processes and conductive heat flow. The more heat used on resurfacing, the less heat can become conductive heat flow and the less thermal stress is caused by heat. The thermal stress is important as it can be generated wherever and whenever volcanic output is less than tidal heating input. Geodynamic models Due to a strong tidal heating, Io is very geologically active and is volcanically resurfaced by lavas and plume deposits at a high rate (about per year). Several models have been proposed to related this resurfacing process to accumulation of stress in the lithosphere. Many mountains higher than have been observed on Io. This implies that Io has a thick crust. In O'Reilly and Davies' 1981 paper, they proposed that part of the heat in Io is transported by advection. Magma from depths rises to the surface through isolated vents, and spreads out and cools at surface. The solid lithosphere subsides under the continuously generated new lava flow. Solid material is heated by conduction at the base of lithosphere and melts again. The thrust faulting and uplifting of large crust blocks on Io are interpreted by a model proposed by Schenk and Bulmer's 1998 paper. In the model, Io's crust keeps recycling. Violent volcanic activity brings lava to the surface and older, buried layers are forced to subside. The old volcanic crust materials are compressed laterally as they sink. A later model presents more details. Io consists of a continuous stack of mafic and ultramafic deposits. After new erupted volcanic materials cool down and are buried, the stack of rocks become indurated and form bedrocks. The bedrocks are fractured due to tidal flexing, compression at depth, volcanic intrusion and other mechanisms, and then are broken into large blocks a hundred kilometers across. Products of magmatism like sills, dikes and batholiths may intrude into layers of stacking volcanics to form a composite crust. Occasionally, the large blocks of crust are rotated and thrusted along deep-rooted thrust faults. This process may expose a cross-section of crust to the surface, as at Euboea Montes. Later, these blocks can also be eroded by mass wasting and reburied by subsequent volcanism. At the base of the crust, materials are met again by heat. Compression at depth due to global burial and subsidence can also form ductile deformation like folding of crust. Mountains and paterae Paterae and mountains are observed to appear near each other