Understanding Volcano Formation from Lava Flows

The question asks about the type of volcano built up over a long period by repeated, thin lava flows. This specific mode of formation leads to a particular shape and structure for the volcano.

Let's analyze the process described: individual lava flows are thin, only a few feet thick. However, over a significant amount of time, layer upon layer of these thin flows accumulate. This accumulation builds up a large volcanic structure.

Characteristics of Volcano Types

Volcanoes are classified based on their shape, size, and the materials they erupt. The options provided represent different types:

• Shield Volcano: These are large volcanoes with gentle slopes. They are built almost entirely from fluid lava flows. The lava is typically basaltic, which is low in viscosity (it flows easily). Because the lava is very fluid, it spreads out widely before solidifying, creating thin layers. Repeated eruptions of this fluid lava build a broad, shield-like shape over time.
• Composite Volcano (also known as Strato-volcano): These volcanoes are characterized by their conical shape with steep slopes. They are built from alternating layers of lava flows and volcanic ash, cinders, and bombs (collectively called tephra). The lava is usually more viscous than basaltic lava, leading to shorter, thicker flows and steeper slopes. Eruptions can be explosive due to the build-up of gas pressure.
• Cinder-cone Volcano: These are the simplest type of volcano and are typically the smallest. They are built from pyroclastic materials (rock fragments, ash, cinders) ejected from a single vent. These materials pile up around the vent, forming a cone with steep sides and a large crater at the summit. They do not have extensive lava flows building their main structure.

Analyzing the Question Description

The question explicitly mentions "individual lava flows are normally only a few feet thick" and "over a long period of time, repeated flows may build up a volcano". This description perfectly matches the formation process of a shield volcano, where thin, fluid lava flows accumulate layer by layer over many eruptions to create a large, gently sloped mountain.

Composite or Strato-volcanoes are built from a mix of lava and ash and have steeper slopes. Cinder-cone volcanoes are built primarily from ejected rock fragments (cinders) and are much smaller.

Therefore, the type of volcano built by repeated thin lava flows is a shield volcano.

Revision Table: Comparing Volcano Types

Additional Information on Volcanoes and Lava

The characteristics of volcanic eruptions and the resulting volcano shape are heavily influenced by the type of magma (and thus lava) involved, particularly its viscosity and gas content.

• Lava Viscosity: Low viscosity lava (like basaltic) flows easily and spreads out, building wide, shallow slopes typical of shield volcanoes. High viscosity lava (like andesitic or rhyolitic) is thick and sticky; it flows slowly and builds steeper slopes, often associated with composite volcanoes.
• Gas Content: The amount of dissolved gases in magma affects the explosiveness of an eruption. High gas content, especially combined with high viscosity magma that prevents gases from escaping easily, can lead to explosive eruptions that produce large amounts of ash and tephra. Low gas content and low viscosity magma allow gases to escape more gently, resulting in effusive eruptions dominated by lava flows.
• Volcanic Provinces: Shield volcanoes are commonly found in areas with 'hot spots' (like Hawaii) or divergent plate boundaries, where large volumes of fluid basaltic magma are produced. Composite volcanoes are often located at convergent plate boundaries, particularly along the Pacific Ring of Fire, where subduction creates magmas with higher silica content and gas. Cinder cones can occur in various volcanic settings, often on the flanks of larger volcanoes or in volcanic fields.