A composite volcano, also known as a stratovolcano, presents the classic image most people associate with a volcano. These immense structures are among the tallest and most visually striking landforms on Earth, often rising thousands of feet above the surrounding terrain. They are built from the accumulation of materials ejected during a long history of alternating eruptive styles.
Defining Physical Shape and Profile
The most distinguishing visual feature of a composite volcano is its towering conical shape. These volcanoes are often symmetrical, though later eruptions or erosion can alter this. The profile features a concave slope that becomes steeper as it approaches the summit.
The flanks typically maintain slopes between 30 and 35 degrees near the peak. This steepness results from the nature of the magma involved in their formation. Because the lava is thick and viscous, it does not flow far from the vent before cooling and solidifying.
This rapid solidification causes the material to pile up directly around the central vent, building the characteristic high-angle cone. Famous examples, such as Mount Fuji or Mount Rainier, exemplify this form.
Internal Structure and Composition
The term “composite” refers to the layered, or stratified, construction of the volcano’s interior. The structure is an alternating sequence of hardened lava flows and fragmented pyroclastic material. Eruptions typically alternate between effusive phases, which produce lava, and explosive phases, which create debris.
The lava flows, often composed of andesite, have a high silica content, making them thick and slow-moving. These viscous flows travel short distances before cooling, creating thin but structurally sound layers. These hardened sheets provide a supporting framework for the edifice.
Interspersed with the lava are layers of fragmental rock known as tephra, including ash, cinders, and volcanic bombs. This material is ejected during gas-driven explosions and settles on the slopes. The combination of strong lava layers and loose pyroclastic layers builds the steep, high profile.
Key Surface Features
The peak is typically capped by a summit crater, a depression that forms around the main vent. This often results from explosive eruptions blasting out material. A central vent or cluster of vents within the crater serves as the primary conduit for erupting material.
In some cases, the summit structure may collapse inward after an eruption empties the underlying magma chamber, leaving a much larger depression called a caldera. Lava domes, which are bulges of solidified, viscous lava, may also grow within the summit crater during quieter periods.
The flanks can exhibit secondary features, such as parasitic cones or fissures, where magma bypasses the main conduit. Water runoff and gravity acting on the steep ash layers create deep, radiating valleys or gullies down the sides. These erosional features become more pronounced on older volcanoes.
Comparison to Other Volcano Types
The appearance of a composite volcano contrasts sharply with shield and cinder cone volcanoes. Shield volcanoes, like those in Hawaii, are built from fluid, low-viscosity lava that flows great distances. This results in a broad, gently sloping dome with slopes generally less than 10 degrees.
Composite volcanoes are significantly taller and steeper than these broad shields. Cinder cones are the simplest and smallest form, built entirely from loose, ejected fragments piled around a single vent. They possess steep sides, sometimes exceeding 35 degrees, but rarely rise more than a thousand feet and lack the complex structure of a composite volcano.