The constant rise of mantle material (magma) along an axis of a mid - oceanic ridge causes divergence of lithosphere plates. Collision of oceanic and continental plates. The subducting of an oceanic plate under a continental plate creates a deep trench adjacent to the continental coast and a great mountain range on land. Down-drag of an oceanic plate in the mantle results in the formation of an volcanic island-arc system in the ocean and deep trench seaward of the island-arc.
1 Core 2 Earth's crust 3 Mid-ocean ridge 4 Convection currents 5 Island-arc 6 Trench The arrows show the directions of movement of the lithosphere plates moved by convection currents in the mantle.
Underwater Volcanism
One of the greatest natural phenomena is underwater volcanism, which at great depths is almost imperceptible, and in shallow regions has an explosive character. In the Philippine Sea, there are several cases on record where vessels have been lost as a result of submarine volcanic eruptions. However, the greatest danger from underwater volcanism is seen in the products of the eruption: pyroclastic ash flows and steam. Upon reaching the Ocean surface, they sharply worsen visibility and cause mats to form on the Ocean surface, which impedes surface transportation.
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