**Continental Margins and Ocean Floors**Continental margins occupy the perimeters of ocean basins where they meet the continents. Many continental margins exhibit three distinct segments: a shelf, slope, and rise. Shelves (most landward segments) are flooded extensions of the continents. Slopes (middle segment) are steep as the name implies, and represent the approximate boundary between oceanic and continental crust. The rise (most seaward segment) is a gradual descent to the deep basin.Ocean basin floors consist of flat plains (called abyssal plains) dotted with seamounts and also include deep trenches. Seamounts are underwater volcanoes. Most seamounts are extinct. Often they form at mid-ocean ridges (discussed below). Embedded in moving plates of lithosphere, seamounts migrate away from mid-ocean ridges and gradually subside. Guyots are seamounts with flat tops, formed by waves eroding a volcano that once rose above sea level.Ocean trenches are long narrow features of the ocean floor, which form where moving plates plunge into the mantle below. This setting is called a subduction zone. Trenches are the deepest parts of oceans; the deepest trenches are approximately 11 km beneath sea level.Extensive faulting and volcanic structures characterize mid-ocean ridges (sometimes called spreading centers). Plates on either side of a mid-ocean ridge spread apart from one another. Mid-ocean ridges average about 1,300 km wide, with a 20-80 km wide central depression called the rift valley, a site of active volcanism. In some places, the volcanoes extend above sea level to form islands such as Iceland.**Assignment**1. **Sketch and label areas of erosion and deposition** resulting from the groin (top), jetty (middle), and breakwater (bottom) in these three maps: - **Groin:** Shows a barrier perpendicular to the shore. The longshore current moves along the shore, depositing material on one side of the groin (facing the current) and causing erosion on the other. - **Jetty:** Consists of two parallel structures extending into the sea, typically at a river mouth, affecting sand movement along the coast similar to groins. - **Breakwater:** Parallel to the shore, this structure reduces wave energy, encouraging deposition behind it while erosion may occur at its ends.2. **Rising seas inundate

Jetties are rock walls that are designed to protect the entrance of a harbor from sediment deposition and storm waves. They are usually built in pairs and stand above the surface of the water. Figure a) shows areas of erosion and deposition of sand near Jetties. Groins are short walls that are built perpendicular to shore to trap moving sand and widen a beach. They are built to protect beaches that are losing sand from longshore drifting. Figure b) shows areas of erosion and deposition of sand near Groins. The breakwater is an offshore structure built to absorb the force produced by large, breaking waves. It provides quiet water conditions near shore. Figure c) shows areas of erosion and deposition of sand near breakwater.