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When an asteroid strikes Earth, its energy produces a rapid shock that compresses and heats rock, fractures and displaces the ground, and excavates a crater. Material thrown out becomes ejecta; some rock melts and mixes with broken target rock. As the crater changes shape, minerals preserve evidence of the pressure and heat long after the event.
How impact energy starts a crater
An impact crater forms when a fast-moving asteroid or meteorite collides with a larger solid body. The collision deposits energy quickly, so it is not simply a projectile punching a hole. Shock waves travel through the target and the incoming object, producing intense pressure and heat. NASA explains that this shock can melt and recrystallize ground rock (NASA Space Place).
As the target fractures, material is driven outward and upward. This displacement excavates a cavity, while some of the material is thrown beyond the developing crater. The balance of shock, heating, fragmentation, and excavation depends on the impact and the target; a single example should not be treated as a universal outcome.
How an excavated cavity becomes a crater
Crater formation includes an excavation phase and a later modification phase. Excavation creates a transient cavity as target material moves away. The rim and crater floor can then continue to move, including through gravitational rebound.
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A 1987 simulation by D. J. Roddy and colleagues modeled a 10-kilometer asteroid striking either a continental or an oceanic target. In those modeled cases, shock fronts in the air preceded much of the crater and ejecta formation, and target material and rim uplift entered the hot, low-density air above the cavity before the craters underwent rebound. These are results for that simulation, not a timetable or set of dimensions that applies to every impact (Roddy et al., 1987, indexed by USGS).
What one large-impact simulation illustrates
The model specified a vertical impact speed of 20 kilometers per second and kinetic energy of 2.6 × 1030 ergs, equivalent in the report to 6.2 × 107 megatons of TNT. Its scale and assumptions matter when interpreting the figures:
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- At about 29 seconds, the modeled transient craters were about 62 kilometers across and 39 kilometers deep; the rim reached nearly 40 kilometers altitude around 30 seconds.
- At 120 seconds, the continental and oceanic modeled transient craters were about 80 and 105 kilometers across, respectively, with depths around 27 kilometers.
- By 60 seconds, the modeled continental case had ejected about 2 × 1014 tonnes, roughly twice the ejecta mass in the modeled oceanic case.
These figures describe two particular modeled targets and one specified large impact. They are useful for showing that target setting can change modeled outcomes, not for predicting a typical crater.
What ejecta and impact melt contain
Ejecta are material expelled from the developing crater. They consist chiefly of excavated target material, though some impactor material can be incorporated. At Meteor Crater, the USGS describes impact melt made from melted target rock and projectile material and transported with fractured target rock, mixing with rock that experienced varying degrees of shock.
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That mixture can leave several kinds of evidence near a crater, including glassy ballistic particles, metallic spherules, impact-melt breccias, and sheets or intrusions of melt called dikes and sills. Ejecta deposits can therefore preserve clues about both the impact process and the geology of the target (USGS Open-File Report 2024-1032).
How shocked rock records the event
Shocked rock is target rock whose minerals record the rapid pressure and temperature pulse of an impact. The evidence is microscopic as well as visible in the rock’s composition: USGS lists closely spaced planar microstructures in minerals, solid-state glass in silica and plagioclase, and high-pressure forms of silica such as coesite and stishovite among documented shock effects (USGS Professional Paper 575-D).
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Meteor Crater’s quartz-bearing target rocks show a range of effects, from fractured quartz to high-pressure silica phases and shock-melted glass. A specimen need not show every feature, and no single clue should automatically be treated as conclusive. Geologists interpret mineral evidence in the context of the rock and the site. NASA’s educational overview also describes impact effects and glass droplets (NASA Johnson Space Center).
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A real-world scale example: Meteor Crater
NASA Space Place says Meteor Crater formed about 50,000 years ago. The impactor may have been up to about 150 feet wide and traveling more than 28,000 miles per hour, according to NASA’s page; these are estimates for that example, not a general threshold for crater formation (NASA Space Place).
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