## 5 Discussion  
When a subducting plate descends into the mantle, the components within a specific depth range under the volcanic arc can undergo partial melting to form magma, given the right conditions of temperature, pressure, and fluid. This magma then moves upwards, accumulates, and eventually erupts, creating arc volcanoes. The magmatic activity associated with subduction is predominantly influenced by factors such as temperature, pressure, and fluid presence (Figure 6a,b). To begin with, an increase in fluid content can significantly lower the melting point and solidus temperature of rocks. This factor also effects the melt's viscosity and the magma's average density [33,34]. Generally, volcanic arcs are believed to develop during subduction due to the dehydration of the descending slab, which leads to melting and magmatic eruptions after the mantle above the subducted slab becomes saturated with fluids [35]. Importantly, the depth at which the slab dehydrates plays a key role in influencing magmatic processes. If hornblende in the oceanic crust dehydrates shallowly, the partial melting of mantle rocks may be reduced even when the temperature of the mantle wedge exceeds 1000 \({}^{\circ}\)C at certain depths beneath the arc. This reduction occurs because a drier rock has a higher melting point, diminishing subduction-related magmatism [36]. Additionally, pressure affects rock melting, as it decreases the melting point. During the ascent of magma, decompression further melts the partial melt beneath the volcanic arc, which enhances the production of melting magma and speeds up its rise [37,38]. Lastly, elevated temperatures facilitate partial melting. In episodic fissure eruptions, the melting temperatures range between 700 and 900 \({}^{\circ}\)C for rhyolite and 1000 and 1300 \({}^{\circ}\)C for basalt. Thermal variations or disturbances during subduction can boost partial melting and modify magma composition. The depth at which the plate undergoes dehydration is influenced by the thermal structure of the subduction zone. When the depth of dehydration is optimal, the temperature profile of the mantle wedge controls arc magmatism. The thermal structure of the subduction zone is influenced by several factors, including subduction rates, the shape of the subducting slab, and shear friction heating [37]. In contrast, the temperature distribution within the mantle wedge is mainly governed by the overriding plate's movement rate, the properties of the lithosphere, and cooling rates. Thermal simulations have been employed to assess the temperature structures of the subduction zone and the mantle wedge beneath the Halmahera volcanic arc, accounting for different rates of subduction and movement of the overriding plate (Figure 6c-f). As subduction progresses, the slab dehydrates at depths ranging from 100 km to 150 km, initiating partial melting of the mantle wedge material located above the subduction zone. This melting occurs at temperatures ranging from 800 to 1100 \({}^{\circ}\)C [35; 39]. It follows that higher overriding plate movement rates (Case 1) or lower subduction rates (Case 3) are conducive to magma generation (Figure 6). The overriding plate movement rate is identified as the primary factor influencing the thermal structure of the mantle wedge. With a high movement rate of the overriding plate (Figure 6c), temperatures in the mantle wedge near the subduction zone rise significantly along the subduction slope, reaching  
Figure 6: The analysis of volcanic arc magmatism is presented as follows: (**a,b**) illustrate factors affecting arc magmatism, including fluid effects, temperature effects, and pressure effects. (**c,d**) show the possible magmatism temperature ranges under constant subduction rates but varying overriding plate movement rates. (**e,f**) display the possible magmatism temperature ranges under constant overriding plate movement rates but varying subduction rates. up to 900-1300 \({}^{\circ}\)C at 50-85 km above the subduction zone.