Magma and lava are the same material: molten rock. The only difference is location. Scientists call it magma when it’s underground and lava once it breaks through the Earth’s surface. Beyond that naming convention, the two behave quite differently depending on their chemical makeup, temperature, and gas content.
How Magma Forms Underground
Deep inside the Earth, temperatures are high enough to partially melt solid rock into a thick, flowing substance. Because this molten rock is lighter than the solid rock surrounding it, it rises and collects in underground pools called magma chambers. These chambers can sit at various depths. Beneath Yellowstone National Park, for example, scientists have mapped two separate magma bodies using a technique similar to a medical CT scan: a shallower one stretching from about 5 to 17 kilometers below the surface and a deeper one extending from 20 to 50 kilometers down.
Magma doesn’t just sit still. It moves upward through cracks and weak points in the crust, sometimes pooling for thousands of years before finding a path to the surface. When it finally pushes through vents or fissures and reaches open air, it becomes lava.
Temperature and Composition
Not all molten rock is created equal. The key variable is silica content, the proportion of silicon and oxygen in the melt. Silica-rich magmas behave very differently from silica-poor ones, and geologists group them into a few broad categories.
Basaltic magma has the least silica (roughly 45 to 55 percent by weight) and runs the hottest, between 1,000°C and 1,200°C (about 1,830°F to 2,190°F). It flows relatively easily, almost like thick honey. This is the type that produces the glowing rivers of lava you see in footage from Hawaii or Iceland. On the other end of the spectrum, rhyolitic magma contains 70 to 77 percent silica and erupts at cooler temperatures, around 650°C to 800°C. Despite still being hot enough to melt most metals, it’s far more sluggish and paste-like. Andesitic magma falls in the middle, with temperatures between 800°C and 1,000°C.
To put these numbers in perspective, a high-end pizza oven operates at about 370°C to 540°C, hot enough to cook a pizza in a minute or two. Even the coolest magmas are significantly hotter than that.
Why Some Eruptions Explode and Others Flow
Silica content controls more than temperature. It determines viscosity, which is essentially how thick and resistant to flow the molten rock is. Low-silica basaltic magma is runny, so dissolved gases can escape from it relatively gently, like bubbles leaving a pot of thin soup. The result is the kind of eruption where lava pours out steadily and spreads across the landscape.
High-silica rhyolitic magma is so thick that gas bubbles get trapped inside. Pressure builds until the magma reaches the surface, where those gases expand violently. This is what drives explosive eruptions, the towering ash columns, pyroclastic flows, and widespread destruction associated with volcanoes like Mount St. Helens. The magma’s chemistry essentially determines whether a volcano oozes or detonates.
Types of Lava Flows
Once lava is on the surface, it takes on different forms depending on how fast it’s being erupted and how quickly it cools. Two Hawaiian terms describe the most common types.
Pahoehoe forms when lava is erupted slowly. It travels through insulated underground tubes that keep it hot over long distances, and it advances by oozing out in small lobes or “toes.” Once it cools, pahoehoe has a smooth, ropy surface that’s been compared to the top of a pan of brownies. Because it moves gently, gas bubbles stay trapped inside and form small, round voids throughout the rock.
Aa (pronounced “ah-ah”) forms when a large volume of lava pours out quickly through open surface channels. Exposed to air, it loses heat fast, thickens, and grows increasingly pasty. Crystals form throughout the flow as it cools, making it even stiffer. The result is a rough, jagged, clinkery surface. An advancing aa flow looks less like a river and more like a steep wall of broken rock chunks bulldozing forward, crashing through whatever stands in its path.
Both types can come from the same eruption. If conditions change (the eruption rate increases, for instance) a pahoehoe flow can transition into aa, though the reverse almost never happens.
What Happens When Molten Rock Cools
The speed of cooling determines what kind of solid rock you end up with, and this is where the magma-versus-lava distinction has lasting consequences.
Magma that stays trapped deep underground cools very slowly, sometimes over millions of years. Individual mineral crystals have plenty of time to grow, producing rocks with large, visible grains. Granite is the most familiar example of this type, called intrusive or plutonic rock. Diorite, gabbro, and peridotite also form this way.
Lava that erupts onto the surface cools quickly, sometimes in hours or days. Mineral crystals barely have time to form, so the resulting rock has a very fine-grained texture. Basalt, andesite, and rhyolite are common examples. If cooling is extremely rapid, no crystals form at all and the rock is essentially volcanic glass. Obsidian, the shiny black rock prized for making sharp tools throughout human history, forms exactly this way.
So the same chemical mixture can produce very different-looking rocks depending on whether it cooled underground as magma or on the surface as lava. A geologist can pick up a rock, look at the crystal size, and immediately tell where in the Earth it solidified.
Magma Beneath Your Feet
Magma chambers exist in more places than most people realize. The two reservoirs beneath Yellowstone are massive: the shallower one alone is about 90 kilometers long and 40 kilometers wide. Scientists detect these hidden pools using seismic tomography, which tracks how earthquake waves speed up or slow down as they pass through different materials underground. Molten rock slows the waves, creating a kind of shadow that reveals the chamber’s size and shape.
The shallower Yellowstone chamber is made of silica-rich rhyolite, while the deeper one is silica-poor basalt. This layering is common at large volcanic systems and helps explain why a single volcano can produce different types of eruptions over its lifetime, depending on which magma body feeds a given event.