Mount Washington has a diameter of about . It has a summit cone, a volcanic plug comprised by cinder, lava flows, and intrusive rock that covers Washington's volcanic conduit. This plug is made of micronorite with a diameter of . There are dikes exposed throughout the summit cone, mostly oriented from north to south, with another dike swarm trending north from the central plug. The summit formed over a platform of basaltic andesite lavas from early eruptions at Mount Washington, made of thinner flows combined with pyroclastic rock.