Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Causes of Coastal Variability

Causes of Coastal Variability 

Coastal variability depends upon following factors.

Plate Tectonic Setting 

The tectonic setting of a coast plays a role in determining whether the coast has steep-sided mountain slopes or a broad plain that borders the sea. Along an active margin, compression squeezes the crust and pushes it up, creating mountains like the Andes along the western coast of South America. Along a passive margin, the cooling and sinking of the lithosphere may create a broad coastal plain, a flatland that merges with the continental shelf, as exists along the Gulf Coast and south-eastern Atlantic coast of the United States. 
Not all passive margins have coastal plains. The coastal areas of some passive margins were uplifted during the rifting event that preceded establishment of the passive margin. For example, highlands formed during rifting border the Red Sea and portions of the Brazilian and southern African coasts. Highlands also rise along the east coast of Australia.

Relative Sea-Level Changes 

Because of sea-level drop during the ice age, there was more dry land.
Sea level, relative to the land surface, changes during geologic time. Some changes develop due to vertical movement of the land. These may reflect plate-tectonic processes or the addition or removal of a load (such as a glacier) on the crust. Local changes in sea level may reflect human activity when people pump out groundwater or oil, for example, the pores between grains in the sediment beneath the ground collapse, and the land surface sinks. Some relative sea-level changes, however, are due to a global rise or fall of the ocean surface. Such eustatic sea-level changes may reflect changes in the volume of mid-ocean ridges. An increase in the number or width of ridges, for example, displaces water and causes sea level to rise. Eustatic sea-level changes may also reflect changes in the volume of glaciers, for glaciers store water on land (figure above). As glaciers grow, sea level falls, and as glaciers shrink, sea level rises. 

Features of emergent coastlines (relative sea level is falling) and submergent coastlines (relative sea level is rising).
Geologists refer to coasts where the land is rising or rose relative to sea level as emergent coasts. At emergent coasts, steep slopes typically border the shore. A series of step-like terraces form along some emergent coasts (figure above a). These terraces reflect episodic changes in relative sea level and/or ground uplift. Those coasts at which the land sinks relative to sea level become submergent coasts (figure above b). At submergent coasts, landforms include estuaries and fjords that  developed when the rising sea flooded coastal valleys. 

Sediment Supply and Climate 

The quantity and character of sediment supplied to a shore affects its character. That is, coastlines where the sea washes sediment away faster than it can be supplied (erosional coasts) recede landward and may become rocky, whereas coastlines that receive more sediment than erodes away (accretionary coasts) grow seaward and develop broad beaches. 
Climate also affects the character of a coast. Shores that enjoy generally calm weather erode less rapidly than those constantly subjected to ravaging storms. A sediment supply large enough to generate an accretionary coast in a calm environment may be insufficient to prevent the development of an erosional coast in a stormy environment. The climate also affects biological activity along coasts. For example, in the warm water of tropical climates, mangrove swamps flourish along the shore, and coral reefs form offshore. The reefs may build into a broad carbonate platform such as appears in the Bahamas today. In cooler climates, salt marshes develop, whereas in arctic regions, the coast may be a stark environment of lichen-covered rock and barren sediment.

Source: Essentials of Geology; book by Stephen Marshak

Coastal Landforms

Where Land Meets Sea: Coastal Landforms

Tourists along the Amalfi coast of Italy thrill to the sound of waves crashing on rocky shores. But in the Virgin Islands sunbathers can find seemingly endless white sand beaches, and along the Mississippi delta, vast swamps border the sea. Large, dome-like mountains rise directly from the sea in Rio de Janeiro, Brazil, but a 100-m-high vertical cliff marks the boundary between the Nullarbor Plain of southern Australia and the Great Southern Ocean. As these examples illustrate, coasts, the belts of land bordering the sea, vary dramatically in terms of topography and associated landforms. 

Beaches and Tidal Flats 

Characteristics of beach, barrier islands and tidal flats.
For millions of vacationers, the ideal holiday includes a trip to a beach, a gently sloping fringe of sediment along the shore. Some beaches consist of pebbles or boulders, whereas others consist of sand grains (figure above a, b). This is no accident, for waves winnow out finer sediment like silt and clay and carry it to quieter water, where it settles. Storm waves, which can smash cobbles against one another with enough force to shatter them, have little effect on sand, for sand grains can’t collide with enough energy to crack. Thus, cobble beaches exist only where nearby cliffs supply large rock fragments. 
The composition of sand varies from beach to beach because different sands come from different sources. Sands derived from the weathering and erosion of silicic-to- intermediate rocks consist mainly of quartz; other minerals in  these rocks chemically weather to form clay, which washes away  in waves. Beaches made from the erosion of limestone, or of  coral reefs and shells, consist of carbonate sand, including masses of sand-sized chips of shells. And beaches derived from  the erosion of basalt boast black sand, made of tiny basalt grains.
A beach profile, a cross section drawn perpendicular to the shore, illustrates the shape of a beach (figure above c). Starting from the sea and moving landward, a beach consists of a foreshore zone, or intertidal zone, across which the tide rises and falls. The beach face, a steeper, concave-up part of the foreshore zone, forms where the swash of the waves actively scours the sand. The backshore zone extends from a small step, cut by high-tide swash to the front of the dunes or cliffs that lie farther inshore. The backshore zone includes one or more berms, horizontal to landward-sloping terraces that receive sediment only during a storm. 
Geologists commonly refer to beaches as “rivers of sand,” to emphasize that beach sand moves along the coast over time it is not a permanent substrate. Wave action at the shore moves an active sand layer on the sea floor on a daily basis. Inactive sand, buried below this layer, moves only during severe storms or not at all. Longshore drift, discussed earlier, can transport sand hundreds of kilometres along a coast in a matter of centuries. Where the coastline indents landward, beach drift stretches beaches out into open water to create a sand spit. Some sand spits grow across the opening of a bay, to form a baymouth bar (figure above d). 
The scouring action of waves sometimes piles sand up in a narrow ridge away from the shore called an offshore bar, which parallels the shoreline. In regions with an abundant sand supply, offshore bars rise above the mean high-water level and become barrier islands (figure above e), and the water between a barrier island and the mainland becomes a quiet-water lagoon, a body of shallow seawater separated from the open ocean. Though developers have covered some barrier islands with expensive resorts, in the time frame of centuries to millennia, barrier islands are temporary features and may wash away in a storm.
Tidal flats, regions of clay and silt exposed or nearly exposed at low tide but totally submerged at high tide, develop in regions protected from strong wave action (figure above f). They are typically found along the margins of lagoons or on shores protected by barrier islands. Here, sediments accumulate to form thick, sticky layers. 

Rocky Coasts 

Erosion landforms of rocky shorelines.
More than one ship has met its end, smashed and splintered in the spray and thunderous surf of a rocky coast, where bedrock cliffs rise directly from the sea. Lacking the protection of a beach, rocky coasts feel the full impact of ocean breakers. The water pressure generated during the impact of a breaker can pick up boulders and smash them together until they shatter, and it can squeeze air into cracks, creating enough force to push rocks apart. Further, because of its turbulence, the water hitting a cliff face carries suspended sand and thus can abrade the cliff. The combined effects of shattering, wedging, and abrading, together called wave erosion, gradually undercut a cliff face and make a wave-cut notch (figure above a). Undercutting continues until the overhang becomes unstable and breaks away at a joint, creating a pile of rubble at the base of the cliff that waves immediately attack and break up. In this process, wave erosion cuts away at a rocky coast, so that the cliff gradually migrates inland. Such cliff retreat may leave behind a wave-cut bench, or platform, that becomes visible at low tide (figure above b). 
Other processes besides wave erosion break up the rocks along coasts. For example, salt spray coats the cliff face above the waves and infiltrates into pores. When the water evaporates, salt crystals grow and push apart the grains, thereby weakening the rock. Biological processes also contribute to erosion, for plants and animals in the intertidal zone bore into the rocks and gradually break them up. 
Many rocky coasts are irregular with headlands protruding into the sea and embayments set back from the sea. Wave energy focuses on headlands and disperses in embayments, a result of wave refraction. The resulting erosion removes debris at headlands, and sediment accumulates in embayments (figure above c). In some cases, a headland erodes in stages (figure above d). Because of refraction, waves curve and attack the sides of a headland, slowly eating through it to create a sea arch connected to the mainland by a narrow bridge. Eventually the arch collapses, leaving isolated sea stacks just offshore (figure above d). Once formed, a sea stack protects the adjacent shore from waves. Therefore, sand may collect in the lee of the stack, slowly building a tombolo, a narrow ridge of sand that links the sea stack to the mainland.

Estuaries 

The Chesapeake Bay estuary formed when the sea flooded river valleys. The region is sinking relative to other coast areas because it overlies a buried meteor crater.
Along some coastlines, a relative rise in sea level causes the sea to flood river valleys that merge with the coast, resulting in estuaries, where seawater and river water mix. You can recognize an estuary on a map by the dendritic pattern of its river-carved coastline (figure above). Oceanic and fluvial waters interact in two ways within an estuary. In quiet estuaries, protected from wave action or river turbulence, the water becomes stratified, with denser oceanic salt water flowing upstream as a wedge beneath less-dense fluvial freshwater.  In turbulent estuaries, oceanic and fluvial water combine to create nutrient-rich brackish water with a salinity between that of oceans and rivers. Estuaries are complex ecosystems inhabited by unique species of shrimp, clams, oysters, worms, and fish that can tolerate large changes in salinity.

Fjords 

Fjord landscapes form where relative sea-level rise drowns glacially carved valleys.
During the last ice age, glaciers carved deep valleys in coastal mountain ranges. When the ice age came to a close, the glaciers melted away, leaving deep, U-shaped valleys. The water stored in the glaciers, along with the water within the vast ice sheets that covered continents during the ice age, flowed back into the sea and caused sea level to rise. The rising sea filled the deep valleys, creating fjords, or flooded glacial valleys. Coastal fjords are fingers of the sea surrounded by mountains; because of their deep-blue water and steep walls of polished rock, they are distinctively beautiful (figure above).

Coastal Wetlands 

Examples of coastal wetlands.
A flat-lying coastal area that floods during high tide and drains during low tide, but does not get pummeled by intense waves, can host salt-resistant plants and evolve into a coastal wetland. Wetland-dominated shorelines are sometimes called “organic coasts.” Researchers distinguish among different types of coastal wetlands based on the plants they host. Examples include swamps (dominated by trees), marshes (dominated by grasses; figure above a), and bogs (dominated by moss and shrubs). So many marine species spawn in wetlands that as a whole, wetlands account for 10% to 30% of marine organic productivity. 
In tropical or semitropical climates (between 30 north and 30 south of the equator), mangrove trees may become the dominant plant in swamps (figure above b). Some mangrove species form a broad network of roots above the water surface, making the plant look like an octopus standing on its tentacles, and some send up small protrusions from roots that rise above the water and allow the plant to breathe. Dense mangrove swamps counter the effects of stormy weather and thus prevent coastal erosion.

Coral Reefs 

The character and evolution of coral reefs.
Along the azure coasts of Hawaii, visitors swim through colorful growths of living coral. Some corals look like brains, others like elk antlers, still others like delicate fans (figure above a). Sea anemones, sponges, and clams grow on and around the coral. Though at first glance coral looks like a plant, it is actually a colony of tiny invertebrates related to jellyfish. An individual coral animal, or polyp, has a tubelike body with a head of tentacles. 
Coral polyps secrete calcite shells, which gradually build into a mound of solid limestone whose top surface lies from just below the low-tide level down to a depth of about 60 m. At any given time, only the surface of the mound lives the mound’s interior consists of shells from previous generations of coral. The realm of shallow water underlain by coral mounds, associated organisms, and debris comprises a coral reef. Reefs absorb wave energy and thus serve as a living buffer zone that protects coasts from erosion. Corals need clear, well-lit, warm (18–30C) water with normal oceanic salinity, so coral reefs grow only along clean coasts at latitudes of less than about 30 (figure above b). 
Marine geologists distinguish three different kinds of coral reef, on the basis of their geometry (figure above c). A fringing reef forms directly along the coast, a barrier reef develops offshore, and an atoll makes a circular ring surrounding a lagoon. As Charles Darwin first recognized back in 1859, coral reefs associated with islands in the Pacific start out as fringing reefs and then later become barrier reefs and finally atolls. This progression reflects the continued growth of the reef as the island around which it formed gradually sinks. Eventually, the reef itself sinks too far below sea level to remain alive and becomes the cap of a flat-topped seamount known as a guyot.

Ocean waters and currents

Ocean waters and currents

Ocean waters and currents depends upon lots of things as below.

Composition and Temperature 

If you've ever had a chance to swim in the ocean, you may have noticed that you float much more easily in ocean water than you do in freshwater. That’s because ocean water contains an average of 3.5% dissolved salt; in contrast, typical freshwater contains less than 0.02% salt. The dissolved ions fit between water molecules without changing the volume of the water, so adding salt to water increases the water’s density, and you float higher in a denser liquid. 
There’s so much salt in the ocean that if all the water suddenly evaporated, a 60-m-thick layer of salt would coat the ocean floor. This layer would consist of about 75% halite (NaCl) with lesser amounts of gypsum (CaSO4s(2O), anhydrite (CaSO4), and other salts. Oceanographers refer to the concentration of salt in water as salinity. Although ocean salinity averages 3.5%, measurements from around the world demonstrate that salinity varies with location, ranging from about 1.0% to about 4.1%. Salinity reflects the balance between the addition of freshwater by rivers or rain and the removal of freshwater by evaporation, for when seawater evaporates, salt stays behind; salinity also depends on water temperature, for warmer water can hold more salt in solution than can cold water. 
When the Titanic sank after striking an iceberg in the North Atlantic, most of the unlucky passengers and crew who jumped or fell into the sea died within minutes because the seawater temperature at the site of the tragedy approached freezing, and cold water removes heat from a body very rapidly. Yet swimmers can play for hours in the Caribbean, where sea-surface temperatures reach 28C (83F). Though the average global sea-surface temperature hovers around 17C, it ranges between freezing near the poles to almost 35C in restricted tropical seas. The correlation of average temperature with latitude exists because the intensity of solar radiation varies with latitude. 
Water temperature in the ocean varies markedly with depth. Waters warmed by the Sun are less dense and tend to remain at the surface. An abrupt thermocline below which water temperatures decrease sharply, reaching near freezing at the sea floor appears at a depth of about 300 m in the tropics. There is no pronounced thermocline in polar seas, since surface waters there are already so cold.

The Coriolis Effect 

Imagine that you have a huge cannon aim it due south and fire a projectile from the North Pole to a target on the equator (figure below a). If the Earth were standing still, the shot would follow a line of longitude. But the Earth isn't standing still. It rotates counter-clockwise around its “axis” (an imaginary line that passes through the planet’s centre and its geographic poles). To an observer in space, an object at the pole doesn't move at all as the Earth spins because it is sitting on the axis, but an object  at the equator moves at about 1,665 km/h (1,035 mph). Because of this difference, the target on the equator will have moved by the time the projectile reaches it. In fact, to an observer standing on the Earth and moving with it, the projectile follows a curving trajectory. The same phenomenon happens if you place the cannon on the equator and fire the projectile due north (figure below b)the projectile’s path curves because the projectile moves eastwards progressively faster than the land beneath while moving north. (The same phenomenon, of course, happens in the southern hemisphere, but in reverse.) This behaviour is called the Coriolis effect, after the French engineer who, in 1835, described its consequences. Because of the Coriolis effect, north-flowing currents in the northern hemisphere deflect to the east, and south flowing currents deflect to the west.

The Coriolis effect because the velocity of a point at the equator, in the direction of the Earth's spin, is greater than that of a point near the poles.

Currents: Rivers in the Sea 

Since first setting sail on the open ocean, people have known that the water of the ocean does not stand still, but rather flows or circulates at velocities of up to several kilometers per hour in fairly well-defined streams called currents. Oceanographic studies demonstrate that circulation in the sea occurs at two levels: surface currents affect the upper hundred meters of water, and deep currents keep the remainder of the water column in motion. 

 The major surface currents of the world’s oceans.
Surface currents occur in all the world’s oceans (figure above). They result from interaction between the sea surface and the wind as moving air molecules shear across the surface of the water, the friction between air and water drags the water along with it. The Earth’s rotation, however, generates the Coriolis effect, a phenomenon that causes surface currents in the northern hemisphere to veer toward the right and surface currents in the southern hemisphere to veer toward the left of the average wind direction.  Across the width of an ocean, the Coriolis effect causes surface currents to make a complete loop, known as a gyre. Surface water may become trapped for a long time in the centre of the gyre, where currents hardly exist, so these regions tend to accumulate floating plastics, sludge, and seaweed. The “Sargasso Sea,” named for a kind of floating seaweed, lies at the centre of the North Atlantic gyre, and the “Great Pacific Garbage Patch,” an accumulation of floating plastic and trash, lies at the centre of the North Pacific gyre. Figure above is a simplification of currents interactions of currents with coastlines create chains of eddies, in which water circulates in small loops (figure below a–c).

 The complexity of the ocean’s currents. An animation by NASA, based on data collected over a two-year period, shows the details of eddies and swirls in the ocean, and emphasizes that currents interact with the coasts.
Surface water and deeper water in the ocean exchange at a number of locations. Specifically, in downwelling zones, surface water sinks, and in upwelling zones, deeper water rises. Downwelling and upwelling occur for a number of reasons. For example, in places where winds blow surface water shoreward, an oversupply of water develops along the coast, so surface water must sink to make room. And where winds blow surface water away from the shore, a deficit of water develops along the coast, so deeper water must rise to fill the gap. Upwelling of deeper water also occurs near the equator, where winds blow steadily from east to west, because the Coriolis effect deflects surface currents to the right in the northern hemisphere and to the left in the southern hemisphere, thereby leading to the development of a water deficit along the equator. The resulting rise of cool, nutrient-rich water fosters an abundance of life in equatorial water. 

Global-scale upwelling and downwelling of ocean currents.
Contrasts in water density, caused by differences in temperature and salinity, can also drive upwelling and downwelling. We refer to the rising and sinking of water driven by such density contrasts as thermohaline circulation. During thermohaline circulation, denser (cold and/or saltier) water sinks, whereas water that is less dense (warm and/or less salty) rises. As a result, the cold water in polar regions sinks and flows back along the bottom of the ocean toward the equator. This process divides the ocean vertically into a number of distinct water masses, which mix only very slowly with one another. In the Atlantic Ocean, for example, the Antarctic Bottom Water sinks along the coast of Antarctica, and the North Atlantic Deep Water sinks in the north polar region (figure above a). The combination of surface currents and thermohaline circulation, like a conveyor belt, moves water and heat among the various ocean basins and moderates global climate (figure above b).
Figures credited to Stephen Marshak.

Landscapes Beneath the Sea

Landscapes beneath the sea

Contrasts between continental lithosphere and oceanic lithosphere.
There are lot of landscapes beneath the sea, if the surface of the lithosphere were completely smooth, an ocean would surround the Earth as a uniform, 2.5-km deep layer. But because about 30% of this planet’s surface is dry land, most seawater resides in distinct ocean basins (Contrasts between continental lithosphere and oceanic lithosphere).

  • The distinction between land and sea exists because continental and oceanic lithosphere differ markedly from one another in terms of composition and thickness (Contrasts between continental lithosphere and oceanic lithosphere).
  • Due to isostasy, the denser and thinner oceanic lithosphere “floats” with its surface at a lower elevation, relative to that of the relatively buoyant, thicker continental lithosphere. On the present-day map of the world, cartographers divide the global ocean into several major parts (oceans and seas), with somewhat arbitrary boundaries and significantly different volumes (figure below). 

The oceans of the world. The Pacific is the largest, covering almost half the planet. The Arctic region is an ocean covered by a thin coating of ice, whereas the Antarctic region is a continent surrounded by an ocean.
Have you ever wondered what the ocean floor would look like if all the water evaporated? Marine geologists can now provide a clear image of the ocean’s bathymetry, or variation in depth. These measurements were first obtained by using a plumb line (a weight at the end of a cable). Then, in the 20th century, sonar measurements (made by bouncing sound waves off the ocean floor) became available. Today, satellites survey the ocean. Such studies indicate that the ocean contains bathymetric provinces, distinguished from each other by their water depth. Let’s now examine each of these provinces.

Continental Shelves, Slopes, and Rises 

Bathymetric features of the seafloor. The maps are produced by computer using measurements from satellites or submersibles.
Imagine you’re in a submersible cruising just above the floor of the western half of the North Atlantic. If you start at the shoreline of North America and head east, you will cross the 200- to 500-km-wide continental shelf, a relatively shallow portion of the ocean that fringes the continent. Water depth over the continental shelf does not exceed 500 m. At its eastern edge, the continental shelf merges with the continental slope, which descends to depths of nearly 4 km. From about 4 km down to about 4.5 km, a province called the continental rise, the slope angle decreases until at 4.5 km deep, you find yourself above a vast, nearly horizontal surface, the abyssal plain
Broad continental shelves, like that of eastern North America, form along passive continental margins, margins that are not plate boundaries and thus lack seismicity (figure above a). Passive margins originate after rifting breaks a continent in two. When rifting stops and sea-floor spreading begins, the stretched lithosphere at the boundary between the ocean and continent gradually cools and sinks. Sand and mud that washed off the continent, along with the shells of marine creatures that grow on the sea floor or settle from the water above, bury the sinking crust, slowly producing a pile of sediment up to 20 km thick.
If you were to take your submersible to the western coast of South America and cruise out into the Pacific, you would find a very different continental margin. After crossing a narrow continental shelf, the sea floor drops down to a depth of over 8 km. South America does not have a broad continental shelf because it is an active continental margin, a margin that coincides with a plate boundary and thus hosts many earthquakes (figure above b). In the case of South America, the edge of the Pacific Ocean is a convergent plate boundary. The narrow shelf along a convergent plate boundary forms where an apron of sediment eroding from the continent spreads out over the top of an accretionary prism, the pile of sediment and basalt scraped off the downgoing subducting plate. 
At many locations, relatively narrow and deep valleys called submarine canyons downcut into continental shelves and slopes (figure above c). Some submarine canyons start offshore of major rivers, and for good reason: rivers cut into the continental shelf at times when sea level was low and the shelf was exposed. But river erosion cannot explain the total depth of these canyons some slice almost 1,000 m down into the continental margin, far deeper than the maximum sea-level change. Much of the erosion of submarine canyons results from the flow of turbidity currents, submarine avalanches of sediment mixed with water.

The Bathymetry of Oceanic Plate Boundaries 

Sea-floor spreading at a divergent boundary yields a mid-ocean ridge, a 2-km-high submarine mountain belt. Because crust stretches and breaks as sea-floor spreading continues, the axis of a ridge may be bordered by escarpments, a result of normal faulting. Oceanic transform faults, strike-slip faults along which one plate shears sideways past another, typically link segments of mid-ocean ridges. Transforms are delineated by fracture zones, narrow belts of steep escarpments, and broken-up rock. These fracture zones can be traced into the oceanic plate away from the ridge axis where they are not seismically active but still form the boundary between plates of different ages. 
Subduction at convergent boundaries yields a trench, a deep, elongate trough bordering a volcanic arc. Some trenches reach depths of over 8 km the deepest point in the ocean, 11,035 m, lies in the Mariana Trench of the western Pacific.  Only three people have descended to the floor of the Mariana Trench two in 1960 and the third in 2012. Some trenches border continents as we described earlier; others border island arcs, which are curving chains of active volcanic islands.

Abyssal Plains and Seamounts 

As oceanic crust ages and moves away from the axis of the mid-ocean ridge, two changes take place. First, the lithosphere cools, and as it does so, its surface sinks. Second, a blanket of sediment gradually accumulates and covers the basalt of the oceanic crust. This blanket consists mostly of microscopic plankton shells and fine flakes of clay, which slowly fall like snow from the ocean water and settle on the sea floor. Because the ocean crust gets progressively older away from the ridge axis, sediment thickness increases away from the ridge axis. 
In dozens of locations around the world, hot-spot eruptions on oceanic lithosphere produced volcanoes. Presently active oceanic hot-spot volcanoes, as well as the remnants of extinct ones, rise above sea level as oceanic islands; those that lie below sea level are called seamounts. Particularly voluminous eruptions have produced broader buildups of basalt that are known as submarine plateaus.
Figures credited to Stephen Marshak.