Question 8 of 12 Why do shield volcanoes have weak eruptions? A. They have excess magma. B. They form calderas. C. They feature magma with high silica content. D. They feature magma with very low viscosity.

## Step 1<br />The question asks why shield volcanoes have weak eruptions. The options provided are:<br />A. They have excess magma.<br />B. They form calderas.<br />C. They feature magma with high silica content.<br />D. They feature magma with very low viscosity.<br /><br />## Step 2<br />The key to answering this question lies in understanding the characteristics of shield volcanoes and the nature of their eruptions. Shield volcanoes are known for their gentle, broad slopes and are built up over time by the flow of low-viscosity lava. This type of lava flows easily and spreads out over a large area, resulting in a wide, gently sloping volcano.<br /><br />## Step 3<br />The viscosity of magma is a crucial factor in determining the nature of a volcanic eruption. Magma with high viscosity tends to trap gases, leading to explosive eruptions. In contrast, magma with low viscosity allows gases to escape more easily, resulting in gentler eruptions.<br /><br />## Step 4<br />Option A, "They have excess magma," is not directly related to the strength of the eruption. The amount of magma does not necessarily determine the nature of the eruption.<br /><br />## Step 5<br />Option B, "They form calderas," is not relevant to the question. Calderas are large, depression-like features that can form after a volcanic eruption, but they do not directly influence the strength of the eruption.<br /><br />## Step 6<br />Option C, "They feature magma with high silica content," is incorrect. High silica content in magma typically leads to higher viscosity, which would result in stronger, more explosive eruptions, not weaker ones.<br /><br />## Step 7<br />Option D, "They feature magma with very low viscosity," is the correct answer. The low viscosity of the magma in shield volcanoes allows gases to escape more easily, leading to gentler, less explosive eruptions.