margins consist of broad coastal plains bordered offshore by wide continental shelves and gentle continental slopes. These relatively stable coastlines are characterized by depositional landforms such as broad sandy beaches and offshore barrier bars and at low latitudes by broad coral reefs. The southeast coast of North America and the northeast coast of Australia are typical passive-margin coastlines with low-to-moderate topographic relief and subdued depositional landforms. Much of the former is bordered offshore by barrier bars (Oaks and Du Bar, 1974), and most of the latter by extensive coral reefs (Hopley, 1983).
Long-term tectonic stability along most passive-margin coastlines is expressed stratigraphically by undeformed continental and marine sediments that underlie flat coastal plains and continental shelves and geomorphically by broad accretionary strandline terraces that consist of subdued beach ridges separated by abandoned tidal flats (Oaks and Du Bar, 1974). However, rapid sediment accumulation, such as at the mouth of a large river, may isostatically depress an otherwise stable passive-margin coastline (Figure 6.4C) (Fisk and McFarlan, 1955; Hicks and Crosby, 1974). Also, occasional large earthquakes, such as the 1886 Charleston, South Carolina, seismic event on the Atlantic coast of North America (Hays and Gori, 1983), indicate that passive continental margins are not completely aseismic, even though there is little stratigraphic or geomorphic evidence in these areas of recent, near-surface crustal deformation.
In contrast to the subdued coastlines along passive plate boundaries, most coastlines along or near active continental margins consist of coastal hills or mountains bordered offshore by narrow continental shelves and