The Chicxulub crater is a large impact crater located beneath the Yucatán Peninsula in Mexico. Its center is under the ocean, but the crater is named after a nearby town called Chicxulub Pueblo, which is different from the larger town of Chicxulub Puerto. The crater formed about 66 million years ago when an asteroid, approximately ten kilometers (six miles) wide, hit Earth. The crater is estimated to be 200 kilometers (120 miles) wide and is buried about 1 kilometer (0.62 miles) under younger rock layers. It is one of the largest impact structures on Earth, along with the older Sudbury and Vredefort craters. It is the only crater with a peak ring that remains intact and available for scientific study.
The crater was discovered by Antonio Camargo and Glen Penfield, geophysicists searching for oil in the Yucatán Peninsula during the late 1970s. Penfield could not find proof it was a crater at first and stopped his search. Later, in 1990, he contacted Alan R. Hildebrand and received samples that showed the feature was caused by an impact. Evidence supporting this includes shocked quartz, an unusual gravity reading, and tektites found nearby.
The time of the impact matches the Cretaceous–Paleogene boundary, also called the K–Pg or K–T boundary. Scientists now believe the damage and climate changes from the impact were the main causes of the Cretaceous–Paleogene extinction event. This event led to the extinction of 75% of Earth’s plant and animal species, including all non-avian dinosaurs.
Discovery
In the late 1970s, geologist Walter Alvarez and his father, Luis Walter Alvarez, a Nobel Prize-winning scientist, proposed a theory that the Cretaceous–Paleogene extinction was caused by an asteroid impact. Their main evidence was a thin layer of clay found at the Cretaceous–Paleogene boundary (K–Pg boundary) in Gubbio, Italy. This layer contained unusually high amounts of iridium, a rare element on Earth but common in asteroids. Iridium levels in this layer were up to 160 times higher than normal. Scientists thought the iridium was spread into the atmosphere when the asteroid hit Earth, then settled across the planet’s surface, creating the layer of iridium-rich clay. At the time, scientists had no agreement on what caused the extinction. Some theories included a nearby star explosion, climate change, or changes in Earth’s magnetic field. Many paleontologists disagreed with the Alvarez impact theory, believing that the lack of fossils near the K–Pg boundary suggested a slower extinction process.
In 1980, Walter Alvarez and his colleagues, Frank Asaro and Helen Michel from the University of California, Berkeley, published a paper in Science about the high levels of iridium found in the K–Pg boundary. Around the same time, scientists Jan Smit and Jan Hertogen published similar findings in Nature from Caravaca, Spain. These discoveries led to many other studies showing iridium spikes at the K–Pg boundary worldwide, sparking widespread interest in the cause of the extinction. Over 2,000 papers were written about the topic in the 1980s. Scientists searched for an impact crater of the right age and size but found none. In 1981, Lee Hunt and Lee Silver organized a meeting in Snowbird, Utah, to bring together experts from different fields to study the issue. Unbeknownst to them, evidence of the crater they were looking for was presented at the same time but went largely unnoticed.
In 1978, geophysicists Glen Penfield and Antonio Camargo worked for Pemex, a Mexican oil company, conducting a magnetic survey of the Gulf of Mexico near the Yucatán Peninsula. Penfield used data to find possible oil drilling sites. He noticed unusual magnetic patterns and compared them with older gravity data from the 1940s. When the data was matched, Penfield saw a circular shape, 180 kilometers (110 miles) wide, suggesting an impact crater. A decade earlier, another scientist had seen similar patterns but could not share his findings due to company rules. Penfield shared his discovery with Pemex, but they dismissed the crater idea, believing the feature was caused by volcanic activity. Pemex allowed Penfield and Camargo to present their findings at a conference, but the event had few attendees, and their report received little attention. A journalist, Carlos Byars, wrote about their discovery, but it did not gain much attention.
Penfield had geophysical data but no physical evidence like rock samples to support his theory. He knew Pemex had drilled wells in the area. One well from 1951 had found a thick layer of andesite, a type of rock that could form from an impact, but it was thought to be a lava dome. Penfield asked for samples but was told they had been lost. Unable to find proof, he gave up and returned to his work. He wrote to Walter Alvarez about the Yucatán structure, but Alvarez did not respond.
Alvarez and others continued searching for the crater, mistakenly believing the impact had occurred in the ocean based on glassy spherules found at the K–Pg boundary. In Texas, graduate student Alan Hildebrand and his adviser William Boynton looked for a crater near the Brazos River. They found greenish-brown clay with extra iridium, shocked quartz, and small glass beads that looked like tektites. They also found jumbled rock deposits, which could have been thrown by an impact. Hildebrand suggested that a volcano on Haiti might be linked to a nearby impact. Tests showed tektite glass, which forms only from asteroid impacts or nuclear explosions.
In 1990, Carlos Byars told Hildebrand about Penfield’s earlier discovery. Hildebrand contacted Penfield, and they obtained samples from Pemex wells stored in New Orleans. Tests showed shock-metamorphic materials, confirming an impact. Researchers using satellite images found a ring of sinkholes near Chicxulub Pueblo, matching Penfield’s earlier findings. The crater is now known to be 300 kilometers (190 miles) wide, with the 180-kilometer ring being its inner wall. In 1991, scientists named the crater after the nearby town of Chicxulub Pueblo.
In 2010, 41 experts reviewed evidence from many fields and concluded that the Chicxulub impact caused the K–Pg extinction. Some scientists, like Gerta Keller, argue that volcanic eruptions in India, called the Deccan Traps, may have played a role. These eruptions occurred before and after the impact. A 2013 study compared the ages of impact glass from Chicxulub and volcanic ash from the K–Pg boundary, finding they were nearly the same age.
Impact specifics
A 2013 study in the journal Science estimated the age of the impact as 66,043,000 years ago, with a possible error of 11,000 years. This estimate considered other errors, which could add up to 43,000 years. Scientists used several methods, including argon–argon dating of tektites from Haiti and bentonite layers above the impact site in northeastern Montana. A 2015 study supported this date by using argon–argon dating of tephra found in lignite beds in the Hell Creek and Fort Union formations in northeastern Montana. A 2018 study, which used argon–argon dating of spherules from Gorgonilla Island, Colombia, found a slightly different date of 66,051,000 years ago, with a possible error of 31,000 years. Scientists believe the impact happened during the Northern Hemisphere’s spring, based on isotope patterns in fish bones found at the Tanis site in North Dakota. This sediment layer formed quickly after the impact.
At the time of the impact, the crater site was a marine carbonate platform. Water depth varied from 100 meters (330 feet) on the western edge to over 1,200 meters (3,900 feet) on the northeastern edge, with an estimated depth of 650 meters (2,130 feet) at the center. The seafloor had layers of Jurassic–Cretaceous marine sediments, about 3 kilometers (1.9 miles) thick. These sediments included mostly carbonate rocks like dolomite (35–40%) and limestone (25–30%), along with evaporites (anhydrite, 25–30%) and smaller amounts of shale and sandstone (3–4%). These layers rested on about 35 kilometers (22 miles) of continental crust, including igneous rocks like granite.
The impactor was about 10 kilometers (6.2 miles) wide. If placed at sea level, it would have been taller than Mount Everest. A 2021 study estimated the impactor’s speed as 20 kilometers per second (12 miles per second) and its angle of impact as 45–60 degrees from horizontal, coming from the northeast. The impact released energy equal to 72 teratonnes of TNT (300 ZJ). Winds near the blast’s center reached over 1,000 kilometers per hour (620 miles per hour). The impact created a temporary crater 100 kilometers (62 miles) wide and 30 kilometers (19 miles) deep, which later collapsed. This crater is now under the sea and covered by about 1,000 meters (3,300 feet) of sediment. The impact caused megatsunamis over 100 meters (330 feet) tall, with one model suggesting waves may have reached 1.5 kilometers (0.93 miles) high. These waves left ripples on the seafloor, some as large as 600 meters (2,000 feet) long and 16 meters (52 feet) high. Material from the impact and earthquakes may have reached Texas and Florida, with effects as far as 6,000 kilometers (3,700 miles) from the impact site. The impact caused a seismic event with a magnitude of 9–11.
A cloud of hot dust, ash, and steam spread from the crater. The blast ejected about 25 trillion metric tons of material into the atmosphere. Some of this material escaped Earth’s orbit, while some fell back to Earth and vaporized upon re-entry. The heat from the impact started wildfires that may have burned nearly 70% of the planet’s forests. Fossil evidence in New Jersey shows that many animals died suddenly and were buried quickly by debris, even though the site was 2,500 kilometers (1,600 miles) away. Studies from the Hell Creek Formation in North Dakota show that many species went extinct at the same time, with features matching the impact event.
The impact site had shallow water, so sulfur-rich gypsum from the Cretaceous layer was vaporized and released into the atmosphere. This caused global climate changes, including sudden temperature drops and harm to the food chain. Scientists believe the impact also created a subsurface hydrothermal system that helped life recover. In 2008, seismic images showed the impactor landed in deeper water than previously thought, which may have increased sulfate aerosols in the atmosphere. This could have made the climate cooling worse and caused acid rain.
Dust and particles from the impact may have covered Earth for years, creating harsh conditions for life. The destruction of carbonate rocks released carbon dioxide, causing a greenhouse effect. For a decade or more, dust blocked sunlight, cooling Earth’s surface and stopping photosynthesis. A model by Lomax et al. (2001) suggests that long-term carbon dioxide levels may have increased plant growth.
A long-term effect of the impact was the creation of the Yucatán sedimentary basin. This basin eventually created conditions that helped humans settle in an area where water is scarce today.
Post-discovery investigations
Scientists have collected two sets of seismic data over the offshore area of the crater since its discovery. Older 2D seismic data, originally used for finding oil and gas, has also been used. In October 1996, a group called BIRPS collected three long 2D seismic lines, totaling 650 kilometers (400 miles). The longest line, called Chicx-A, was aligned parallel to the coast, while Chicx-B and Chicx-C ran northwest to southeast and southwest to northeast, respectively. In addition to regular seismic imaging, scientists recorded data on land to create wide-angle refraction images.
In 2005, another set of seismic profiles was collected, increasing the total length of 2D deep-penetration seismic data to 2,470 kilometers (1,530 miles). This survey used ocean bottom seismometers and land stations to help create a 3D map of the crater’s structure. The data focused on the offshore peak ring to identify possible drilling locations. At the same time, gravity data was collected along 7,638 kilometers (4,746 miles) of profiles. This work was supported by the National Science Foundation (NSF) and the Natural Environment Research Council (NERC), with help from the National Autonomous University of Mexico (UNAM) and the Centro de Investigación Científica de Yucatán (CICY).
Some core samples from oil and gas exploration wells drilled by Pemex on the Yucatán peninsula have provided helpful information. In 1995, UNAM drilled eight fully-cored boreholes, three of which reached the ejecta deposits outside the crater’s edge (UNAM-5, 6, and 7). Between 2001 and 2002, a scientific borehole called Yaxcopoil-1 (or Yax-1) was drilled near Hacienda Yaxcopoil to a depth of 1,511 meters (4,957 feet) as part of the International Continental Scientific Drilling Program. This hole was cored continuously and passed through 100 meters (330 feet) of impactites. Three fully-cored boreholes were also drilled by the Comisión Federal de Electricidad (Federal Electricity Commission) with UNAM. One of these, BEV-4, reached the ejecta deposits.
In 2016, a team from the United Kingdom and United States collected the first offshore core samples from the peak ring in the crater’s central area. This borehole, called M0077A, was part of Expedition 364 of the International Ocean Discovery Program. It reached 1,335 meters (4,380 feet) below the seafloor.
Morphology
The shape and structure of the Chicxulub crater are mainly studied using geophysical data. It has a clear, circular structure with multiple rings. The outermost ring was discovered using seismic reflection data. This ring is up to 130 kilometers (81 miles) from the center of the crater and is made of normal faults, which slope downward toward the center. These faults mark the farthest point where the Earth's crust was significantly deformed. This makes the Chicxulub crater one of the three largest impact structures on Earth.
Closer to the center, the next ring is the main crater rim, also called the "inner rim." This ring matches a group of sinkholes on land and a major circular change in gravity measurements. The size of this ring ranges from 70 to 85 kilometers (43 to 53 miles) in radius. Inside this ring is the peak ring, which is about 80 kilometers wide. Its height varies: it is 400 to 600 meters (1,300 to 2,000 feet) above the crater’s base in the west and northwest, and 200 to 300 meters (660 to 980 feet) in the north, northeast, and east.
Between the inner rim and the peak ring lies the "terrace zone," which is made of large blocks of rock separated by normal faults that slope toward the crater’s center. These blocks are sometimes called "slump blocks." The center of the crater is located above an area where Earth’s mantle was pushed upward, making the boundary between the crust and mantle (called the Mohorovičić discontinuity) about 1 to 2 kilometers (0.6 to 1.2 miles) closer to the surface than usual.
The ring structures are most clearly visible to the south, west, and northwest. They become less clear toward the north and northeast. Scientists believe this is because the ocean was deeper in those areas when the impact occurred, making the rings harder to see.
Geology
Before the impact, the geology of the Yucatán area, sometimes called "target rocks," included layers of limestone from the Cretaceous period. These limestones were above red-colored rocks of unknown age, which rested on a layer of mostly granitic basement rock. The basement rock is part of the Maya Block, and details about its composition and age in the Yucatán area have been studied mainly through drilling near the Chicxulub crater and by analyzing basement material found in rocks far from the crater. The Maya Block is one of several crustal blocks found at the edge of the Gondwana continent. Zircon ages suggest the presence of an older Grenville-age crust, along with large amounts of igneous rocks formed during the Pan-African orogeny. Late Paleozoic granitoids, known as "pink granite," were found in the M0077A borehole, with an estimated age of 326 ± 5 million years (Carboniferous). These rocks have an adakitic composition and are thought to form from the detachment of a tectonic slab during the Marathon-Ouachita orogeny, part of the collision between Laurentia and Gondwana that created the Pangaea supercontinent.
Red-colored rocks of varying thickness, up to 115 meters (377 feet), covered the granitic basement, especially in the southern part of the area. These continental clastic rocks are believed to be from the Triassic to Jurassic periods, though they may extend into the Lower Cretaceous. The lower part of the Lower Cretaceous sequence includes dolomite with layers of anhydrite and gypsum, while the upper part consists of limestone with some dolomite and anhydrite. The Lower Cretaceous layer ranges in thickness from 750 meters (2,460 feet) to 1,675 meters (5,495 feet) in boreholes. The Upper Cretaceous sequence is mainly platform limestone, with marl and anhydrite layers. It varies in thickness from 600 meters (2,000 feet) to 1,200 meters (3,900 feet). Evidence suggests a Cretaceous basin called the Yucatán Trough, which runs roughly south to north and widens toward the north, explaining the differences in rock thickness.
The most common impact rocks found in the area are suevites, which are present in many boreholes around the Chicxulub crater. Most suevites were resedimented soon after the impact when ocean water flowed back into the crater, creating a layer of suevite that extends from the crater’s center to its outer edge. Impact melt rocks are thought to fill the crater’s central area, reaching a maximum thickness of 3 kilometers (1.9 miles). These melt rocks have compositions similar to basement rocks, with some evidence of mixing with carbonate materials from Cretaceous carbonates. Analysis of melt rocks from the M0077A borehole shows two types: an upper impact melt (UIM) with a clear carbonate component, and a lower impact melt-bearing unit (LIMB) without carbonate. The difference between these layers is believed to result from the upper melt mixing with materials from the shallow crust, either falling back into the crater or being brought back by water resurgence.
The "pink granite," a granitoid rich in alkali feldspar found in the peak ring borehole, shows signs of extreme deformation caused by the crater’s formation and the development of the peak ring. This granitoid has a lower density and P-wave velocity compared to typical granitic basement rocks. Core samples from M0077A reveal deformation features in the following order: widespread fracturing along grain boundaries, many shear faults, bands of cataclasite and ultra-cataclasite, and some ductile shear structures. This sequence is interpreted as resulting from initial crater formation through acoustic fluidization, followed by shear faulting and the development of cataclasites with fault zones containing impact melts.
Drilling below the seafloor also uncovered evidence of a large hydrothermal system that altered approximately 1.4 × 10 km of Earth’s crust over hundreds of thousands of years. These systems may support the theory that impacts contributed to the origin of life during the Hadean eon, when Earth’s surface was frequently hit by large impactors.
After the immediate effects of the impact, sedimentation in the Chicxulub area returned to its pre-impact shallow water carbonate environment. The rock layers, dating back to the Paleocene, include marl and limestone, reaching a thickness of about 1,000 meters (3,300 feet). The K–Pg boundary within the crater is much deeper than in surrounding areas.
On the Yucatán peninsula, the inner rim of the crater is marked by clusters of cenotes, which are surface features showing where groundwater flows through a karstic aquifer system. These cenotes are linked to the underlying crater rim, possibly due to increased fracturing from differential compaction.
Astronomical origin and type of impactor
Most scientists agree that the Chicxulub impactor was a C-type asteroid with a composition similar to carbonaceous chondrites, not a comet. These asteroids formed in the outer Solar System, beyond Jupiter’s orbit. In 1998, a meteorite about 2.5 millimeters (1/8 inch) in size was found in a deep sea sediment core from the North Pacific. The sediment layer dated to the Cretaceous–Paleogene boundary, with the meteorite located at the base of the K-Pg boundary iridium anomaly. Scientists suggested this meteorite might be a fragment of the Chicxulub impactor. Analysis showed it matched the characteristics of the CV, CO, and CR groups of carbonaceous chondrites. A 2021 study used geochemical evidence, including chromium isotope ratios and platinum group metal ratios in marine impact layers, to conclude the impactor matched CM or CR carbonaceous chondrites. Ruthenium isotope ratios in impact layers also support a carbonaceous chondrite composition.
A 2007 study in Nature proposed that the Chicxulub asteroid originated from a collision in the asteroid belt 160 million years ago. The researchers, William F. Bottke, David Vokrouhlický, and David Nesvorný, suggested a 170 km (110 mi) diameter parent body collided with another 60 km (37 mi) diameter body, creating the Baptistina family of asteroids. The largest member of this family is 298 Baptistina, and they claimed the Chicxulub asteroid was part of this group. Later evidence disproved this theory. A 2009 spectrographic analysis found 298 Baptistina had a composition typical of S-type asteroids, not carbonaceous chondrites. In 2011, data from the Wide-field Infrared Survey Explorer revised the collision’s date to about 80 million years ago, leaving only 15 million years for gravitational interactions and collisions, which typically take much longer. In 2010, another hypothesis linked the asteroid 354P/LINEAR, a member of the Flora family, to the K–Pg impactor. A 2021 simulation study suggested the impactor likely originated in the outer main part of the asteroid belt.
Some scientists have argued the impactor was a comet. Two 1984 studies proposed it originated from the Oort cloud. In 1992, researchers suggested tidal disruption of comets could increase impact rates. In 2021, Avi Loeb and a colleague proposed in Scientific Reports that the impactor was a fragment from a disrupted comet. A rebuttal in Astronomy & Geophysics countered that the amount of iridium deposited globally, 2.0 × 10⁻².⁸ × 10 kg (4.4 × 10⁻⁶.² × 10 lb), was too large for a comet of the size implied by the crater. They also noted Loeb et al. overestimated comet impact rates. The rebuttal concluded that all evidence strongly supports an asteroid impactor, ruling out a comet. Ruthenium isotope ratios in impact layers also support an asteroid origin.