This erosion is both gradual and catastrophic.
Hawaiian lavas are very permeable.
Many are vesicular, and lava tubes, clinker layers, and flow boundaries all provide easy pathways for percolating water. For this reason, even in many of the wettest areas of Mauna Loa and Kilauea, erosion is minimal. During the post-shield alkalic stage, however, the greater explosivity of the eruptions deposits many ash and cinder layers. These pyroclastic layers are much less permeable, and they allow streams to form readily. For example, corals on the shelf off Mauna Kea are about 500,000 years old.