Explain the formation of thousands of islands in Indonesian and Philippines archipelagos.

Explain the formation of thousands of islands in Indonesian and Philippines archipelagos. [10 marks, 2014]

An archipelago is a group or chain of islands. Indonesia and the Philippines contain thousands of islands because they lie in a highly active zone where several tectonic plates converge near the Pacific Ring of Fire.

Their present form is therefore the result of both internal forces—subduction, volcanism, uplift and faulting—and external forces such as sea-level change, erosion and coral growth:

1. Plate convergence provided the basic setting:

1. Indonesia lies mainly near the interaction of the Indo-Australian Plate and the Eurasian Plate.
2. The Philippines lies between the Philippine Sea Plate and the Eurasian margin.

2. Subduction → magma formation → volcanic eruptions on the sea floor → volcanic mountains rise above sea level → island chains form.
3. Uplift and collision added non-volcanic islands.
4. Faulting fragmented the land into numerous blocks. This explains why islands such as Sulawesi and many Philippine islands have highly irregular shapes.
5. During colder periods of the Earth’s history, sea level was lower. Later, sea-level rise separated higher lands.
6. Erosion and coral growth further increased island diversity.

The subduction mechanism explains the striking linearity of these island chains. Where one plate bends beneath another, a deep ocean trench forms on the sea floor; the descending slab melts at depth, and the rising magma builds a curved line of volcanoes parallel to the trench, called an island arc. Indonesia's arc runs through Sumatra and Java and carries active volcanoes such as Krakatoa and Merapi, while the Philippine arc includes Mayon and Pinatubo. Repeated eruptions over millions of years piled lava and ash on the sea floor until submarine volcanoes emerged as islands, which is why Indonesia alone counts more than seventeen thousand islands and the Philippines more than seven thousand.

External forces then multiplied and reshaped this tectonic inheritance. During the Pleistocene glacial periods, much of the shallow Sunda Shelf between the Indonesian islands and mainland Asia stood exposed as dry land; when the ice melted and the seas rose, only the higher ground remained above water, so continuous land became many separate islands. In the warm tropical seas, coral growth added fringing reefs and low coral islands around the volcanic cores, while waves and rivers eroded coastlines into fresh outlines.

The Indonesian and Philippine archipelagos are therefore the exposed peaks and fragments of a vast tectonically active, largely submerged mountain system.