A volcano is a vent in the Earth’s crust where molten rock, ash, and gases escape from below the surface. The materials that construct and are expelled from a volcano exist across all three states of matter: solid, liquid, and gas. These components interact in complex ways, determining the eruption style and the final shape of the landform. Understanding these liquid, solid, and gaseous components provides a complete picture of what a volcano is made of, starting with the molten rock that feeds the system.
The Source Material Magma Composition
The story of a volcano starts with magma, the molten rock mixture found beneath the Earth’s surface. Magma is a complex, high-temperature liquid primarily composed of silicate minerals (combinations of silicon and oxygen). The percentage of silica (SiO2) within the melt is the most important factor determining the material’s behavior.
Magmas are broadly categorized based on their silica content, which directly controls viscosity (internal friction). Felsic magmas, such as rhyolite, have a high silica content, typically greater than 68% by weight. This high concentration causes the magma to be very viscous (thick and sticky) and generally results in a lower temperature range, often between 650°C and 800°C.
Mafic magmas, which form rocks like basalt, are on the opposite end of the spectrum. These magmas have a low silica content (45% to 52%) but are rich in iron and magnesium. Low silica results in much lower viscosity, allowing the melt to flow easily, similar to warm motor oil. Mafic magmas are also hotter, commonly measuring between 1000°C and 1200°C. The magma’s viscosity dictates how easily trapped gases escape, which is a major factor in determining if an eruption will be quiet and flowing or violent and explosive.
Eruption Products Lava Ash and Tephra
When magma reaches the surface, it is known as lava, and its behavior is governed by its original properties. Low-viscosity mafic lava, characteristic of shield volcanoes, can flow great distances before cooling. The surface texture of these flows depends on the cooling rate and flow speed as the lava moves away from the vent.
One form is pahoehoe, which is smooth, ropy-looking lava that forms when the surface crust folds over itself. In contrast, a’a lava has a rough, jagged, and blocky texture, formed when a thicker, more viscous flow breaks apart as it moves. Both forms are chemically the same material, but their physical differences illustrate the dynamic cooling process.
If the magma is highly viscous, trapped gases cannot escape easily, leading to a pressure build-up that fragments the melt upon eruption. This fragmentation produces tephra, a collective term for all airborne fragments of volcanic rock and lava. Volcanic ash is the finest type of tephra, consisting of particles smaller than two millimeters in diameter and often composed of microscopic shards of volcanic glass. Larger fragments, like lapilli and volcanic bombs, are also tephra, and the accumulation of these materials constructs the steep slopes of explosive stratovolcanoes.
Classification of Solidified Volcanic Rocks
The permanent, solid structure of a volcano is built from extrusive igneous rocks. These rocks form when lava and tephra cool rapidly on the Earth’s surface or in the atmosphere. The final rock type is classified based on the chemical composition of the original magma and the cooling conditions. This rapid cooling results in a fine-grained, or aphanitic, texture where individual mineral crystals are too small to be seen without magnification.
Basalt is the most common volcanic rock, originating from mafic, low-silica magma. It is dark-colored and forms vast, flowing plateaus or the broad slopes of shield volcanoes like those in Hawaii. Andesite is an intermediate rock type, forming from magmas with a moderate silica content, and is common in the steep-sided composite volcanoes found along subduction zones.
Rhyolite is the felsic equivalent, formed from high-silica magma and often associated with the most explosive eruptions. Rapid cooling of high-silica lava can produce unique textures. For example, obsidian is a dense volcanic glass that cools too quickly for any crystals to form. Pumice is another felsic rock characterized by a light color and porous structure, formed when gas-rich lava solidifies around escaping bubbles.
Volcanic Gases and Volatiles
Volcanic gases, or volatiles, represent the third state of matter and are the force that drives eruptions. These gases are dissolved within the magma while it is under immense pressure deep beneath the surface. Water vapor (H2O) is the most abundant component, often making up over 70% of the total gas volume.
The next most common gases are carbon dioxide (CO2) and sulfur dioxide (SO2). As magma ascends toward the surface, the confining pressure drops significantly. This causes the dissolved gases to separate from the melt and form bubbles, a process called exsolution, similar to opening a bottle of soda.
The rapid expansion of these bubbles creates enormous internal pressure. This is the primary mechanism that forces magma out of the vent and can lead to violent, explosive eruptions. When the eruption is less explosive, these gases are released into the atmosphere through the vent or through smaller cracks in the volcanic edifice.