Explanation:

Composite volcanoes are found on destructive plate margins, where the oceanic crust sinks beneath the continental crust. Composite volcanoes have the following characteristics:

• Acidic lava, which is very viscous (sticky).
• Steep sides as the lava doesn't flow very far before it solidifies.
• Alternate layers of ash and lava. For this reason, they're also known as stratovolcanoes. (Strato means layers).
• Violent eruptions.
• Longer periods between eruptions.

Hence, composite volcanoes help in formation of new islands/ landmasses. An example of a composite volcano is Mount Pinatubo in the Philippines.

When a composite volcano becomes dormant, erosion begins to destroy the cone. As the cone is stripped away, the hardened magma filling the conduit (the volcanic plug) and fissures (the dikes) becomes exposed, and it too is slowly reduced by erosion. Finally, all that remains is the plug and dike complex projecting above the land surface--a telltale remnant of the vanished volcano. Thus, post erosion, caldera is formed. eg: Crater Lake.