Question

How do a shield volcano, a cinder cone, and a composite cone differ from one another? How are they similar?

Short answer

Shield volcanoes have gentle slopes; cinder cones are small with steep sides; composite cones are large with steep sides. All are volcanic landforms.

Step-by-step solution

Understanding Shield Volcanoes

Shield volcanoes are broad, domed-shaped volcanoes with gently sloping sides. They are primarily built by the flow of low-viscosity basaltic lava that can travel great distances. This type of volcano has large surface areas but is not very steep.

Analyzing Cinder Cones

Cinder cones are the smallest type of volcano. They have steep slopes (usually 30-40 degrees) and are composed mainly of volcanic ash, cinders, and bombs that accumulate around the vent. These volcanoes typically have a crater at the summit and erupt in a more explosive manner compared to shield volcanoes.

Examining Composite Cones

Composite cones, also known as stratovolcanoes, are large, symmetrical volcanoes with steep sides. They consist of alternating layers of solidified lava flows, volcanic ash, and other volcanic debris. These volcanoes have a higher viscosity magma, leading to more violent eruptions.

Comparing Characteristics

While shield, cinder cone, and composite cone volcanoes differ in their size, shape, and eruption styles, they share the commonality of being volcanic structures formed by magma reaching the Earth's surface. All three can produce lava flows, though the nature and volume of these flows vary greatly. Additionally, their formation is directly related to tectonic activity where magma ascends from the mantle.

Key concepts

Shield Volcano

A shield volcano is known for its distinctive broad, domed shape and gentle slopes. These formations arise primarily from low-viscosity basaltic lava that flows over large areas, creating layers that gently pile up like a warrior’s shield. Unlike their steeper counterparts, shield volcanoes do not rise sharply into the sky. This is because the flowing lava is fluid and can traverse long distances before solidifying.
Some famous shield volcanoes include Mauna Loa and Kīlauea in Hawaii. These volcanic giants cover extensive areas and have relatively calm eruptions. The lava flows form vast, flat layers rather than dramatic peaks.

• Diameter can span over dozens of kilometers.
• Effusive eruptions with lava fountains and flows.
• Can be colossal, but they have smooth, rounded profiles.

Cinder Cone Volcano

Cinder cone volcanoes, the smallest among the volcano types, present with striking steep slopes. Often compared to hill-like formations, they are primarily structures made up from volcanic ash, cinders, and bombs—the solidified lava fragments shot into the air during eruptions.
They often feature a crater at their summit and give rise to a more explosive eruption style compared to the slow lava flows of shield volcanoes. Their formation is quick and can occur over months or even days.

• Characteristically steep slopes, often between 30-40 degrees.
• Single-eruption events can create these volcanoes.
• Visibly cratered summit due to explosive material ejection.

Composite Cone Volcano

Composite cone volcanoes, also recognized as stratovolcanoes, are renowned for their majestic, symmetrical peaks with steep sides. They comprise alternating layers of solidified lava flows, volcanic ash, and volcanic debris, making them the ultimate mixed – or composite – volcano.
The magma associated with composite cones tends to have a higher viscosity compared to shield volcanoes. This leads to violent eruptions, often producing ash plumes, pyroclastic flows, and lava domes.

• High-viscosity magma results in more violent eruptions.
• Famous examples include Mount St. Helens and Mount Fuji.
• Can reach significant heights with a more tiered appearance.

Lava Viscosity

Lava viscosity plays a crucial role in determining volcano characteristics and eruption styles. In simple terms, viscosity is the measure of a fluid's resistance to flow. Low-viscosity lava flows easily and spreads across vast distances, characteristic of shield volcanoes. On the other hand, high-viscosity lava is thicker, does not flow easily, and can lead to explosive eruptions seen in stratovolcanoes. This thick consistency often allows gases to build pressure, leading to more violent outcomes.

• Lower viscosity: Lava flows gently (e.g., basalt lava).
• Higher viscosity: Leads to explosive eruptions and debris accumulation.
• Affects the shape and eruption style of the resulting volcano.

Tectonic Activity

Volcanoes owe much of their existence to tectonic activity beneath the Earth's crust. Tectonic plates are massive slabs of solid rock that move and interact with one another. This movement can cause magma from the mantle to surface, forming volcanoes. Three main tectonic settings can result in volcano formation: convergent boundaries, divergent boundaries, and hot spots. At convergent boundaries, one tectonic plate subducts beneath another, melting into magma that may eventually surface as composite cones. In divergent boundaries, plates pull apart, allowing magma to rise and form shield volcanoes. Hot spots occur when magma from deep within the mantle rises through the crust, exemplified by Hawaiian shield volcanoes.

• Volcano formation is closely linked to tectonic boundaries.
• Helps determine the type and behavior of a volcano.
• Fundamental to understanding volcano distribution and eruption patterns.