Volcanoes are geological structures where molten rock, ash, and gases escape from beneath the Earth’s crust. They occur in diverse forms, but the two most recognized types are the composite volcano and the shield volcano. Their names are descriptive labels that capture their unique physical forms. Understanding the origins of these names provides insight into the fundamental differences in their construction and appearance.
How Composite Volcanoes Earned Their Name
The name “composite volcano” directly refers to the varied materials that compose its structure, built up over countless eruptions. This type of volcano forms a steep, symmetrical cone, a shape that results from the accumulation of alternating layers of different substances. These layers include hardened lava flows, fragmented rock pieces, ash, and cinders, all stacked one upon the other.
Every eruption cycle adds a new layer, with the thicker, more viscous lava flows solidifying quickly before spreading far from the central vent. This process of successive layering creates the characteristic sharp, concave slopes. The alternate name for this type of mountain is a stratovolcano, a term derived from the Latin word stratum, meaning a layer or bed.
The term stratovolcano emphasizes the internal stratified, or layered, construction of the mountain. While the name “composite” refers to the mixture of materials, “stratovolcano” more precisely describes the sequence of these alternating layers. Both names accurately reflect the volcano’s growth method.
How Shield Volcanoes Earned Their Name
Shield volcanoes earned their name from their unmistakable resemblance to a warrior’s shield lying face up on the ground. This descriptive label was first coined by Austrian geologist Eduard Suess in 1888, using the German term Schildvulkan. The characteristic shape is a broad, gently sloped dome, which is significantly wider than it is tall.
These mountains have a very low profile, typically featuring slopes that only range from about two to ten degrees near the summit. The vast size and gentle inclination are a direct result of how they are built. They are primarily constructed from successive flows of highly fluid lava that spread out over immense distances before cooling and solidifying.
The continuous, widespread flow of low-viscosity lava builds a vast, low-angle cone that can span hundreds of kilometers across. The resulting convex shape, with a central raised area and long, gentle flanks, perfectly mirrors the appearance of a massive, ancient circular shield.
Eruptive Processes That Define the Names
The distinct structures that gave these volcanoes their names are ultimately a consequence of the type of magma fueling them. Composite volcanoes are fed by magma that is relatively high in silica, which makes it thick and viscous. This high viscosity traps volcanic gases, leading to immense pressure buildup and highly explosive eruptions.
The explosive force of these eruptions sends fragmented rock, ash, and cinders high into the atmosphere, which then rain down to create the loose layers of pyroclastic material. The thick lava flows that follow cannot travel far, quickly solidifying on the steep upper flanks and contributing to the cone’s height and steepness. This cycle of explosive and effusive events is the mechanical process behind the layered, composite structure.
In contrast, shield volcanoes are formed by magma that is low in silica, making it highly fluid and low-viscosity, such as basalt. The runny nature of this magma allows volcanic gases to escape easily, resulting in non-explosive, effusive eruptions. This fluid lava flows quickly and travels great distances across the landscape.
Because the lava spreads out rapidly and efficiently, it creates extremely thin, widespread layers with each event. This process prevents the formation of a steep cone and instead builds the massive, gently sloping dome. The highly fluid nature of the magma is the underlying cause for the shield’s shape, which then gave the volcano its descriptive name.