Sea Floor Exploration

Earth’s scientists have been interested in the ocean as well as the continents for at least the last 150 years. However, “modern oceanography” started after World War II in 1946, when the world’s politicians and scientists realized that in order to have supremacy over other countries and acquire knowledge of our planet it would be necessary to increase the capability of sea floor exploration. Private and government funds were provided to enhance underwater technology. Ocean-going ships began to carry out regular and more extensive expeditions on all the major oceans and seas. Also, we now know that the modern oceans and seas are relatively young (150–170 million years old) when compared to the continental landmasses. Geological records found on the continents indicate that the ocean must have existed since our planet’s creation 4.6 billion years ago, and it seems probable that the Earth has undergone several cycles of ocean formation and retrieval during the past 4 billion years.

Planet Earth was formed from the agglomeration of solid bodies. Earth’s iron-nickel enriched core segregated from a silicate mantle as early 30 million years after its formation 4.1–4.7 billion years ago. Earth is the third planet in our solar system circling around the Sun in an elliptical orbit at a distance of 147–152 million kilometers. Such a distance from the Sun enables our planet’s temperature to be hospitable to animal and plant life. Among the nine planets of our solar system, Earth is the only one whose surface environment is adapted for water to exist in its three states: solid, liquid and gas. The elements necessary for life consisting essentially of oxygen, carbon, hydrogen, nitrogen and sulfur, are also found in comets, stars and probably on other planets in the Universe. 71 % of the Earth’s surface is covered by water, which means that when seen from space, our home could be called the “Blue Planet”.

Most of the Earth’s surface is under water. The hostile environment of the underwater world makes it difficult for direct exploration by human beings. Underwater visibility is limited, it is impossible for humans to breathe in water, its salty nature makes seawater denser than air (about 1300 times heavier than air), and underwater currents tend to displace any man-made engines at depth. Despite these difficulties, humans have always been attracted to the underwater world. Since antiquity men and women have tried to penetrate the sea and elucidate its mysteries. In 1934, exploration with manned submersibles first became a reality. Even if technology has advanced our capabilities, the spirit of underwater exploration has remained the same. Each dive into the unknown is a unique and exciting experience, bringing both joy and fear.

Sea floor rocks can reveal Earth’s history. Based on the various sampling operations conducted by ocean going scientists, it is inferred that the relative distribution of oceanic rocks includes about 50 % basalts/dolerites, 20 % gabbros and 30 % peridotites. Basalt and dolerite are the main volcanic rocks of the upper crust, gabbros are formed during the solidification of a magma chamber or magma conduit, and peridotites are either the heavy mineral residues left within the reservoirs after magma solidification or they could be the remains after a partial melting of mantle material. Basalt is the most common type of volcanic rock found on the sea floor. Basalt and other related rocks have been extruded after partial melting of the Earth’s mantle material. Rocks from the sea floor differ from those encountered on land due to their shape and their chemical composition. The effect of seawater and the pressure it exercises on hot, outpouring lava will fashion the shape of deep-sea volcanic rocks giving the sea floor a different appearance than what we see in subaerial environments. Curved and spherical-shaped pillow lavas are only found on the sea floor due to the fact that seawater pressure is equally applied on all directions of the lava flows and their cooling surfaces. Basalts consist of silicates of magnesium, plus iron and calcium oxides. Less common silica-enriched rocks (SiO₂ > 53 %) such as andesites (SiO₂ = 53–59 %), rhyolites (>70 %) and trachytes (SiO₂ = 59–64 %) are also found on some undersea structures such as domes and seamounts.

Convection currents inside the Earth’s asthenosphere will cause instability at shallow depths in the mantle. Rising material and subsequent decompression melting will form hot, upwelling mantle plumes or diapirs. This phenomenon is more common on slow spreading ridges (total rate <5 cm/yrs), rather than beneath fast (total rate >5 cm/yrs) spreading ridge segments with their extensive fissural magmatism. Magma upwelling after partial melting of the mantle will depend on the force of buoyancy and on the permeability of the lithosphere. The effects of permeability and buoyancy will be modified by tectonic stress-release as well as by compression due to spreading following a cooling of the lithosphere. Instability in the melting zone is related to pressure release during a period without magma extraction. If pressure is released during spreading, the heat supply will generate more melting. With increased tension, the melting zone expands laterally and deepens until enough melt aggregates and accumulates in a confined zone to form a magma chamber. Rapid migration of melt through fissures at shallow depths enables a release of tension in the magmatic zone as it undergoes periods of melting and magma accumulation. When this process is repeated several times, it will trigger successive arrivals of more deep-seated magma, which can replenish the magma reservoir.

The World’s major marine geological structures have been affected by physical and chemical changes during and after their creation at spreading axes and in intraplate regions where islands and seamounts are formed. These changes are largely related to the penetration of the hydrosphere into the rigid lithosphere. A chemical reaction between the hydrosphere (seawater) and the lithosphere (rocks) in the presence of heat gives rise to hydrothermal fluid. The circulation of this fluid is the main cause of the lithosphere’s transformation due to the alteration (hydration) of metallic components from rocks and their subsequent precipitation on the sea floor in the form of ore deposits. A theory about the existence of hydrothermal discharge and metal deposits on the ocean floor was based on geophysical constraints related to the Earth’s heat budget. The deficit of heat discharge on spreading ridges was thought to be the result of a discharge of hydrothermal fluid on the sea floor.

The Spreading Ridge System is one of the most striking features exposed on the seafloor. It is a belt of volcanoes that surrounds the planet both under the oceans and in subaerial terrain. The Spreading Ridge System is where plate divergence occurs when ascending hot material rising from the mantle creates the sea floor. The spreading ridges extend around the World for a total length of 70,000 km and a width of about 1,000–2,000 km. This system is cut by tectonic and magmatic discontinuities such as fracture zones and disrupted spreading ridge segments. When compared to a human body, the spreading ridge system is like a continuous “backbone” and the “rib-bones” are the fracture zones emanating from both sides of the ridge axis.

Fracture zones are major discontinuities disrupting the linearity of spreading ridge segments. They form depressions separating two regions of plates that slip in opposite directions. The motion of the plates has gashed the lithosphere down to more than 4000 m deep, often exposing deep mantle material. Fracture zones vary in length from a few tens to hundreds of kilometers and are narrow, no more than a few tens of kilometers wide. The seismically and tectonically active portion of a fracture zone is also called a “transform fault”. The area of the Equatorial Atlantic between the African and South American continents located between 5°N and 5°S corresponds to the highest concentration of closely spaced fracture zones in the World’s ocean. That is where the St. Peter and St. Paul’s Rocks Fracture Zone crosses the entire Atlantic Ocean from the Amazon basin at the west through the coast of Liberia to the east. Most transforms are regions of intense tectonic events but are deprived of volcanic activity, except for a few transform faults associated with the fast spreading ridge systems in the Pacific Ocean where recent volcanic activity has been detected.

Hotspots are regions of the lithosphere that are fed by the melting of a hot mantle plume, giving rise to volcanic activity on the Earth’s surface. The source of hot mantle material (a “plume”) could have originated in the lower mantle, at the boundary of the molten Earth’s core at 2900 km depth. The mantle’s thermal flow ceases when attaining the colder and more rigid lithosphere boundary, less than 100 km deep. The rising of hot mantle plumes contributes to partial melting and to a thinning of the rigid lithosphere’s plates. Some volcanic edifices created during hotspot volcanism are formed on top of spreading ridges such as the islands of Iceland and Jan-Mayen in the Atlantic or Amsterdam Island in the Indian Ocean. Many other hotspot volcanoes occur in the intra-plate regions of the world’s oceanic basins and form tall submarine or subaerial edifices rising more than 4000 m from the seafloor.

After its creation at spreading ridge axes, the oceanic lithosphere will move with the drift of the plates until it encounters the border of another plate. These “converging plate” areas are the locus of lithosphere shrinking and large amounts of tectonic activity. During convergence, one plate may slide under another plate; this is called “subduction”. Subduction plates sink into the Earth’s mantle as convergence takes place. Deep trenches, island arcs and/or high mountain ranges mark the areas of plate convergence. Depending on the type of rocks that have built the surface of the plates, density differences mean that some plates will be pushed up, while others are subducted. Converging plate regions are highly affected by earthquakes and active volcanism.