the particles inside the molten mass determines how much pressure must be applied to the molten mass to cause it to solidify. If the particles are very small, there is not enough pressure to keep them from sinking back down into the molten mass where they can re-melt and be re-recrystallized into new rock.