Why Do Shield Volcanoes Have Weak Eruptions

Shield volcanoes are broad, gently sloping volcanic landforms that are built by the accumulation of low-viscosity basaltic lava flows. Unlike the explosive eruptions seen in stratovolcanoes, shield volcanoes have relatively weak eruptions characterized by effusive lava flows. There are several reasons why shield volcanoes have these types of eruptions. Let’s explore them in detail:

1. Magma Composition

One of the key factors that influence the eruptive behavior of shield volcanoes is the composition of the magma. Shield volcanoes are primarily made up of basaltic magma, which has a low silica content. Low-silica magma is less viscous compared to high-silica magma found in stratovolcanoes. The low viscosity of basaltic magma allows it to flow more easily, leading to gentle, effusive eruptions rather than explosive ones.

2. Gas Content

In addition to magma composition, the amount and type of gases present in the magma can also impact the eruptive behavior of shield volcanoes. Basaltic magmas have low gas content compared to more silica-rich magmas. This means that shield volcanoes are less likely to have explosive eruptions driven by gas build-up and pressure release.

3. Vent Structure

The vent structure of a volcano plays a crucial role in determining the style of eruption. Shield volcanoes typically have large, open vents that allow lava to flow freely without building up excessive pressure. This design promotes effusive eruptions characterized by steady lava flows instead of explosive events.

4. Sloping Profile

The gentle, sloping profile of shield volcanoes also contributes to their weak eruptions. The broad shape of these volcanoes allows lava to flow more easily down their flanks, preventing the build-up of pressure that can lead to explosive eruptions. The low angle of repose of shield volcanoes promotes the continuous flow of lava rather than sudden, explosive eruptions.

5. Reservoir Size

The size of the magma reservoir beneath a volcano can influence its eruptive behavior. Shield volcanoes typically have large magma reservoirs that can supply lava for extended periods of time. This continuous supply of magma promotes effusive eruptions, as the volcano can release lava steadily without the need for sudden, explosive events.

6. Tectonic Setting

The tectonic setting of a volcano can also play a role in determining the type of eruptions it experiences. Shield volcanoes often occur at divergent plate boundaries or hot spots where mantle plumes bring basaltic magma to the surface. These tectonic settings tend to produce effusive eruptions due to the low viscosity of basaltic magma.

7. Eruption Frequency

Shield volcanoes typically experience frequent but relatively small eruptions compared to other types of volcanoes. The continuous, effusive eruptions at shield volcanoes help build up their characteristic broad, gently sloping shapes over time. These frequent eruptions are a result of the steady supply of magma from the underlying mantle, as well as the volcano’s vent structure and magma composition.


In conclusion, shield volcanoes have weak eruptions characterized by effusive lava flows due to a combination of factors such as magma composition, gas content, vent structure, sloping profile, reservoir size, tectonic setting, and eruption frequency. These factors work together to create the typical behavior of shield volcanoes, where lava flows steadily and gently rather than explosively. Understanding why shield volcanoes have weak eruptions can provide valuable insights into the geology and dynamics of these fascinating volcanic landforms.

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