Earth’s Surface I

Let's start with the solid Earth. The Earth is layered. You may have previously learned that the outer layer is the crust and that it is underlain by the mantle. Below the mantle at the center of the planet is the core. The lithosphere, which is the rigid outermost shell of our planet is composed of all of the crust and the upper mantle.

• Earth is layered

• Lithosphere

• Outermost rigid shell

• All of crust and upper mantle

• Broken into plates

• Move around at speeds of 0 -10 cm/yr

Earth’s Surface II

The lithosphere is broken into tectonic plates that move across the surface of the Earth at very slow speeds. Where the plates meet, their relative motion determines the type of boundary:

• convergent (moving toward one another),

• divergent (moving away from one another), or

• transform (moving alongside one another).

Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. Remember that the relative motion of the plates is very, very slow, typically ranging from 0 to 10 cm/yr.

Earth’s Surface III

Heat from deep inside the planet convects upward and fuels the movement of the plates. New crust is formed along divergent boundaries which are also called spreading centers or mid-ocean ridges. Older lithosphere is destroyed at convergent boundaries in a process called subduction. When two plates collide, the denser of the two sinks below the other. A process called slab pull pulls the plate into the mantle where it is heated and destroyed. Oceanic trenches are created by subduction.

Earth’s Surface IV

Though this process is slow, over geologic time it dramatically changes the shape and size of the ocean basins. The modern ocean is divided by the continents and currents into 5 major basins. There is a short lesson following this one that lists and characterizes the basins.

Ocean Provinces

Three major provinces:

• Continental margin = shallow area close to the continents

• Deep ocean basins = deep areas far from land

• Mid-ocean ridge = underwater mountain ranges formed by spreading centers near the middle of basins

Imagine you had a magic ability to survey the seafloor at will. If you were to leave the beach in NC, you would first find yourself on the continental shelf, a relatively shallow and flat environment. After quite some distance you would find yourself at the edge, which we call the shelf break. Here the seafloor would markedly begin to slope downward- earning the name of continental slope. At the bottom of the slope would be a feature created by eroded sediment falling down the slope called the continental rise. This marks the transition from the continental margin to the deep ocean basin which is also known as the abyssal plain. If you were up to a long trek, you could continue across the deep basin to the mid-ocean ridge. Past that you would find a mirror image: more abyssal plain that ends at the continental margin off Africa.

Oceanographers that we divide these areas of the ocean floor into three major provinces:

• The continental margin which is the shallow area close to the continents;

• The deep ocean basins which are the deep areas far from land, and

• The mid-ocean ridge which is home to the spreading centers near the middle of the ocean basins.

You can see your path as a red line across the map shown here. In the top panel is the ocean profile produced by making a cross section of the seafloor along that red line. The boundaries of the 3 major ocean provinces are labelled. Next, we’ll spend a few minutes exploring each of the provinces.

Continental Margin

Continental margins are the edges of the continents that transition into the deep-water environments of the ocean basins. In general, continental margins have several distinct physiographic regions, including continental shelf, the shelf break, the continental slope, and the continental rise.

The continental shelf includes the seafloor that extends from the shoreline seaward to the shelf break. Continental shelves seem relatively flat although they typically gently slope seaward. The shelf break occurs where the gradient of the slope of the shelf steepens dramatically. Most shelves are relatively shallow with an average depths at the shelf break between 120 to 130 m. Continental shelves often accumulate sediment from continental river systems. Some shelves have accumulations of sediment 10 to 15 km thick. Continental shelves are underlain with granitic continental crust.

Beyond the shelf break is the continental slope. The slope has a steep gradient coincident with a transition from granitic continental crust to basaltic oceanic crust. Continental slopes extend from the shelf break at approximately 120 m to as deep as 3,000 m. Slopes are steepest in locations adjacent to geologically young plate tectonic margins with narrow continental shelves.

The continental rise is the most distal part of the continental margin. It represents the transition from the slope to the deeper, flat province of the deep ocean basin. The continental rise is a wedge of sediment that can be several kilometers thick and several hundred kilometers wide developed because of the seaward transport of sediment from the more shallow water continental shelves and slopes.

Active and Passive Margins

Continental margins are often classified as either active or passive depending on the tectonic conditions along the margin.

Passive margins are not located near any plate boundaries, so they experience no significant tectonic activity. The lack of activity is conducive to the accumulation of thick layers of sediment on the margin. The east coast of the United States is an example of a passive boundary.

Active margins are located near plate boundaries so they are tectonically active regions. Active margins along convergent boundaries usually have a narrow shelf and an offshore trench associated with the subduction zone. The coastline is punctuated with active volcanoes. The western coast of South America is an example of a convergent active margin.

Active margins along transform boundaries are not as common, but when they do occur the coastline is often marked by linear islands and deep basins close to shore. Coastal California along the San Andreas Fault is an example of a transform continental margin.

Active Margins

This map shows the locations of the major ocean trenches. You can see the majority are in the Pacific Basin.

Deep Ocean Basin

The deep ocean basin extends from base of continental rise to the mid-ocean ridge. It is some of the deepest, flattest parts of Earth, and as a result, it is also called the abyssal plain. It is underlain by oceanic crust. As new crust spreads slowly away from the mid-ocean ridge, very fine particles of sediment settle out of the water column and accumulate on the seafloor. Over time, think layers pile up masking the relief of the basaltic crust. This sediment drape is especially well developed in the Atlantic and Indian Ocean basins.

Abyssal Hills, Seamounts and Tablemounts

Some regions of the abyssal plain lack a cover of sediments (wet mud), exposing small hills (see sonar image on the top). These hills reach a few 100's of meters high and have formed by a combination of volcanism and earthquake faulting.

In contrast, seamounts and tablemounts are taller, reaching at least 1 km (0.6 mile) above sea floor. Both are formed by volcanic activity. Table mounts, also called guyots (sonar image left), form from seamounts (sonar image right) that reach the sea surface and become islands. Over time, wind and waves erode the tops until they are flat. Once volcanic activity ceases, cooling and subsidence (isostacy) then drowns the guyot.

• • Abyssal hill = 100s of meters tall

• Seamounts > 1,000m tall

• Table mounts (AKA guyots)

• Begins as seamounts

• Tops eroded flat at sea surface

• Later drowned

Mid-ocean Ridge

The mid-ocean ridge wraps around the globe for more than 65,000 km like the seam of a baseball, with an average depth to the ridge crest of 2500 m. It is the longest mountain range on the planet. In this bathymetric map of a segment of the mid-Atlantic ridge just north of the equator, you can see the highest peaks colored in red (shallower than 2 km) and the deepest parts of the seafloor in dark blue (deeper than 4 km).

As we have discussed already, mid-ocean ridges are the spreading centers that build new oceanic, basaltic crust. Near the axes of ridges where magma chambers are nearest the seafloor, seawater seeps in, is heated, and escapes. Locations where the hot seawater escapes are called hydrothermal vents.

As the hot water seeps through the basalt, it leaches chemicals (just like hot water flowing through coffee grinds leaches the yummy chemicals many of us rely on to kick start our mornings). Which minerals are dissolved by the water is determined by the temperature of the water.

White Smokers

White smokers are vents that release water with a temperature between 30 – 350°C. The water exiting these vents are rich in barium, calcium and silicon. The minerals precipitate out of solution as the vent solution meets the cooler seawater creating a structure called a chimney. These minerals have a white color, which gives the appearance of white "smoke” leaving a chimney.

• Release water at temperatures between 30 – 350°C

• Leached minerals rich in barium, calcium and silicon (look like white smoke)

• Minerals precipitate out of solution creating chimney

Black Smokers

Black smokers are vents that release water with a temperature higher than 350°C. They are the most common type of vent along the mid-ocean ridges. They are named for the black-colored water that comes out of them, like the picture on the left. The black "smoke" is caused the presence of iron and sulfur, which combine to become iron monosulfide, which has a black color. When the iron monosulfide solidifies, it created the black chimneys. Solutions exiting these vents are acidic (pH ≈ 3.5) and contain up to 300 ppm hydrogen sulfide (H2S).

• Release water at temperatures hotter than 350°C

• Leached minerals rich in iron and sulfur (look like black smoke)

• Vent solutions

• Acidic (pH ≈ 3.5) • Contain up to 300 ppm H2S

Marine Habitats

Oceanographers define the provinces by their geology. Marine biologists have a parallel nomenclature to organize seafloor (benthic) and water column habitats into zones.

The two most generalized are the neritic and pelagic zones. The neritic zone stretches from the intertidal zone to the shelf break. A few biologists may qualify it as the zone over the continental shelf where light penetrates to the seabed. We will use the first, more popular definition. The pelagic zone is the water column of the open ocean beyond the shelf break.

The neritic zone is subdivided into the water column and two benthic zones: The littoral and the sublittoral. The littoral zone is also called the intertidal zone, and it represents the area between high and low tide. The sublittoral zone is the subtidal zone.

The water column and seabed of the pelagic zone are subdivided into multiple zones. In general, I would advise you not focus on memorizing the depths that delineate between each zone in the pelagic, but instead try to list them and order them from shallow to deep. If you are dead set on memorizing details, this may be one of those times when notecards are useful. As we work through the rest of the semester, you will practice using many of these term. As we do, they will become familiar and you will feel more comfortable.

Epipelagic zone: Water column from the surface to around 200 m depth

This zone is characterized as having enough sunlight for photosynthesis. It is sometimes called the photic zone. Nearly all primary production in the ocean occurs here. Examples of organisms living in this zone are plankton, floating seaweed, jellyfish, tuna, many sharks and dolphins.

Bathyal zone: Seafloor at depths between 200m and 4,000m

Mesopelagic zone: Water column from 200 m to 1,000 m

The most abundant organisms thriving into the mesopelagic zone are heterotrophic bacteria. Many organisms that live in this zone are bioluminescent. Some creatures living in the mesopelagic zone migrate to the epipelagic zone at night to feed.

Bathypelagic zone: Water column from 1,000 m to 4,000 m

No sunlight penetrates to this zone. Most animals living here are predators or detritivores (consumers of dead organic matter).

Abyssal zone: Seafloor at depths between 4,000m and 6,000m

Abyssopelagic zone: Water column from 4,000 m down to 6,000m

Many of the species living at these depths are transparent and eyeless because of the total lack of light in this zone.

Hadal zone: Water column and seabed below 6,000m

The name is derived from the realm of Hades, the Greek underworld. The hadal zone is generally found at the bottom of the trenches.

Watch this video footage from the University of Aberdeen’s Hadal-Lander deployed in the Mariana Trench from the Schmidt Ocean Institute’s Research Vessel Falkor. This video shows an aggregation of snailfish (Liparidae) at 7485m.

https://youtu.be/EuaAMHuAfuA

• Organize marine habitats into zones based on environmental conditions

• Nomenclature can be messy

• Seafloor habitats = benthic

• Neritic zone is the water column of the coastal ocean (intertidal zone to shelf break)

• Pelagic zone is the water column of the open ocean beyond the shelf break

Ocean Circulation

The water in the ocean is in constant motion. Ocean currents are flowing masses of water set in motion either by the wind or density differences. Wind-driven currents, or surface currents, move water horizontally in the uppermost layer of the ocean. Vertical and deep-water circulation are by differences in density.

Surface currents affect about 10% of the ocean and are generated by wind. The transfer of energy between wind and water is relatively inefficient (about 2%), so currents never flow as fast as the winds that generate them.

Surface currents generally follow the major wind belts (see figure below), but their flow is interrupted by the continents. Their path is also influenced by gravity and the Coriolis effect.

As you looked at the figure above, you may have noticed that the major ocean surface currents form large circular loops centered around 30° latitude. These are the sub-tropical gyres and they are each bounded by 4 major currents:

• Equatorial current

• Western Boundary currents

• Northern or Southern Boundary currents

• Eastern Boundary currents.

Western Boundary

• fast

• narrow

• deep

• large transport volume

• move warm water from equator toward poles

• gulf stream, kuroshio current, east Australian current, brazil current, agulhas current

Eastern Boundary

• slow

• wife

• shallow

• small transport volume

• move cold water from poles toward equator

• canary current, benguela current, California current, humboldt current, west austrlian current

Ocean Circulation II

Boundary currents can create biological boundaries between the center of the gyres and the surrounding sea. Western and eastern boundary currents are notable in their contrast. The table on the slide compares the two.

The currents of the subtropical gyres have a large impact on climate, impacting temperatures and humidity levels on land, especially along coasts. The major wind belts are responsible for 2/3 of the heat transferred from the tropics to the poles. Ocean currents are responsible for the remaining 1/3.

Upwelling and Downwelling

We generally think of surface currents moving laterally, but there are places where and times when water moves vertically in the surface ocean.

Upwelling is when winds and the Coriolis effect drive the vertical movement of cold, nutrient-rich water to surface. Upwelling zones are areas of high biological productivity and are usually rich in fisheries resources.

Downwelling is when winds and the Coriolis effect cause the vertical movement of surface water to depth. Downwelling can occur where currents converge, at the centers of the subtropical gyres and along coastlines.

Deep-water Circulation

Deep ocean circulation driven by density differences and is called thermohaline circulation because the density of seawater is primarily controlled by temperature (thermo) and salinity (haline). Ninety percent of all ocean water occurs below the surface layer. Deep ocean water is uniformly cold - on average around 4°C. Deep water is cold because it forms at high latitude and circulates at depths where the sun's energy cannot reach. The deepest waters in the ocean sink in the Weddell Sea (near Antarctica) and in areas near Greenland in the North Atlantic.

In both areas the atmosphere is cold so heat leaves the surface ocean. Sea ice often forms in these regions. As the hydrogen bonds in the ice lock into the crystal lattice, salts in the water are excluded so remaining unfrozen sea water becomes saltier. This cold brine is extremely dense and it sinks to the seafloor and spreads throughout the ocean basins.

Deep water masses are layered by density with the densest at the bottom. Water masses are usually named for the region where they originate, but occassionally they are named for their position in the stratified layers (e.g. Intermediate water in the figure below).

• Driven by density differences

  • Temperature

  • Salinity

• Cold salty water sinks near poles

• Deep ocean is uniformly cold (4°C)

• Water masses named for area of origination or stratified position

Global Conveyor Belt

As the deep water masses spread around the globe, some are returned to the surface through mixing often caused by internal waves. The deep and surface ocean are connected in a global pattern called the global conveyor belt.

Waves

Waves are different from currents. Currents are masses of moving water. Waves represent the propagation of energy through matter. Even though mass motion is associated with wave propagation, mass is not transported with the wave. For example, if you were to knock on the end of a table, the

sound waves would travel to the opposite end but the table would not move noticeably. In ocean waves, the water mass moves in an orbital motion as the wave passes but returns to the same relative position in which is started.

Waves are characterized by their wavelength, height, frequency, period, and speed. You can use the chart on the slide to review wave characteristics.

Causes and Restoring Forces

Waves in the ocean are caused by what oceanographers call a perturbing force. A restoring force returns the water to a still state. In most cases the restoring force is gravity. You can see examples of perturbing forces of waves in the figure.

Approaching Shore

Waves can have big impacts on organisms living in shallow water and the littoral zone (intertidal). Wave energy can disrupt the environment and has driven fascinating adaptations in organisms that inhabit the shoreline. Wave characteristics change as waves approach a shoreline:

• their height increases,

• their wavelength decreases and

• they slow down.

As height increases at the same time wavelength decreases, a wave become steep. Waves begin to break when the ratio of wave height to wavelength is 1 to 7 (H/L = 1/7).

Tides

Tides are the periodic rise and fall of sea level due to Earth’s rotation and the gravitational effects of the moon and sun. Tidal range is difference in height between high and low tide, which can be quite large in certain areas of the coastal ocean.

We'll briefly review what is called equilibrium tidal theory. It is a highly idealized explanation of the tides based on Newtonian physics. Tides are more complicated than this. You can always take physical oceanography if you would like to learn more about the dynamic theory of tides.

Forces

Newton’s law of universal gravitation states that the gravitational attraction between two bodies is directly proportional to their masses, and inversely proportional to the square of the distance between the bodies. Therefore, the greater the mass of the objects and the closer they are to each other, the greater the gravitational attraction between them.

Tidal forces are based on the gravitational attractive force. With regard to tidal forces on the Earth, the distance between two objects usually is more critical than their masses. Our sun is 27 million times larger than our moon. Based on its mass, the sun's gravitational attraction to the Earth is more than 177 times greater than that of the moon to the Earth. If tidal forces were based solely on comparative masses, the sun should have a tide-generating force that is 27 million times greater than that of the moon. However, the sun is 390 times further from the Earth than is the moon. Thus, its tide-generating force is reduced by 3903, or about 59 million times less than the moon. Because of these conditions, the sun’s tide-generating force is about half that of the moon.

Tides II

The gravitational attractive force creates bulges of water under which the Earth rotates. One bulge faces the Moon (or Sun) and an equal but opposite bulge faces directly away from the Moon (or Sun).

The geometry of the Earth-Moon-Sun system impacts tidal range. Spring tides occur at full and new moons and result in large tidal ranges because the gravitational impacts of the Sun and Moon align. Neap tides occur at the quarter moons and result in smaller tidal ranges because the gravitational effects of the Sun and Moon are offset at an angle.

Types of Tides

Newtonian physics would predict that every point in the ocean would experience two high tides and two low tides each day. Some places do experience tides this way. Tides with that periodicity are called semidiurnal or mixed. Tides interact with the seafloor and the continents so the periodicity is different in other locations. Diurnal tides result in one high and one low tide per day. The map below summarizes the types of tides the coasts of the continents experience.

• Diurnal tides = 1 high + 1 low per day

• Semidiurnal = 2 highs + 2 lows of equal height each day

• Mixed = 2 highs + 2 lows of unequal height each day