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The Birth of a Volcanic Island Arc

As these volcanic islands emerged above the ocean surface, erosion began destroying them. Result: sedimentary rock (like conglomerate) interlayered with tuffs and lavas.

Conglomerate

At the same time that the volcanoes were eroded, they were actively spewing ash and lava. The conglomerate and other rocks that are the evidence of erosion are interlayered with volcanic tuffs and lava, indicating eruptions.