Most of Earth’s surface is underwater, and most of its geology happens there. New crust is born at mid-ocean ridges, old crust sinks into trenches, volcanoes erupt unseen at depth, and sediments pile up over millions of years like pages in a book. Geological oceanographers are the scientists who read that book. Their field is one of the four main branches of oceanography, alongside physical, chemical and biological oceanography, and it has produced some of the biggest discoveries in the history of science, including plate tectonics and the hydrothermal vents that support life without sunlight.

For a reef keeper, marine geology is not as remote as it sounds. Every reef tank I’ve built has started with geology: live rock, which is pieces of old reef limestone, and sand or crushed coral substrate. Choosing how much rock and sand to use, and later deciding to take the sand bed out of my 75-gallon system to improve flow, taught me how much the physical structure of a reef shapes everything that lives on it. Marine geologists study that structure on the scale of whole ocean basins.

What marine geologists study

The field covers everything from the shape of the coastline to the deepest trenches, and from rocks formed yesterday to sediments laid down 100 million years ago.

Seafloor morphology is the shape of the ocean floor: continental shelves, slopes, canyons, abyssal plains, seamounts, ridges and trenches. Plate tectonics explains why those features exist, with new crust forming at spreading ridges and sinking at subduction zones into trenches such as those that form the hadal zone. Marine sediments come in several kinds: material washed off the land, the shells and skeletons of plankton (the calcareous and siliceous “oozes” that blanket much of the deep sea), minerals that precipitate from seawater such as manganese nodules, and volcanic ash. Hydrothermal vents are hot springs on the seafloor where seawater heated by magma pours out rich in minerals; see hydrothermal vents. Paleoceanography uses sediment cores to reconstruct past ocean temperatures, ice ages and sea levels. And coastal geology deals with beaches, erosion, deltas and the effects of rising seas.

Discoveries that changed geology

Marine geology has a short history but a remarkable one.

In the 1950s, Marie Tharp and Bruce Heezen at Columbia University’s Lamont Geological Observatory used echo-sounding profiles to draw maps of the Atlantic floor, and Tharp noticed a rift valley running down the centre of the Mid-Atlantic Ridge. That observation became key evidence for seafloor spreading, which, together with magnetic stripes on the seafloor discovered in the early 1960s, led to the theory of plate tectonics. Their famous painted map of the world ocean floor, published in 1977 with the artist Heinrich Berann, showed the global ridge system for the first time.

In 1977 scientists diving in the submersible Alvin on the Galápagos Rift found hydrothermal vents surrounded by giant tube worms, clams and other animals living on chemical energy rather than sunlight. The expedition was led by geologists looking for hot springs, not biologists looking for life, which shows how geological questions can open entirely new fields. Our profile of Robert Ballard, a marine geologist who took part in the vent discoveries before finding the Titanic, follows that story.

Ocean drilling has been another engine of discovery. The Deep Sea Drilling Project, which began in 1968 with the drill ship Glomar Challenger, and its successors brought up thousands of cores that confirmed seafloor spreading, dated the ocean crust and recorded the history of Earth’s climate in remarkable detail.

Tools of the marine geologist

Because the seafloor is hidden under kilometres of water, almost everything is measured remotely or sampled from a ship.

Multibeam echo sounders fire a fan of sound pulses across the seafloor to build detailed depth maps. Side-scan sonar produces images of the bottom texture, useful for finding wrecks, pipelines and habitats. Sub-bottom profilers and seismic reflection systems send lower-frequency sound into the sediments and rock, revealing buried layers and faults. To get physical samples, geologists use grabs, dredges and corers: gravity and piston corers that punch tubes into the mud, and drilling ships that can recover cores hundreds of metres long. ROVs, autonomous underwater vehicles and crewed submersibles such as Alvin allow close-up observation and precise sampling. Satellites help too: tiny bumps in the sea surface, measured from space, reveal the gravity of seamounts and ridges below. Our page on what oceanographers use echo-sounding devices to study explains sonar mapping in more detail.

A day in the life

Like other oceanographers, marine geologists divide their time between the field, the lab and the computer.

At sea, a marine geologist might spend weeks running sonar surveys around the clock, watching maps build up line by line, or supervising the recovery of sediment cores and describing them as they come on deck. Back on land, the cores go to refrigerated repositories where they are split, photographed, sampled and analysed for grain size, chemistry, microfossils and magnetic properties. Map data has to be cleaned and processed into grids and charts. Much of the work is then interpretation: building models of how a fault moved, how a canyon formed or how climate changed. Writing reports and papers, applying for funding and collaborating with international teams round out the job.

In industry, the day can look different. A marine geologist at a survey company might spend much of the year at sea mapping routes for undersea cables or sites for offshore wind farms, and the rest of the time writing reports for clients and engineers.

How to become a geological oceanographer

The usual path starts with a solid undergraduate degree in geology, geophysics or earth science.

Courses in mineralogy, sedimentology, structural geology, geophysics, physics, chemistry and mathematics are the foundation. Programming and GIS skills are increasingly important, as is experience with data processing software. Field camps on land are a standard part of geology training and are good preparation for fieldwork at sea. For research or senior positions, a master’s or PhD in marine geology, geophysics or oceanography is usually needed. Leading programmes are found at oceanographic institutions such as Scripps, Woods Hole, Lamont-Doherty, the University of Rhode Island, Oregon State and the University of Washington, as well as at universities in Europe, Asia and Australia. Getting time at sea as a student, on a research cruise or in a summer programme, is one of the best ways to find out whether the life suits you.

Where marine geologists work

Marine geologists are employed across research, government and industry.

In the United States, the US Geological Survey runs a major coastal and marine geology programme studying hazards, sediments and resources, and NOAA maps the seafloor for navigation and exploration. The Bureau of Ocean Energy Management and other agencies need geologists to assess offshore energy and minerals. Universities and research institutes employ many marine geologists as researchers and teachers. In the private sector, oil and gas companies have long employed marine geologists and geophysicists, and the growth of offshore wind, undersea cables and coastal engineering has created many new roles. Deep-sea mining is a newer and controversial area that has brought both jobs and intense scientific debate about environmental impacts. For pay in a related field, see oceanographic engineering salaries.

Why marine geology matters now

Some of the most pressing problems of the coming decades are partly geological.

Coastal communities need to know how shorelines will respond to rising seas. Tsunami warning systems depend on understanding which faults and slopes can generate waves. Undersea internet cables, which carry most international data, need safe routes away from landslides and earthquakes. Offshore wind farms need stable foundations. Sediment cores remain one of the best records of how quickly the climate has changed in the past, which helps put today’s changes in context. And the seafloor itself is still poorly mapped: only around a quarter has been charted in high resolution, and an international effort called Seabed 2030 aims to complete the map. See why we have better maps of Mars than of the ocean for the scale of the task, and our guides to what oceanographers do and chemical oceanographers for the other branches.