The Chicxulub crater is an impact crater located beneath the Yucatán Peninsula in Mexico. Its center is underwater, but the crater is named after the nearby town of Chicxulub Pueblo, which should not be confused with the larger town of Chicxulub Puerto. The crater formed about 66 million years ago when an asteroid, approximately 10 kilometers (6 miles) wide, hit Earth. The crater is estimated to be 200 kilometers (120 miles) wide and lies about 1 kilometer (0.62 miles) beneath younger sedimentary rocks. It is one of the largest impact structures on Earth, along with the older Sudbury and Vredefort craters, and is the only one with a peak ring that remains intact and available for scientific study.
The crater was discovered by Antonio Camargo and Glen Penfield, scientists who were searching for oil in the Yucatán Peninsula during the late 1970s. Penfield initially could not prove the geological feature was a crater and stopped his search. Later, in 1990, Penfield worked with Alan R. Hildebrand and obtained samples that suggested the feature was caused by an impact. Evidence supporting the crater’s impact origin includes shocked quartz, a gravity anomaly, and tektites found in nearby areas.
The time of the impact matches the Cretaceous–Paleogene boundary (also called the K–Pg or K–T boundary). Scientists now agree that the damage and changes to Earth’s climate from the impact were the main causes of the Cretaceous–Paleogene extinction event. This event led to the extinction of about 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 levels 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 spread into the atmosphere when the asteroid hit, then settled across Earth’s surface, forming the clay layer. At the time, scientists had no agreement on what caused the extinction. Some theories included a nearby star explosion, climate changes, or shifts in Earth’s magnetic field. Many paleontologists disagreed with the impact theory, believing the lack of fossils near the K–Pg boundary suggested a slower extinction process.
In 1980, Walter Alvarez and others published a paper in Science about the high levels of iridium found in the clay layer. Around the same time, scientists in Spain also reported similar findings in Nature. These discoveries led to widespread interest in the cause of the extinction, with over 2,000 scientific papers published on the topic in the 1980s. No impact craters of the right age or size were known, so scientists searched for one. A meeting in Utah in 1981 was organized to discuss the issue. Unbeknownst to the organizers, evidence of the crater was presented at the same time but went unnoticed.
In 1978, geophysicists Glen Penfield and Antonio Camargo, working for a Mexican oil company, studied magnetic data from the Gulf of Mexico near the Yucatán Peninsula. Penfield noticed unusual patterns that suggested a large crater. He compared this data with older gravity maps and found a circular shape, 180 kilometers wide, that looked like an impact site. Earlier, another scientist had noticed this feature, but the company did not allow him to share his findings. Penfield shared his results with his employer, but they dismissed the idea, thinking the feature was caused by volcanic activity. He presented his findings at a conference, but few people attended, and the report received little attention. A journalist wrote about Penfield’s work, but the news did not spread widely.
Penfield had no physical evidence, like rock samples, to prove his theory. He learned that oil companies had drilled wells in the area in 1951. One well had reached a layer of andesite, a type of rock that could form from an asteroid impact, but it was mistaken for a lava dome. Penfield asked for samples from these wells but was told they were lost. Unable to find proof, he gave up and returned to his work. He later wrote to Walter Alvarez about the Yucatán structure, but Alvarez did not respond.
Alvarez and others continued searching for the crater, believing the impact had occurred in the ocean based on incorrect analysis of glassy spherules. Meanwhile, a graduate student named Alan Hildebrand and his adviser studied the Brazos River in Texas. They found clay with extra iridium, shocked quartz, and tektites—glass-like beads formed by impacts. They also found rock fragments scattered by an impact. Hildebrand suggested that a volcano in Haiti might be linked to an impact nearby. Tests later confirmed tektite glass from the K–Pg boundary.
In 1990, a journalist reminded Hildebrand about Penfield’s earlier discovery. Hildebrand contacted Penfield, and they obtained samples from old oil wells. Tests showed the samples contained materials changed by extreme pressure and heat, suggesting an impact. Scientists later used satellite images to find a ring of sinkholes near Chicxulub Pueblo, matching Penfield’s observations. The crater was found to be 300 kilometers wide, with the 180-kilometer ring as its inner wall. In 1991, scientists named the crater after the nearby town of Chicxulub Pueblo.
In 2010, 41 scientists reviewed evidence and concluded the Chicxulub impact caused the mass extinction. Some scientists, like Gerta Keller, argued that volcanic activity from the Deccan Traps in India might have played a role. Studies in 2013 compared the timing of the Chicxulub impact and volcanic eruptions, finding they occurred almost at the same time.
Impact specifics
A 2013 study in the journal Science estimated the age of the impact as about 66 million years ago, with some uncertainty. Scientists used multiple methods, including a type of dating called argon–argon dating, to analyze materials like tektites from Haiti and layers of rock in Montana. A 2015 study confirmed this date by examining similar materials in Montana. A 2018 study found a slightly different result, estimating the impact occurred about 66.05 million years ago. Scientists believe the impact happened during the Northern Hemisphere's spring season, based on clues in fish bones found in sediment layers in North Dakota. These layers formed very quickly after the impact, within hours.
At the time of the impact, the site of the crater was a shallow underwater rock formation called a marine carbonate platform. The water depth varied, with the deepest part near the center of the crater being about 650 meters (2,130 feet). The rocks below the surface were thick layers of ancient marine sediments, mostly made of carbonate rock like dolomite and limestone, along with some evaporites and smaller amounts of shale and sandstone. These layers rested on a thick layer of hard, igneous rock beneath the ocean floor.
The object that caused the impact was about 10 kilometers (6.2 miles) wide—larger than Mount Everest if placed at sea level. A 2021 study estimated it traveled at 20 kilometers per second (12 miles per second) and hit the Earth from the northeast. The impact released energy equal to 72 teratonnes of TNT, creating winds faster than 1,000 kilometers per hour (620 mph) and forming a temporary crater 100 kilometers (62 miles) wide and 30 kilometers (19 miles) deep. This crater later collapsed and was covered by about 1,000 meters (3,300 feet) of sediment. The impact also caused massive waves, some over 100 meters (330 feet) tall, which may have reached heights of up to 1.5 kilometers (0.93 miles). These waves left large ripples on the ocean floor, now found in Louisiana, and disturbed sediments as far as 6,000 kilometers (3,700 miles) away from the impact site. The event likely caused a powerful earthquake with a strength estimated between 9 and 11 on the moment magnitude scale.
A cloud of hot dust, ash, and steam spread from the crater, with as much as 25 trillion metric tons of material ejected into the atmosphere. Some of this material escaped Earth’s gravity and traveled through space, while other pieces fell back to Earth, burning up as they re-entered the atmosphere. The heat from the impact ignited wildfires that may have burned nearly 70% of the planet’s forests. Evidence of sudden extinction was found in a thin layer of soil in New Jersey, far from the impact site, showing that many animals died quickly and were buried by debris. Studies from North Dakota’s Hell Creek Formation show that many species went extinct at the same time, matching the effects of the impact.
The impact site had shallow water, so the rock that was vaporized included sulfur-rich gypsum. This sulfur was released into the atmosphere, causing a sudden drop in global temperatures and harming the food chain. Scientists believe the impact also created a large underground system of hot water that helped life recover over time. Later research using seismic images showed the impact occurred in deeper water than previously thought, which may have increased the amount of sulfur in the atmosphere, making the climate changes even more severe.
The dust and particles from the impact could have covered Earth for years, creating harsh conditions for life. The destruction of carbonate rocks also released carbon dioxide, leading to a temporary greenhouse effect. For many years, sunlight would have been blocked by dust in the atmosphere, cooling the planet and stopping plants from photosynthesizing. Over time, high carbon dioxide levels might have increased plant growth.
A long-term effect of the impact was the creation of the Yucatán sedimentary basin, which later helped humans settle in an area where water is scarce.
Post-discovery investigations
Two seismic reflection data collections have been gathered over the offshore area of the crater since its discovery. Older 2D seismic data, originally used for oil and gas exploration, have also been included. In October 1996, a group called BIRPS collected three long 2D seismic lines covering 650 kilometers (400 miles). The longest line, named Chicx-A, was positioned parallel to the coast, while the other two lines, Chicx-B and Chicx-C, were aligned northwest to southeast and southwest to northeast, respectively. In addition to standard seismic reflection imaging, data was recorded on land to study how seismic waves travel through different layers of rock.
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 create 3D models of the crater’s velocity structure. The data focused on the offshore peak ring to help identify potential drilling sites. At the same time, gravity data was collected along 7,638 kilometers (4,746 miles) of profiles. This work was funded by the National Science Foundation (NSF) and the Natural Environment Research Council (NERC), with support from the National Autonomous University of Mexico (UNAM) and the Centro de Investigación Científica de Yucatán (CICY).
Core samples from oil and gas exploration wells drilled by Pemex on the Yucatán peninsula have provided some useful information. In 1995, UNAM drilled eight fully-cored boreholes, three of which reached deep enough to sample ejecta deposits outside the main crater rim (UNAM-5, 6, and 7). Between 2001 and 2002, a scientific borehole named 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 borehole was continuously cored and passed through 100 meters (330 feet) of impactite rock. 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 the United States collected the first offshore core samples from the peak ring in the central part of the crater. This was done during drilling of a borehole called M0077A, as part of Expedition 364 of the International Ocean Discovery Program. The borehole 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 found 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 that slope downward toward the center, showing the farthest point where the Earth's crust was deformed. This makes the Chicxulub crater one of the three largest impact craters on Earth.
Closer to the center, the next ring is the main crater rim, also called the "inner rim." This ring matches a ring of cenotes on land and a major circular gravity change. The size of this ring ranges from 70 to 85 kilometers (43 to 53 miles) in radius. Inside this ring is the peak ring. The area between the inner rim and the peak ring is called the "terrace zone." This area has many fault blocks, which are pieces of rock tilted toward the center of the crater, sometimes called "slump blocks."
The peak ring has a diameter of about 80 kilometers and varies in height. 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. The center of the crater is located above a region where Earth's mantle was pushed upward, making the Mohorovičić discontinuity, a boundary between Earth's crust and mantle, about 1–2 kilometers (0.6–1.2 miles) shallower than nearby areas.
The ring structures are most clearly visible to the south, west, and northwest. They become less clear toward the north and northeast. This is believed to be caused by differences in water depth when the crater formed. Areas where the water was deeper than 100 meters (330 feet) show less-defined rings.
Geology
Before the impact, the geology of the Yucatán area, sometimes called "target rocks," included a series of mainly Cretaceous limestones. These limestones were above red beds of uncertain age, which rested on an unconformity with the underlying granitic basement. The basement is part of the Maya Block, and information about its composition and age in the Yucatán area has come only from drilling near the Chicxulub crater and from analysis of basement material found in ejecta at distant K–Pg boundary sites. 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 late Ediacaran arc-related igneous rocks, which formed during the Pan-African orogeny. Late Paleozoic granitoids, known as "pink granite," were found in the peak ring borehole M0077A. These granitoids are estimated to be about 326 ± 5 million years old (Carboniferous) and have an adakitic composition. They are thought to have formed due to slab detachment during the Marathon-Ouachita orogeny, which was part of the collision between Laurentia and Gondwana that created the Pangaea supercontinent.
Red beds of varying thickness, up to 115 meters (377 ft), cover the granitic basement, especially in the southern part of the area. These continental clastic rocks are likely Triassic- to Jurassic-age, though they may extend into the Lower Cretaceous. The lower part of the Lower Cretaceous sequence includes dolomite with interbedded anhydrite and gypsum, while the upper part is limestone with some dolomite and anhydrite. The thickness of the Lower Cretaceous layer ranges from 750 meters (2,460 ft) to 1,675 meters (5,495 ft) in boreholes. The Upper Cretaceous sequence is mainly platform limestone, with marl and interbedded anhydrite. Its thickness varies from 600 meters (2,000 ft) to 1,200 meters (3,900 ft). Evidence suggests a Cretaceous basin called the Yucatán Trough exists in the region, running approximately south–north and widening toward the north, which explains the observed thickness variations.
The most common impact rocks found are suevites, which are present in many boreholes around the Chicxulub crater. Most suevites were resedimented shortly after the impact when ocean water flooded the crater, creating a layer of suevite that extends from the inner crater to the outer rim. Impact melt rocks are believed to fill the central part of the crater, with a maximum thickness of 3 kilometers (1.9 mi). These melt rocks have compositions similar to the 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), which contains a clear carbonate component, and a lower impact melt-bearing unit (LIMB), which lacks carbonate. The difference between these two types is thought to result from the upper part of the initial melt mixing with materials from the shallow crust, either falling back into the crater or being brought back by water resurgence to form the UIM.
The "pink granite," a granitoid rich in alkali feldspar found in the peak ring borehole, shows many deformation features that record the extreme forces involved in forming the crater and the peak ring. This granitoid has an unusually low density and P-wave velocity compared to typical granitic basement rocks. Core samples from M0077A reveal deformation features in the following order: pervasive fracturing along grain boundaries, a high density of shear faults, bands of cataclasite and ultra-cataclasite, and some ductile shear structures. This sequence is interpreted to result from initial crater formation involving 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 massive hydrothermal system that altered approximately 1.4 × 10 km of Earth's crust and lasted for hundreds of thousands of years. These systems may support the impact origin of life hypothesis for the Hadean eon, a time when Earth's surface was frequently struck by large impactors.
After the immediate effects of the impact, sedimentation in the Chicxulub area returned to a shallow water platform carbonate environment, similar to what existed before the impact. The sequence, dating back to the Paleocene, includes marl and limestone, reaching a thickness of about 1,000 meters (3,300 ft). The K–Pg boundary inside 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 preferentially from a recharge zone in the south to the coast through a karstic aquifer system. The location of cenotes is clearly linked to the underlying crater rim, possibly due to higher fracturing caused by 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 originally 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. This sediment layer included material from the time of the Cretaceous–Paleogene (K–Pg) boundary, with the meteorite located at the base of an iridium-rich layer. Scientists suggested the meteorite might be a piece of the Chicxulub impactor. Analysis showed it matched the characteristics of CV, CO, or CR groups of carbonaceous chondrites. A 2021 study used chemical evidence, such as chromium isotope ratios and platinum group metal levels in marine impact layers, to support that the impactor matched CM or CR carbonaceous chondrites. Ruthenium isotope ratios in impact layers also support a carbonaceous chondrite origin.
In 2007, a study in Nature proposed that the Chicxulub asteroid came from a collision in the asteroid belt 160 million years ago. The collision involved a 170 km (110 mi) diameter object and a 60 km (37 mi) diameter object, creating the Baptistina family of asteroids. The largest member of this family is asteroid 298 Baptistina. The study suggested the Chicxulub asteroid was also part of this group. However, later evidence disproved this idea. A 2009 analysis showed that 298 Baptistina has a composition more 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 processes that take much longer. In 2010, another hypothesis linked the asteroid 354P/LINEAR, part of the Flora family, to the K–Pg impactor. A 2021 simulation suggested the impactor likely came from the outer part of the asteroid belt.
Some scientists have argued the impactor was a comet, not an asteroid. Two 1984 studies proposed it originated from the Oort cloud. A 1992 study suggested tidal forces could increase comet impacts. In 2021, Avi Loeb and a colleague proposed in Scientific Reports that the impactor was a fragment from a disrupted comet. However, a rebuttal in Astronomy & Geophysics argued that the amount of iridium found globally (2.0 × 10⁻².8 × 10 kg or 4.4 × 10⁻6.2 × 10 lb) was too large for a comet of the size implied by the crater. They also said the comet impact rate was overestimated. 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.