Magma Viscosity Quiz: What Controls Lava Flow?

Magma is a mixture of molten rock, dissolved gases, and mineral crystals found beneath Earth's surface. When magma reaches the surface, it is called lava. Its composition varies depending on where it forms within Earth's interior and the surrounding rock types.

High silica content in magma increases its viscosity, making it thick and resistant to flow. Silica-rich magmas, such as rhyolitic magma, trap gases more easily and are associated with more explosive volcanic eruptions compared to low-silica, low-viscosity basaltic magmas.

Basaltic magma has the lowest silica content among common magma types, which gives it low viscosity. This allows it to flow easily and quickly. Basaltic lava is commonly found at shield volcanoes, such as those in Hawaii, and at mid-ocean ridges.

Magma viscosity is primarily controlled by silica content, temperature, and dissolved gas content. Higher silica makes magma thicker, while higher temperatures reduce viscosity. Dissolved gases can affect how magma flows and erupts. The color of surrounding rock has no effect on viscosity.

As temperature increases, magma becomes less viscous and flows more easily. This is similar to how heating honey makes it runnier. Higher-temperature magmas, such as basaltic magmas, flow more freely, while cooler, silica-rich magmas are much thicker and slower-moving.

Rhyolitic magma has very high silica content, which makes it extremely thick and slow-moving. It has high viscosity, the opposite of basaltic magma. Because gases cannot escape easily from rhyolitic magma, it is associated with highly explosive volcanic eruptions.

Magma forms primarily in the mantle and lower crust where heat and pressure conditions cause rock to melt partially or fully. This molten material then moves upward through the crust due to its lower density compared to surrounding solid rock.

Shield volcanoes are built from repeated eruptions of low-viscosity basaltic lava that spreads widely and creates broad, gently sloping structures. They are among the largest volcanoes on Earth and are common in places like Hawaii and Iceland.

High-viscosity magma flows slowly, contains high levels of silica, and tends to produce explosive eruptions because trapped gases build up pressure. It does not flow like water. Rhyolitic and andesitic magmas are examples of high-viscosity magmas found at convergent plate boundaries.

Dissolved gases in magma play a major role in volcanic eruptions. As magma rises and pressure decreases, gases expand rapidly. In high-viscosity magma, these gases cannot escape easily, leading to explosive eruptions. In low-viscosity magma, gases escape more gently, producing calmer lava flows.

When magma exits a volcano or a fissure and reaches Earth's surface, it is called lava. Lava can flow as a liquid or erupt explosively as fragments. The term magma is used only when the molten rock is still underground. Pumice and obsidian are rocks that form when lava cools.

Silica, or silicon dioxide, forms long chain-like molecular structures in magma that make it thick and resistant to flow. The higher the silica content, the more viscous the magma. Rhyolitic magma can contain over 70 percent silica, while basaltic magma contains around 50 percent or less.

Andesitic magma is intermediate in composition, with silica content typically between 57 and 63 percent. This places it between low-silica basaltic magma and high-silica rhyolitic magma. Andesitic volcanoes are commonly found along subduction zones and can produce moderately explosive eruptions.

Basaltic, andesitic, and rhyolitic are the three main types of magma classified by silica content. Basaltic has the least silica, rhyolitic has the most, and andesitic falls in between. Crystalline is not a magma type but refers to the structure of certain minerals and rocks after cooling.

High-viscosity magma traps dissolved gases, building up enormous pressure. When that pressure is released, it results in explosive eruptions that can send ash, rock fragments, and pyroclastic material high into the atmosphere. This type of eruption is associated with volcanoes like Mount St. Helens and Krakatoa.