Though ocean floors are thousands of miles below the water surface, they also experience many volcanic activities.

In shallow waters, eruptions tend to involve cold water coming into contact with extremely hot lava from beneath that ocean floor. The steam escapes high above the water surface. If the eruptions occur deeper in the ocean, the weight and pressure of the water above it suppresses its expulsion and keeps it mild. This means the ocean floor is not necessarily affected in any way.

Sometimes, lava that erupts into shallow water cools off very quickly and breaks down into débris or sand. It is believed that Hawaii’s black sands beaches are a result of lava interacting with seawater.
Sometimes, lava settles on the water in the form of débris, which is carried farther away by ocean currents.