What best describes a shield volcano?

What best describes a shield volcano?

• A. A wide, gently sloping mountain made of layers of lava and formed by quiet eruptions
• B. A tall, steep-sided cone formed by explosive eruptions
• C. A mountain built primarily from ash and pumice deposits during explosive events
• D. A plateau of cooled lava with no eruption

Answer: (A)

The idea being tested is how the eruption style and lava properties shape a volcano. A shield volcano forms because the lava that erupts is low in viscosity, like basalt, so it flows easily and travels long distances. These fluid lava flows spread out in all directions, building up a broad, gently sloping mountain over time. Eruptions are typically effusive and relatively quiet, with lava pouring out and covering wide areas rather than blasting upward in explosions. That combination—wide, shallow slopes and many flowing eruptions—defines a shield volcano. This contrasts with explosive, steep-sided cones that come from viscous lava and ash-rich eruptions, which produce tall, narrow shapes built from alternating layers of lava and tephra. The idea of a mountain made mainly from ash and pumice points to that explosive, layered construction, not the smooth, wide profile of a shield volcano. Likewise, a plateau of cooled lava with no eruption would resemble a lava surface or flood lava feature rather than a shield formed by ongoing, gentle lava flows.

The idea being tested is how the eruption style and lava properties shape a volcano. A shield volcano forms because the lava that erupts is low in viscosity, like basalt, so it flows easily and travels long distances. These fluid lava flows spread out in all directions, building up a broad, gently sloping mountain over time. Eruptions are typically effusive and relatively quiet, with lava pouring out and covering wide areas rather than blasting upward in explosions. That combination—wide, shallow slopes and many flowing eruptions—defines a shield volcano.

This contrasts with explosive, steep-sided cones that come from viscous lava and ash-rich eruptions, which produce tall, narrow shapes built from alternating layers of lava and tephra. The idea of a mountain made mainly from ash and pumice points to that explosive, layered construction, not the smooth, wide profile of a shield volcano. Likewise, a plateau of cooled lava with no eruption would resemble a lava surface or flood lava feature rather than a shield formed by ongoing, gentle lava flows.