Which gas is primarily associated with the effusive eruptions of shield volcanoes?

The gas primarily associated with the effusive eruptions of shield volcanoes is water vapor. Shield volcanoes are characterized by their broad, gently sloping sides and typically have lava flows that are relatively low in viscosity, allowing them to travel long distances. During effusive eruptions, large volumes of lava are expelled, and water vapor plays a crucial role in this process.

As magma rises towards the surface, it undergoes degassing, which involves the release of dissolved gases. Water vapor is one of the most abundant gases released from magma because it originates from both the magma itself and from the surrounding environment where water is present. This vapor can expand dramatically as the pressure decreases when the magma reaches the surface, contributing to the eruption's dynamics.

This significant presence of water vapor in the gas mixture during effusive eruptions helps to produce the characteristic fluid lava flows associated with shield volcanoes, which are less explosive compared to other types of volcanic eruptions. Understanding the role of water vapor in these eruptions helps clarify why it is the gas primarily linked with shield volcano activity.