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Pillow lava forms when eruptions are underwater or when lava flows enter a body of water. The abundance of pillows and deltas made of pillows and hyaloclastite indicates that rivers and lakes were common features during the formation of the Columbia River Flood Basalt Province. Pillows tend can be up to 5 m long and about 0.5 to 1 m thick. They are elongate in the direction of flow and dip 20-30 degrees. Dips decrease higher up in the sequence. Photo by Thor Thordarson.
A delta in the Grande Ronde basalt made of pillow lava and hyaloclastite. Photo by Thor Thordarson.
Hyaloclastite is angular pieces of volcanic glass formed as lava fragments and shatters as it enters the water. The glass is altered or weathered to palagonite. Photo by Thor Thordarson.
Numerous lava deltas formed during the current eruption of Kilauea in Hawaii are very similar to those in the Columbia River Flood Basalt Province.