The words "lava" and "magma" both refer to molten rock that has been melted by the extreme temperatures miles below the earth's surface.
When it's underground, liquid rock is called "magma." If this red-hot liquid make its way to the earth's surface and flows out on the ground, it's given a different name: "lava." Lava is also the name of rocks formed when molten rock cools and hardens, especially lava flows. Volcano experts give different names to hardened lava based on the shapes formed by the hardening rock; using terms like blocky, ropey, and pillow lavas.
Hawaiian names are also used, including fun words like aa (AH-ah, blocky lava) and pahoehoe (puh-HOY-hoy, ropey lava). Whether blocky or ropey, all rocks that form by hardening of lava are called extrusive igneous rocks. Rocks that form underground through the cooling of magma are given a different name, intrusive.
Though Indiana Jones and Terminator movies picture them as glowing red liquid flowing like water, magma and lava have a wide range of viscosity (resistance to flow). Molten rock's thickness changes depending on temperature and the amount of dissolved gases, like water vapor, present. The most important thing that controls viscosity, though, is which elements are present.
Melted rock with a high amount of silicon dioxide, or silica, is thicker than melted rocks with less silica, so it does not flow as smoothly - it's thick and gooey, like oatmeal. Rocks of high silica content are called “acidic” rocks by geologists. Extrusive rocks that form from hardened acidic lava are usually light-colored. Because acidic lava moves more slowly and is less likely to flow, eruptions where the lava is acidic are often violent and full of explosions.
Melted rock with less silica is runnier, like syrup or motor oil. Geologists call low-silica igneous rocks like these “basic,” and the best-known example is basalt. Basic rocks are usually black or dark brown.
Basalt lava makes up most of the Hawaiian Islands, where they have reached the surface at a series of volcanoes. Basaltic lava is thinner than acidic lavas, so it flows more easily. Near active volcanoes in Hawaiian, flowing lava may run as fast as a man can walk. Because the lava is runnier, basaltic lava eruptions are less violent than acidic eruptions.
As you might have guessed, lava and magma are both very, very hot. The temperature needed to melt rock also depends on its composition, but all lava flows have a temperature of at least 700°C (1300°F) and can be as hot as 1200°C (2200°F). So it's no wonder that lava flows destroy just about anything in their path!