The brilliant whiteness of the White Cliffs of Dover comes from chalk, and that chalk is built almost entirely from the remains of microscopic ocean-dwelling algae that lived during the Late Cretaceous period, roughly 100 to 66 million years ago. These single-celled organisms, called coccolithophores, armored themselves in tiny plates of calcium carbonate known as coccoliths. Trillions upon trillions of those plates drifted to the seafloor over tens of millions of years, piling up into a sediment hundreds of meters thick that eventually hardened into the soft, dazzling limestone we see today. The whiteness itself is a product of how calcite crystals interact with light, but the real story starts with the creatures that manufactured each grain.
Tiny Architects of a Giant Cliff
Coccolithophores are phytoplankton, meaning they float near the ocean surface and photosynthesize like plants. What sets them apart is that they build intricate armor out of calcite, a crystalline form of calcium carbonate. Each organism surrounds itself with a sphere of interlocking plates, and the precision of the construction is remarkable. In the species Gephyrocapsa oceanica, for example, individual coccolith units interlock tightly because the crystal layers grow at non-vertical angles, causing neighboring mineral units to extend over one another and lock into place.
1PubMed. Control of crystal growth during coccolith formation by the coccolithophore Gephyrocapsa oceanicaWhen a coccolithophore dies, its armor disassembles and the individual plates begin sinking. Some sink on their own, but many hitch a ride inside the fecal pellets of zooplankton that graze on them. That packaging matters: when less calcite dissolves inside a copepod’s gut and more gets expelled intact in its waste, the pellets become denser and sink roughly twice as fast, ferrying carbon and calcium carbonate to the deep ocean floor more efficiently.
2PubMed Central. Coccolith dissolution within copepod guts affects fecal pellet density and sinking rateThe term “coccolith” was coined in the mid-1850s by Thomas Henry Huxley, who first described them in sediments dredged from the ocean floor. Scientists later recognized the same structures in the Upper Cretaceous English Chalk, confirming that these organisms were major rock-building agents. Although coccolithophores are now understood to have appeared in the Late Triassic, they have been producing calcite plates and accumulating in marine sediments ever since, a span of over 200 million years.
3International Journal of Earth Sciences. Thomas Henry Huxley, a stone tablet, coccoliths, and deep-sea sediments in the high AlpsFrom Soft Ooze to Solid Rock
When coccolith plates first settle on the seafloor, they form a loose, waterlogged paste called calcareous ooze. It looks more like toothpaste than rock. The transformation into chalk happens over millions of years as more sediment piles on top, increasing the pressure on the layers beneath. This process, broadly called diagenesis, compresses the ooze and squeezes out water. At contact points between calcite grains, the pressure causes tiny amounts of mineral to dissolve and re-precipitate, effectively gluing the grains together. Researchers describe this as pressure dissolution at calcite-particle contacts causing contact cementation, and it marks the key transition from ooze to chalk.
4Sedimentology. Porosity in chalk – roles of elastic strain and plastic strainEven after cementation, chalk remains a relatively soft rock. You can scratch it with a fingernail. It never reaches the dense, hard state of many other limestones because the cementation is limited; most of the original pore space between coccolith fragments is preserved rather than filled in with new mineral. That high porosity is part of why chalk erodes so readily when waves undercut the cliffs, and it’s also why the exposed surface stays bright and fresh-looking. Wave action and weathering constantly strip away the outer layers, revealing unweathered white chalk beneath.
Why Calcite Looks White
Calcium carbonate in its pure form is white because it scatters visible light across the entire spectrum without selectively absorbing particular wavelengths. The chalk in the White Cliffs is extremely pure; by some estimates it is more than 95 percent calcium carbonate, with only minor amounts of clay, quartz, and flint nodules interspersed through it. When light hits the cliff face, it encounters an enormous number of tiny calcite surfaces at random orientations, and each surface reflects and scatters the light in all directions. The result is diffuse white reflectance, the same basic reason snow looks white despite being made of transparent ice crystals.
The microstructure of individual coccoliths contributes to this scattering in a specific way. Studies modeling how light interacts with detached coccoliths from the species Emiliania huxleyi found that the backscattering of light depends strongly on the shape of each particle, largely because light undergoes multiple reflections within the coccolith’s complex curved geometry.
5Limnology and Oceanography. Light scattering by nonspherical particles: Application to coccoliths detached from Emiliania huxleyiThis is why coccolithophore blooms in the modern ocean look milky turquoise from space: the loose coccoliths floating in the water scatter sunlight so effectively that they change the color of the sea surface. Compress and dry those same particles into a cliff face, and you get the brilliant white the Dover cliffs are famous for.
The Chalk as a Sponge
One property that surprises people is how porous chalk is. Despite being a solid rock, chalk in the English formations typically has very high porosity, sometimes exceeding 40 percent. All those gaps between coccolith fragments that never fully cemented? They remain as tiny pore spaces. But “porous” does not mean water moves through it easily. The pore throats connecting those spaces are extremely small, giving the chalk matrix a permeability of roughly 0.1 to 10 millidarcys, which in practical terms means water barely trickles through it.
6Geological Society, London, Special Publications. Fluid flow in the Chalk of EnglandWhat allows water to actually move through chalk aquifers is a separate network of fractures. The English Chalk forms what hydrogeologists call a dual-porosity aquifer: low-permeability matrix full of water that barely moves, threaded with high-permeability fissures that carry most of the flow. These fissures are predominantly stress-relief fractures that have been enlarged over time by dissolution, as slightly acidic groundwater widens the cracks it passes through.
7Geological Society, London, Special Publications. Development of spatial permeability variations in English Chalk aquifersThis dual nature matters because the chalk aquifer beneath southeast England is one of the region’s primary sources of drinking water. It holds a vast reserve of groundwater within its matrix pores, but you can only extract that water efficiently through the fractures. Understanding the interplay between the two is an active area of hydrogeological research.
What Happens When You See Bands of Flint
If you look at photographs of the Dover cliffs, you’ll notice dark horizontal lines running through the white face. Those are layers of flint, a form of silica (silicon dioxide) that is chemically unrelated to the calcite making up the chalk. Flint forms after the chalk is deposited, when silica dissolved in groundwater migrates through the porous rock and precipitates as hard nodules, often around organic remains like sponge skeletons or burrows. The flint bands are much harder than the surrounding chalk, so when the cliff erodes, flint nodules often tumble to the beach below, producing the smooth grey and black pebbles that litter the shoreline at the base of the cliffs.
Flint is relevant to the whiteness question because it’s the main impurity interrupting the chalk’s uniformity. Without flint, the cliffs would be an almost unbroken wall of white. As it stands, the flint layers make up only a small fraction of the total rock volume, so the overwhelming impression from a distance remains brilliant white.
Modern Coccolithophore Blooms Visible from Space
The same organisms whose remains built the Dover cliffs are still thriving in today’s oceans, and when conditions are right they bloom in enormous numbers. Satellite imagery shows that coccolithophore blooms cover vast areas of the sea surface each year, with average annual bloom area more than twice as large in the Southern Hemisphere compared to the Northern Hemisphere. One global estimate using multiple satellite sensors put the total average annual bloom area at roughly 2.75 million square kilometers, dominated by contributions from the Southern Ocean.
8Remote Sensing of Environment. Detection of coccolithophore blooms in ocean color satellite imagery: A generalized approach for use with multiple sensorsThese blooms are visible because the shed coccoliths suspended in the upper water column scatter light so strongly that they turn the sea surface a vivid milky blue-green. A consistently calibrated 40-year satellite record has been used to track regions affected by highly reflective blooms of Emiliania huxleyi, the dominant coccolith-producing species, revealing patterns of bloom occurrence over four decades.
9Earth System Science Data. A 40-year global data set of visible-channel remote-sensing reflectances and coccolithophore bloom occurrence derived from the Advanced Very High Resolution Radiometer catalogueIn the modern ocean, this coccolith material is still settling to the seafloor and slowly accumulating. But the rates are far too slow to build anything like the Dover cliffs in a human lifetime. The Cretaceous chalk took tens of millions of years to reach its current thickness. The conditions that made it possible, a warm global climate, high sea levels flooding continental shelves with shallow seas, and abundant nutrients supporting massive coccolithophore populations, persisted for an unusually long stretch of geological time.
Ocean Acidification and Future Coccolith Production
A question that concerns marine scientists is whether rising carbon dioxide levels will undermine the ability of coccolithophores to build their calcite armor. When COâ‚‚ dissolves in seawater, it lowers the pH, making the water more acidic. More acidic water makes it harder for organisms to precipitate calcium carbonate, and it can even dissolve existing shells and plates. The picture for coccolithophores turns out to be more complicated than a simple decline, though.
Earth system model simulations suggest that under moderate increases in COâ‚‚, coccolithophore calcification actually goes up in some regions because the extra carbon dioxide alleviates carbon limitation on growth. Global annual calcification was estimated to be about 6 percent higher under present-day COâ‚‚ levels compared to preindustrial conditions. But the trend reverses at higher concentrations: under roughly 900 parts per million of atmospheric COâ‚‚, global annual calcification dropped about 11 percent below preindustrial levels. The models point to a tipping point at approximately 600 parts per million, above which global calcification shifts from increasing to decreasing.
10Journal of Advances in Modeling Earth Systems. Coccolithophore Growth and Calcification in an Acidified Ocean: Insights From Community Earth System Model SimulationsThe nuance matters. Coccolithophores may actually become more abundant as COâ‚‚ rises, because they’re still photosynthesizers that benefit from more dissolved carbon. But they produce less calcite per cell, meaning they grow bigger populations of thinner-armored individuals. The net effect on how much calcium carbonate reaches the deep ocean depends on which trend wins. Lab experiments with the coastal species Pleurochrysis carterae found that acidification interacts with light levels: under low light, acidification reduced organic carbon production, but the negative effect faded as light intensity increased.
11PubMed Central. Ocean Acidification Affects the Response of the Coastal Coccolithophore Pleurochrysis carterae to IrradianceAtmospheric COâ‚‚ is currently around 425 parts per million and climbing. Whether it reaches that 600 ppm tipping point depends on emission trajectories over the coming decades. If it does, the organisms that built the White Cliffs of Dover would still exist but would be producing less of the raw material that made the cliffs possible. No new chalk cliffs would form on the Dover scale regardless, since the shallow epicontinental seas that covered the region during the Cretaceous no longer exist. But the health of coccolithophore calcification has big implications for the ocean’s carbon cycle and for marine food webs.
Climate Clues Locked Inside the Chalk
The chalk isn’t just a picturesque landmark; it’s a detailed climate archive. Because coccoliths are made of calcium carbonate, their chemical composition records the temperature and chemistry of the seawater in which they grew. By measuring the ratios of carbon and oxygen isotopes in chalk samples from southern England, scientists have reconstructed ocean temperature trends spanning millions of years of the Late Cretaceous. One detailed study of multiple coastal chalk sections found evidence of warming leading up to the boundary between the Cenomanian and Turonian stages, roughly 94 million years ago, followed by a long-term cooling trend.
12Geological Magazine. Carbon- and oxygen-isotope stratigraphy of the English Chalk and Italian Scaglia and its palaeoclimatic significanceThat warming interval corresponds to one of the hottest periods in the last 100 million years, when global temperatures were far higher than today and sea levels submerged vast areas of what is now dry land. The chalk itself is evidence of those conditions: it could only accumulate in such thickness because the sea covered southeast England for so long. As the climate shifted and seas eventually retreated, the chalk was uplifted and exposed, eventually reaching the English Channel coast where wave erosion carved the cliffs we see.
Why the Cliffs Keep Retreating
The White Cliffs are not a permanent feature. They are actively eroding, with the cliff edge retreating landward at an average rate estimated at several centimeters per year, though rates vary depending on the stretch of coastline, the intensity of storms, and the local geology. Erosion happens primarily at the base, where wave action undercuts the cliff by dissolving and abrading the soft chalk. Once enough material has been removed from below, the unsupported rock above collapses in dramatic rockfalls. After each collapse, fresh white chalk is exposed, which is part of why the cliffs maintain their striking appearance rather than graying over with age.
Chalk weathers relatively quickly on the surface because rainwater, which is naturally slightly acidic, slowly dissolves calcium carbonate. The high porosity of chalk means that water also penetrates into the rock, and cycles of freezing and thawing can crack it apart from within. Despite all this, the sheer thickness of the chalk formation, hundreds of meters in places, means the cliffs will persist for a long time yet. But any structure built too close to the cliff edge is eventually at risk, and the English coastline has a long history of buildings and land disappearing as the cliffs crumble.
The interplay between marine erosion at the base and the chalk’s own physical properties keeps the cliff face vertical and clean, which amplifies the visual drama. A cliff made of harder rock might erode more slowly but would also develop soil cover, vegetation, and staining. Chalk cliffs stay white precisely because they’re soft enough to constantly shed their outer skin, exposing fresh material underneath. The whiteness that draws visitors from around the world is, in a sense, a byproduct of the rock’s fragility.