1. The islands form arcs which are commonly arcuate-shaped with a length of the order of 1000 kms and width of about 200 to 300 kms. They are invariably concave towards the nearest continent and convex towards the deep ocean-basin.

2. All the arcs are marginal to continents. They are situated just at the contact of the continental crust and the oceanic crust.

3. The island arcs are invariably associated with deep-sea trenches which are commonly found along the ocean-facing side of island arcs.

They contain the greatest depths found anywhere in the oceans. These trenches are elongated fur­rows extending sometimes up to 12600 metres below the sea- level. The trenches are quite asymmetric in nature.


4. The trenches are characterized by large negative gravity anom­aly (i.e. the force of gravity over the trench is less than would be predicted for a theoritically homogenous earth).

It indicates that there is a mass deficiency in the section of the earth beneath these anomalies. A strong positive anomaly zone is found towards the concave side.

5. They are the sites of active volcanism, which is characteristi­cally andesitic. There is usually noticed a lateral variation in the composition of the volcanic rocks from tholeiitic basalt on the oceanic side of an island are system to increasingly alkaline and andesitic lavas on the continental side.

The line separating these compositional differences is called the Andesite-line.


6. They are characterized by active seismicity. The belts of greatest volcanic activity of the world run parallel to the Almost all the earthquakes are very deep-seated and come from depths as great as 7C0 km.

Shallow quakes are heavily concentrated along the arcs, also. If the positions of the earthquake-epicenters and their depths are plotted, it is found that the deepest quakes are on the continental side and that more shallow quakes line up in the vicinity of the bottom of trenches.

The line of quake foci commonly dip about 45′ bit more towards the continents and is generally thought to a major fault over which ocean basins and continents moving in relation to one another. They are the sites of active deformation.

7. The island arcs exhibit the penological features as follows: