How much of Earth's surface is covered by oceans?
Approximately 71% of the planet is covered by oceans. There are three major oceans (Indian, Pacific, Atlantic) that are connected along their southern margins by the Southern Ocean circling Antarctica.

How does the depth of the oceans vary?
The average depth of the ocean floor is nearly 4 km and a maximum depth of 11 km has been recorded along the Mariana Trench in the western Pacific Ocean.  Four principal depth zones can be identified ranging from the shallow shelf along the continental margins (~100s meters), to the near horizontal floor of the abyssal plain (4-5 km), rising to the crest of the oceanic ridge (~3 km), and descending to the narrow depths of the oceanic trenches (7-11 km).

What factors control variations in the salinity of the oceans?
Seawater contains dissolved salts. The concentration of salt in seawater is salinity. Salinity is measured in parts per thousand (ppt; 1 ppt = 0.1%, 10 ppt = 1%) of salt in water. Salinity varies depending on temperature and the mixing action of ocean currents. Salinity is higher at low latitudes because high temperatures at these locations promote evaporation which removes water but leaves the salt it contains behind. However, the mixing action of ocean currents ensures a consistent salinity range of 33 to 37 parts per thousand for much of the open ocean.

Where are salinity values highest and lowest?
Salinity values are most extreme in restricted ocean basins where the effects of evaporation or stream inflow are exaggerated. Salinity values of over 40 ppt occur in the narrow tropical Red Sea basin between north Africa and the Arabian peninsula. Salinity is lower at high latitudes because of the lack of evaporation, high precipitation, and the influx of freshwater from melting ice sheets. Salinity values of less than 10 ppt are recorded from the Baltic Sea in northern Europe.

How does salinity change with depth?
Salinity increases with depth in the restricted northern ocean waters (Arctic Ocean) but decreases slightly with depth in the tropical open ocean. Salinity is much more consistent at depth (2 km) below the halocline.

What factors control the temperature of the oceans?
Solar radiation is distributed over a wider area and must penetrate a greater thickness of atmosphere at the poles, reducing the amount of solar energy reaching Earth's surface. Consequently, ocean temperatures are greater near the equator. The highest ocean temperatures (~27^oC) are present along the equator and temperatures decrease symmetrically to the north and south approaching 0^oC at high latitudes. 

How does temperature vary with depth?
Temperature decreases significantly with depth. The effects insolation and the surface mixing of currents diminish with depth. Temperature declines steadily to a depth of approximately 1,000 meters. Deeper waters have a uniform temperature of 1 to 2^oC.

What controls the direction of ocean currents?
Winds generated by atmospheric circulation patterns represent the principal control on ocean currents but the distribution of continents and the Coriolis effect also affect currents. Circulation patterns known as gyres control currents in the open oceans. Currents form a clockwise pattern in gyres of the Northern Hemisphere and a counterclockwise pattern south of the equator.

Which currents are important in global climate?
Although all currents contribute to global climate patterns, the western boundary currents such as the Gulf Stream and Brazil currents have an especially significant role as they transport warm tropical waters to higher latitudes.

What is the Coriolis effect?
The Coriolis effect represents the deflection of currents to the right of their course in the Northern Hemisphere and to the left of their course in the Southern Hemisphere. The Coriolis effect results from the contrast in Earth's rotation velocity with latitude.

What is thermohaline circulation?
Thermohaline circulation occurs in deeper ocean waters and is driven by density contrasts related to differences in water temperature and salinity. Thermohaline circulation drives the global conveyer belt that causes surface waters to sink in the northern Atlantic Ocean and sends cold, deep currents through the world's oceans before upwelling in the northern Pacific and Indian Oceans.

What factors influence the development of coastal landforms?
The coastline is a dynamic environment that advances or retreats depending upon the balance between the supply of sediment and the material removed by wave erosion. Seasonal variations in stream flow and storm activity affect the volume of sediment supplied to the coast and the rate of erosion. Climate cycles that result in increasing or decreasing sea levels will have long-term effects measured in decades or centuries. Finally, tectonic cycles measured in hundreds or thousands of years may continually revitalize rugged coastlines by periodic uplifts.

How does water move in waves?
The waveform is a shape that moves across the open ocean but the water particles don't move with the wave but instead trace out a circular path while remaining essentially in place.

What happens when waves approach the coast?
Material is eroded and redeposited by turbulent flow that occurs in the surf zone as waves break along the shoreline. Wave refraction results in wave action being concentrated on headlands. Deposition occurs in the calmer waters of sheltered bays.

Where does coastal erosion occur?
Coastal erosion occurs where erosion by wave action is not balanced by local deposition of the eroded material and the supply of sediment from streams. Erosion exceeds deposition, resulting in a loss of shoreline, along most of the U.S. coastline.

What is a longshore current?
A longshore current is generated when waves strike the coast at an angle. The current forms in the surf zone and transports sediment laterally along the shoreline. Longshore currents can result in the formation of characteristic depositional landforms parallel to the coastline.

What is the sediment budget?
The sediment budget is the balance between material added to the shore by deposition and the material removed by erosion. The construction of dams will reduce the volume of sediment reaching the coast. Drought conditions may also reduce streamflow and thus diminish sediment transported to the shore by streams. Coastal development may result in the construction of structures designed to reduce erosion (e.g., breakwaters, seawalls) or to control the local depositional patterns to prevent infilling of navigation channels (jetties).

Why are shoreline protection structures regarded as a mixed blessing?
Seawalls, groins, and breakwaters act to prevent erosion and/or encourage deposition. However, structures built to protect coastlines may prevent erosion of part of the shoreline but can result in accelerated erosion elsewhere. Artificial beach nourishment occurs when sand is dredged and pumped onto the beach from offshore but has a limited life span unless the processes that caused the original erosion are stopped.