Dynamic Planet
Seawater is a complex mixture of 96.5 percent water, 2.5 percent salts, and smaller amounts of other substances, including dissolved inorganic and organic materials, particulates, and a few atmospheric gases.
On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, or 599 mM), which is a specific gravity of about 1.025. This means that every kilogram (roughly one litre by volume) of seawater has approximately 35 grams (1.2 oz) of dissolved salts (predominantly sodium (Na+) and chloride (Cl−) ions).
Although the vast majority of seawater has a salinity of between 3.1% and 3.8%, seawater is not uniformly saline throughout the world. Where mixing occurs with fresh water runoff from river mouths or near melting glaciers, seawater can be substantially less saline. The most saline open sea is the Red Sea, where high rates of evaporation, low precipitation and river inflow, and confined circulation result in unusually salty water. The salinity in isolated bodies of water (for example, the Dead Sea) can be considerably greater still.
The density of surface seawater ranges from about 1020 to 1029 kg/m3, depending on the temperature and salinity. Deep in
the ocean, under high pressure, seawater can reach a density of 1050 kg/m3 or higher. Seawater pH is limited to the range 7.5
and 8.4. The speed of sound in seawater is about 1,500 m/s, and varies with water temperature, salinity, and pressure. Seawater is denser than both fresh water and pure water (density 1.0 g/ml at 4 °C (39 °F)) because the dissolved salts increase the mass by a larger proportion than the volume.
Sources of salt
Scientific theories behind the origins of sea salt started with Sir Edmond Halley in 1715, who proposed that salt and other minerals were carried into the sea by rivers after rainfall washed it out of the ground. Halley termed this process "continental weathering".
Halley's theory was partly correct. Sodium also leached out of the ocean floor when the ocean formed. The presence of salt’s other dominant ion, chloride, results from outgassing of chloride (as hydrochloric acid) with other gases from Earth's interior via volcanos
and hydrothermal vents. The sodium and chloride ions subsequently became the most abundant constituents of sea salt.
Ocean Water Structure
Continental margin: Words to Know
Accretionary wedge: A mass of sediment and oceanic rock that is transferred from an oceanic plate to the edge of the less dense plate under which it is subducting.
Active continental margin: A continental margin that has a very narrow, or even nonexistent, continental shelf and a narrow and steep continental slope that ends in a deep trench instead of a continental rise; it is marked by earthquake and volcanic activity.
Asthenosphere: The section of the mantle immediately beneath the lithosphere that is composed of partially melted rock.
Continental drift: The hypothesis proposed by Alfred Wegener that the continents are not stationary, but have moved across the surface of Earth over time.
Continental margin: The submerged outer edge of a continent, composed of the continental shelf and the continental slope. Continental rise: The gently sloping, smooth-surfaced, thick accumulation of sediment at the base of certain continental slopes. Continental shelf: The gently sloping region of the continental margin that extends seaward from the shoreline to the continental shelf break.
Continental shelf break: The outer edge of the continental shelf at which there is a sharp drop-off to the steeper continental slope. Continental slope: The steeply sloping region of the continental margin that extends from the continental shelf break downward to the ocean basin.
Convection current: The circular movement of a gas or liquid between hot and cold areas. Crust: The thin, solid outermost layer of Earth.
Erosion: The gradual wearing away of Earth surfaces through the action of wind and water. Lithosphere: The rigid uppermost section of the mantle combined with the crust.
Mantle: The thick, dense layer of rock that lies beneath Earth's crust.
Passive continental margin: A continental margin that has a broad continental shelf, a gentle continental slope, and a pronounced continental rise; it is marked by a lack of earthquake and volcanic activity.
Plates: Large sections of Earth's lithosphere separated by deep fault zones.
Plate tectonics: The geologic theory that Earth's crust is composed of rigid plates that "float" toward or away from each other, either directly or indirectly, shifting continents, forming mountains and new ocean crust, and stimulating volcanic eruptions.
Seafloor spreading: The process by which new oceanic crust is formed by the upwelling of magma at mid-ocean ridges, resulting in the continuous lateral movement of existing oceanic crust.
Subduction zone: A region where two plates come together and the edge of one plate slides beneath the other.
Submarine canyon: A steep-walled, V-shaped canyon that is cut into the rocks and sediments of the continental slope and, sometimes, the outer continental shelf.
Trench: A long, deep, narrow depression on the ocean basin with relatively steep sides.
Age Pattern of Ocean Floor
Sea floor spreading. In the 1960s, geologist Harry Hess proposed that the sea floor was moving outward from the midoceanic ridges. His theory of sea floor spreading maintained that new basaltic oceanic crust forms at a midoceanic ridge and is slowly pushed away on both sides toward the continents as more new crust is produced. (Measurements indicate that new crust moves away from a ridge at rates from 2 to 10 cm/year.)
Reefs & Atolls
A fringing reef is one of the three main types of coral reefs recognized by most coral reef scientists. It is distinguished from the other two main types (barrier reefs and atolls) in that it has either an entirely shallow backreef zone (lagoon) or none at all. If a fringing reef grows directly from the shoreline (see photo, right) the reef flat extends right to the beach and there is no backreef. In other cases (e.g., most of the Bahamas), fringing reefs may grow hundreds of yards from shore and contain extensive backreef areas with numerous seagrass meadows and patch reefs. This type of coral reef is the most common type of reef found in the Caribbean and Red Sea.
Barrier reef - Sometimes it is hard to tell the difference between fringing reefs and another type of reef called a barrier reef. One of the ways that these two types of reefs are separated is based on the depth of the lagoon in the back reef which is the area near to shore. Barrier reefs have at least some deep portions; fringing reefs do not. Another major difference is that barrier reefs tend to be much farther away from shore than fringing reefs.
An atoll (/ ˈ æ t ɒ l / or / ə ˈ t l / ɒ ),[1] sometimes called a coral atoll, is a ring-shaped coral reef including a coral rim that
encircles a lagoon partially or completely. There may be coral islands/cays on the rim.[2](p60)[3] The coral of the atoll
often sits atop the rim of an extinct seamount or volcano which has eroded or subsided partially beneath the water. The lagoon forms over the volcanic crater or caldera while the higher rim remains above water or at shallow depths that permit the coral to grow and form the reefs. For the atoll to persist, continued erosion or subsidence must be at a rate slow enough to permit reef growth upwards and outwards to replace the lost height.
Atoll Barrier Reef
Reef development
The most important determinant of reef growth is available space as determined by sea level changes. Sea level changes are mostly due to glaciation or plate tectonics. There are six different major ways in which fringing reefs grow and develop.[1]
Reefs can develop vertically as far as the space below the surface allows. The reef generally grows upward from a starting point towards the surface. Once the reef crest reaches sea level the reef may begin growing seaward. Growth begins after flooding, mostly from parts of the reef that have died. Because the reef grows upward, the oldest sediments are found lower in the reef. The reef flat's age indicates when the reef reached sea level. Catch-up reefs have younger surfaces than keep-up reefs of this type.
Reefs can expand seaward from the shore. This requires a fairly constant sea level. If the sea level drops, the reef flat in more seaward areas slopes downward.
Reefs can grow atop muddy sediments which can predate the reef or accrete along with the reef's growth. These reefs also grow seaward from the shore. Older sediments are closest to shore and are not buried. Coral, seagrass and algae filter sediment before it is placed on the reef crest.
Reefs can form in a gradual, sporadic manner, with alternate vertical and horizontal growth episodes. In this type of fringing reef formation there are multiple separate reefs that are found parallel to the shore and the original fringing reef. These reefs become a single, large reef when reef sediments fill in the spaces between the different reefs.
Offshore reefs can form their barrier using storms to move coral and other debris inwards. The recurring storms continually reshape the seaward side of such reefs.
Waves
Ocean waves transport energy over vast distances, although the water itself does not move, except up and down.
If they’re not stopped by anything, waves can travel across entire ocean basins and so the waves at your beach might be from a storm half a world away.
The most familiar ocean waves are caused by the wind. These are wind-driven waves.
Waves caused by underwater disturbances such as earthquakes, landslides, or volcanic eruptions are called tsunamis. These waves are typically tens to hundreds of kilometers long.
The gravitational pull of the sun and moon on the earth causes the tides which are actually tidal waves.
crest-the very top of the wave
trough-the hollow between two crests
wave height-the vertical distance between the top of one wave crest and the bottom of the next trough
wavelength-the horizontal distance between any one point on one wave and the corresponding point on the next wave steepness-the ratio of height to length
amplitude-the maximum vertical displacement of the sea surface from still water level (half the wave height) period-the time it takes for one complete wavelength to pass a stationary point
wave speed-the velocity with which waves travel
deep water waves-waves that are in water that is deeper than half their wavelength shallow water waves-waves that are in water that is shallower than 1/20 their wavelength
One waves motion is completely independent of any other wave motion. When two groups of waves meet, they pass
right through each other. Waves can either add up or cancel each other out as they pass through one another. This property is called superposition. If a crest from one wave happens to line up with the trough of another, they cancel each other out. This is called destructive interference. If two waves line up crest to crest or trough to trough, they add up. This is called constructive interference. This is why waves at the beach are all different sizes.
When a wave hits a hard vertical surface it is reflected. In other words, the wall pushes the water back just as hard as it got pushed, and sets up waves in the other direction. With constructive interference, you end up with bigger and therefore stronger waves.
Waves are also refracted. When you’re at the beach, it appears as if the waves are mostly coming ashore in a straight line. They may come in at an angle, but the side that hits shallow water first gets slowed down by friction and the other side "catches up" bending around until they’re parallel with shore.
Longshore drift is a consequence of these refracting waves. Along with you, a lot of sand is getting moved along, and this is one way that barrier islands migrate up and down the coastline.
Interestingly, because tsunamis have such long wavelengths, they are shallow water waves and so the seafloor steers them around.
The biggest waves in our oceans are the tides. These are caused by the gravitational forces between the earth and the sun and the moon. The moon has the biggest influence because it is close. It essentially pulls up a bulge in the ocean on the side of the earth closest to it. It actually pulls up the land too, but not as much. There is also a bulge on the side opposite the moon.
Currents
Coastal currents are affected by local winds. Surface ocean currents, which occur on the open ocean, are driven by a complex global wind system. To understand the effects of winds on ocean currents, one first needs to understand the Coriolis force and the Ekman spiral.
Coriolis Effect - If the Earth did not rotate and remained stationary, the atmosphere would circulate between the poles (high pressure areas)
and the equator (a low pressure area) in a simple back-and-forth pattern. But because the Earth rotates, circulating air is deflected. Instead of circulating in a straight pattern, the air deflects toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere, resulting in curved paths. This deflection is called the Coriolis effect. It is named after the French mathematician Gaspard Gustave de Coriolis (1792-1843), who studied the transfer of energy in rotating systems like waterwheels. (Ross, 1995).
Longshore Currents - The speed at which waves approach the shore depends on sea floor and shoreline features and the depth of the
water. As a wave moves toward the beach, different segments of the wave encounter the beach before others, which slows these segments down. As a result, the wave tends to bend and conform to the general shape of the coastline. Also, waves do not typically reach the beach perfectly parallel to the shoreline. Rather, they arrive at a slight angle, called the “angle of wave approach.” When a wave reaches a beach or coastline, it releases a burst of energy that generates a current, which runs parallel to the shoreline. This type of current is called a “longshore current.”
Upwelling - Winds blowing across the ocean surface often push water away from an area. When this occurs, water rises up from
beneath the surface to replace the diverging surface water. This process is known as “upwelling.”
Rip Currents - As longshore currents move on and off the beach, “rip currents” may form around low spots or breaks in sandbars, and also
