Plate Tectonics is the process by which the series of tectonic plates, into which the entire surface crust of the Earth is divided, slowly moves about due to the convective forces of heat rising up to the surface from the Earth’s core and then spreading out laterally, pushing the plates along. The boundaries of these plates are where the main tectonic actions take place. These tectonic actions include earthquakes, volcanoes, mountain building and sea floor spreading.
Thus, an analysis of the rock types comprising the ocean floor should best begin where ocean floor itself begins… along the Mid-Ocean Ridges. For example, the Mid Atlantic Ridge runs roughly north and south along the entire length of the Atlantic Ocean from Iceland all the way to Antarctica and forms the plate boundary for the adjacent continental plates.
These linear volcanic features exude a dense, dark, iron and magnesium rich (mafic) volcanic magma from the Earth’s mantle called basalt. Similarly, such volcanic ridges form the boundaries of a number of the Earth’s other tectonic plates. Iceland is merely a highly volcanic portion of the Mid Atlantic Ridge that rises above the surface of the ocean.
Sometimes, chimney-like structures, known as black smokers, form near mid-ocean ridges where seawater infiltrates to some depth, gets heated by the mantle, and returns to the seabed through mineral-rich hydrothermal vents. When the mineral-laden water returns to the surface of the sea floor, dissolved minerals precipitate from solution, thus forming the chimney-like structures and accumulations of some valuable mineral sulfides, including iron pyrite, copper, lead, silver and even gold.
Incidentally, rich and diverse biological ecosystems that do not require sunlight for energy are frequently found in association with these deep ocean hydrothermal vents, as well as organisms that use sulfur instead of oxygen for their respiratory functions. This has implications for the origin of life as well as for the potential for life on other planets.
As basalt is added to each side of the mid-ocean ridges and convection from the rising heat of the Earth’s core and mantle disperses, the sea floor spreads outward from the ridges. New sea floor is formed by the addition of basalt as the sea floor spreads outward and away from the ridges, thus leaving room for the addition of new basalt along the length of the ridges. This spreading motion is the main driving force for all of plate tectonics.
In addition to forming along the mid-ocean ridges, basaltic magma is exuded volcanically at certain weaknesses in the Earth’s crust, known as hotspots, such as the islands of Hawaii and the Galapagos. Whenever magma from the Earth’s mantle reaches the surface of the Earth (either above or below water), the rock type basalt accumulates.
Molten basalt cools quickly underwater and forms bulbous mounds known as pillow lavas or pillow basalts. Basalt comprises the deepest parts of the world’s ocean floors as well as volcanic islands (Iceland, Hawaii, Galapagos) wherever magma piles up from great depths and reaches the ocean’s surface.
As new ocean floor forms and spreads continentward from the ridges, a constant rainout of sedimentary particles accumulates on top of the basaltic oceanic basement. This sedimentary layer of mud particles and organic material reaches great thicknesses further away from the ridges, as it has had more time to accumulate.
Given enough time and depth of burial, these sediments lithify (turn to stone) and give rise to the sedimentary rock types: black shale, siltstone, and micritic (muddy) limestone. Geologists believe that petroleum forms from the organic material in deeply buried and heated ocean ooze.
The great weight of huge thicknesses of sedimentary rock on top of the basaltic ocean floor causes the entire ocean floor to compress and sink back into the mantle below, thus giving rise to the great reaches of the relatively flat abyssal plains of the deepest expanses of the world’s oceans.
As you investigate the ocean floor closer to the continents and along the continental shelves, you find sedimentary rock types more directly derived from continental erosion and subsequent oceanic deposition. River deltas are one such source of oceanic sediments.
Occasionally, great underwater landslides called turbidites occur in which layers of mixed size sediment particles sort themselves out as the energy of the turbidite (underwater landslide) decreases basinward down the continental slope under the influence of gravity. This leaves tongue-like trails beginning with the coarsest and thickest sediments (boulders, cobbles, pebbles and gravel) and tapering out to decreasingly finer sediments (sand, mud and silt) extending from the continental shelf, down the continental slope and out onto the abyssal plain.
Sometimes repeated mass movements of turbidite materials downslope leave great thicknesses of these cyclic sedimentary deposits, each ranging from coarse to fine as you examine them, both laterally basinward and vertically upward through the sequence. Given enough time and depth of burial, these sediments lithify into conglomerates, sandstones, mudstones and siltstones, respectively.
In the relatively shallow waters nearest the continental shelf, sunlight penetrates and allows photosynthesis to occur. Here, and especially in warm shallow tropical seas, diverse and abundant marine ecosystems occur. Dissolved calcium carbonate in the water is taken up by an abundance of marine organisms to form seashells and skeletal structures, giving rise to great coral reefs.
As millennia of corals and shells accumulate, great thicknesses of these carbonate remains cement themselves together, forming the chemical sedimentary rock limestone. Limestone (natural concrete) is chemically identical to the shells and coral skeletons from which it is derived, calcium carbonate, and often contains well-preserved fossils of the organisms of which it is comprised.
Similarly, coquina is a light, porous and rough-textured limestone composed of broken bits of shells and corals (also known as beach rock) and has been used as building stone in Florida, the Florida Keys, the Bahamas and throughout the Caribbean.
Completing your exploration of the rock types found on the ocean floor and simultaneously completing this discussion of the cycle of erosion and deposition within the context of plate tectonics beginning at the mid-ocean ridges is the process of oceanic plate (ocean floor) subduction at the distal plate boundaries… Whatever comes up must ultimately go back down.
Wherever two tectonic plates converge (push toward each other), one of two possible outcomes occurs. Where two light (in both color and density) granitic continental plates converge, mountain ranges are thrust upward as tectonic forces drive the two tectonic plates together. This begins anew the cycle of erosion as mountains wear down and their sediments ultimately return to the ocean at great river deltas and as the constant rainout of sediments over all of the world’s oceans.
When two oceanic plates converge or when one oceanic and one continental plate converge, one dark heavy basaltic oceanic plate sinks beneath the other plate in deep oceanic trenches known as subduction zones. As the upper plate acts as a giant scraper scraping off layers of sediment from the other plate sliding beneath it, a mixed and convoluted jumble of rock types called a mélange forms along the leading edge of the upper plate.
As an oceanic plate subducts (drives downward) along another plate boundary, it plunges (over millions of years) back toward the Earth’s mantle, where it is heated, melted, mixed and forced back to the surface of the Earth, where volcanoes occur along volcanic island arcs such as Japan or the Pacific Northwest of North America.
Finally, plumes of ash rising from these chains of volcanoes, as well as sediments derived from their subsequent erosion, all ultimately return to the world ocean, contributing to the accumulation of sediments and the formation of new rocks upon the ocean floor.