Deep trenches are often formed where tectonic plates are being subducted and earthquakes are common. As the sinking plate moves deeper into the mantle, fluids are released from the rock causing the overlying mantle to partially melt.he new magma (molten rock) rises and may erupt violently to form volcanoes, often building arcs of islands along the convergent boundary. These island arcs are always landward of the neighboring trenches. When two plates are moving away from each other, we call this a divergent plate boundary.