This lava quenches into a glass and forms a small cone. Basaltic lava flows form Shield volcanoes . Slightly domed structures form from the successive build up of lava flows. Can be very large, like Mauna Kea in Hawaii. Largest mountain on earth.
Gas is prevented from escaping by the viscous magma, and pressure builds.
When released, the euption tends to be violent. As ash falls to the ground, it compresses into a hot ASH-FLOW TUFF . These hot ash clouds and gases can race down the side of a mountain at near 125 mph wiping out everything in its path. These are called Nuee Ardentes . When ash becomes saturated with water, by rain, it can flow down valleys.