Deep sea exploration

Deep ocean basins cover almost 70% of Earth's surface, and they contain 96% of the planet's life-sustaining water . The oceans support the biosphere by modulating global climate and hydrology, and are home to the marine organisms that form the diverse base of the global ecosystem. Geology's unifying paradigm, the theory of plate tectonics , arose from deep-sea discoveries of the 1950s and 1960s. The title of marine geologist Philip Kuenen's 1958 paper, "No Geology without Marine Geology," rings especially true with hindsight of late twentieth century advancements in the field of marine geology.

The nature of the seafloor was an unrevealed mystery until the mid-nineteenth century; scientists and artists alike envisioned the deep sea as a lifeless soup of placid water, contained in a bowl of static rock . By the late 1860s, however, controversial theories of the origin of life by evolution and the vastness of geologic time had created a climate of scientific curiosity and piqued a general interest in marine science. The Royal Society of London thus mounted an ambitious oceanographic expedition to augment a sparse collection of existing marine data that included Charles Darwin's observations during the voyage of the Beagle (1831–1836), a bathymetric map created by U.S. Navy Lt. Matthew Maury to aid installation of the first trans-Atlantic telegraph cables in 1858, and a few examples of deep marine life. The HMS Challenger expedition (1872–1876) covered almost 70,000 miles, and shipboard scientists collected hundreds of sediment samples, hydrographic measurements, and specimens of marine life. They also dredged mafic basalt from the seafloor, critical evidence that oceanic crust is compositionally different from continental crust, and took almost 500 soundings that revealed the depth and basic physiography of the ocean basins.

While the Challenger expedition provided critical data that led to rapid advances in marine biology and oceanography , the Victorian concept of a geologically inert seafloor persisted for another seventy years. German meteorologist, Alfred Wegener , proposed his continental drift theory in 1912, but the idea was generally discarded because of inadequate knowledge of seafloor geology. During the 1920s and 1930s, development of sonar echosounding and detection of seafloor gravity anomalies rapidly improved the accuracy of bathymetric maps. The technology and government funding that accompanied naval and submarine warfare during World War II, however, precipitated marine geology's age of discovery. Princeton geologist and naval officer, Harry H. Hess, collected bathymetric, gravity, and magnetic polarity data during transatlantic troop transfers, and went on to become a founder of modern marine geology.

The scientific infrastructure of oceanographic institutions, instruments, and vessels that arose following WWII led directly to the theory of plate tectonics. Lamont Geological Observatory of Columbia University geologists surveyed the ocean basins with newly-refined depth recorders in the early 1950s. They discovered the globe-encircling mid-ocean ridge system, and suggested that oceanic crust is created at these chains of submarine volcanoes. Geophysical surveys of the deep ocean trenches , and exploration in manned submersibles, including the U.S. Navy's Trieste and Alvin, suggested the complementary process of seafloor consumption at subduction zones, and the theory of plate tectonics was born. Plate tectonics became widely accepted in the late 1960s as further geophysical surveys of the ocean, seafloor sampling and drilling by the Deep Sea Drilling Project (DSDP), and continental geology all corroborated the theory.

The technology of deep sea exploration has advanced from twine and cannon ball soundings, to ocean surveys from space and robotic exploration of the deep ocean floor. Modern sonar instruments provide high-resolution, three-dimensional images of the seafloor. Seafloor sample collections compiled by the DSPD, its successor the international Ocean Drilling Program (ODP), and many other oceanographic institutions, provide rock and sediment data to augment geophysical images. Seismic reflection surveys allow marine stratigraphers and petroleum geologists to investigate strata and potential source rocks beneath the seabed.

Technology, fueled by scientific curiosity, has revealed the deep ocean as a dynamic geological environment. The discoveries of intricate ecosystems at mid-ocean volcanic vents and the unexpected diversity of marine life have revolutionized biological science. Just as marine geology held the keys to understanding earth history, marine science may also be the path to understanding Earth's future. Ocean circulation and currents control Earth's climate and hydrology, the continental margins are home to most of Earth's human population, and the techniques developed for deep sea exploration are often applicable to space exploration.

See also Bathymetric mapping; Mid-ocean ridges and rifts; RADAR and SONAR; Remote sensing; Seismology