jetty

A structure built along the bank of a stream channel or tidal outlet to direct the flow of a stream or tide and keep the sediment moving so that it cannot build up and fill the channel. Jetties are typically built in parallel pairs along both banks of the channel. Jetties that are built perpendicular to a coast tend to interrupt longshore drift and thus widen beaches.

joint

A fracture dividing a rock into two sections that have not visibly moved relative to each other. See also fault.

juvenile water

The steam that accompanies volcanic eruptions.