Final answer:
Lava domes like Lassen Peak are formed by highly viscous lava that piles up around the vent, often resulting in a dome-shaped volcano with infrequent but potentially explosive eruptions that can release ash clouds.
Step-by-step explanation:
Lava domes like Lassen Peak are characterized by the eruption of highly viscous lava that does not flow easily. This causes the lava to pile up around the vent, forming a dome or bulbous shape rather than a typical conical volcano shape. Lassen Peak and other lava domes usually result from magma that is high in silica, which increases the viscosity of the lava. During eruptions, these domes can release ash clouds, but their magma is not as volatile as some other types of volcanoes, such as stratovolcanoes or cinder cones. Instead, the eruptions tend to be less frequent but can still be explosive due to the pressure build-up from the viscous magma that plugs the vent.