Continental margins may be geologically active or
passive. Active margins occur along tectonic plate
boundaries where earthquakes and/or volcanoes are
common. Passive margins are not associated with plate
boundaries, experiencing little volcanism and relatively
fewer earthquakes. The Atlantic Ocean’s continental
shelf is a passive margin so the Ocean Exploration
Hudson Canyon, Deep East and Islands in the Stream
expeditions focused on features of passive margins.
The shelf is generally a broad, gently slopping platform
that extends from the shoreline to the continental slope.
It has thick accumulations of both coarse and fine-grained
sediments. Since fine particles remain suspended in the
water column longer than larger or denser particles, finer
and lighter particles are carried farther, often to the edge
of the shelf, before they settle. Consequently, slope and
deep-sea sediments tend to be finer grained.
The continental slope is an abrupt drop. Sediments on
the steep slope are largely soft mud. The slope flattens
at the bottom where sediment slides pile up, forming
the continental rise. Its thick sediment accumulations
have fallen from the continental shelf—down the slope,
sometimes quite abruptly.
Submarine canyons cut into the shelf. They usually have
v-shaped profiles, steep walls, rock outcrops, flat floors,
strong currents, and deep-sea sediments fans at their
base. Most are on the upper, steeper part of the slope.
They run perpendicular to the shelf, across the continen-
tal shelf and slope. Generally submarine canyons are
associated with major rivers.
Turbidity currents form deep-sea sediment fans at the
base of the slope. Fine sediments suspended by currents
cause water to become murky or turbid. Turbid water is
more dense and sinks. Turbidity currents—down-slope
movements of sediment-laden water—continuously