When Lava Becomes Land

A Shoreline Under Construction

1A coastline may look like a firm boundary between land and sea, but volcanic activity can redraw it. Melted rock below Earth's surface is called magma; once it reaches the surface, it is called lava. When lava cools and solidifies, it becomes rock. Under suitable conditions, repeated additions of this rock extend a coast or build an island.

2Many volcanoes begin their construction far below the ocean's surface. Lava escaping onto the seafloor can cool into rounded masses called pillow lava. As eruptions add material, an underwater volcanic mountain may grow taller. Only when its summit rises above sea level does part of that mountain become an island.

3Volcanoes can also add land along an existing island's shore. A lava flow may travel downhill and enter the ocean, where its exposed surfaces cool rapidly. Continued flows can add rock beyond the former shoreline, producing a coastal extension often called a lava delta. The visible surface may conceal loose volcanic fragments and layers built over a steep underwater slope.

4That fresh extension is not necessarily secure ground. A lava delta may rest on unstable material that shifts or gives way. Part of the new coastline can collapse into the ocean, carrying solid-looking ground with it. The appearance of a dark, hardened surface therefore does not establish that the whole structure beneath it is stable.

5The ocean also works against the buildup. Waves can break volcanic rock apart and carry fragments away. Loose ash and other small volcanic particles are generally more easily removed than coherent, solid rock. Whether a new patch of land persists depends partly on how the addition of material compares with its removal.

6A photograph taken just after an eruption records one stage of this contest. A later survey may show a different shoreline as erosion, collapse, or further lava flows change its position. Comparing observations made at different times can reveal whether a coastal extension is growing, shrinking, or holding its shape. A single outline on a map cannot show that history of change.

7Volcanic land building includes changes that occur on very different schedules. An eruption can alter a shoreline quickly, while an underwater mountain may require many eruptions before it reaches the air. Some additions remain, and others disappear beneath the waves. To understand a newly drawn coastline, readers must consider both the rock being supplied and the processes that can take it away.