Answer: 

Festoons are islands arcs forming an archipelago in the shape of a loop around the edge of the mainland, marking the continuation of mountain ranges which can be traced on the continent, e.g. the East Indies, the Aleutian Islands, Ryukyu Islands etc. 

They’re  volcanic in origin formed at convergent tectonic plate boundaries, typically associated with subduction zones and volcanic activity.

Mechanism of Island Arc Formation:

• Subduction Zone: Oceanic crust subducts beneath another tectonic plate, generating magma that rises to form volcanic arcs. [ex- Marianas Trench]
• Magma Generation: Water and other volatiles released from the subducting slab melt overlying mantle, creating magma chambers. [ex- Aleutian Islands]
• Volcanic Activity: Magma rises through fractures in the overriding plate, erupting as volcanoes along the arc's curvature. [ex- Java Trench]
• Crustal Deformation: Compression and folding of the overriding plate contribute to the arc's curvature and island formation. [ex- Tonga Trench]
• Arc Migration: Island arcs may migrate over time due to changes in subduction angle or plate motion, altering their geological features. [ex- Philippine Sea Plate]

Higher Occurrence in Indian and Pacific Ocean:

• Subduction Zones: The Indian and Pacific Oceans feature numerous active subduction zones, providing ideal conditions for island arc formation. [e.g., Pacific Ring of Fire]
• Tectonic Activity: High levels of tectonic activity, including subduction and volcanic activity, characterize these ocean basins, promoting island arc development. [e.g Indonesian archipelago.
•  Large Oceanic Plates: The presence of large oceanic plates facilitates long-lasting subduction and the formation of extensive island arcs. [e.g Pacific Plate]
•  Hotspot Activity: Hotspots like the Hawaiian hotspot contribute to island arc formation in the Pacific Ocean through volcanic activity. [e.g Hawaiian Islands]
• Continental Fragmentation: Fragmentation of continental landmasses in the Indian and Pacific Oceans results in numerous small islands and island arcs. [e.g., Andaman Islands]

Festoons offer one of the best scenic beauty promoting the tourism economy and are home to the indigenous populations. They’re facing the threats of submergence due to increasing sea levels, hence global collaboration is needed to preserve them.