Controlled Assessment – Impact Craters
Part 1 - Research
1. Meaning of the terms asteroid and meteorite
Asteroid – A small rocky object that orbits the sun and can vary greatly in its size. These can be found in large numbers between the orbits of Mars and Jupiter (known as the asteroid belt) with the exception of some asteroids that have more irregular orbits.1
Meteorite – A piece of rock or metal that fell to the Earth’s surface as a meteor from outer space. Majority of meteorites consist of only rock but very few meteorites partially or fully contain iron and nickel.2
2. How impact craters are formed
Impact craters form from a process known as cratering. An impactor (such as a meteor) hits a target (such as the Earth) and releases a massive amount of kinetic energy (which depends on the mass of the impactor and its velocity). However, the energy from the impact will not be lost but transferred between the two bodies because energy is contained when two bodies strike each other. The crater formation then proceeds in three consecutive stages:
1. Compression Stage: The impactor creates a relatively small hole on the target which releases a shock wave. The energy from the impactor is converted into heat and electric energy in the target along with the generation of great pressure. Very little material is ejected from the forming crater at this stage with the exception of a plume of vapour generated by the impact which expands above the crater. This stage is relatively quick and can be calculated by this formula:
Time = Diameter (of impactor)/Speed at impact OR T= D/V
2. Excavation Stage: The shock wave continues to spread outwards through the material. The wave spreads upwards due to originating in a point below the surface of impact and causes some of the material to be ejected up and out of the impact surface. This material is known as “ejecta” and can eventually form an ejecta blanket around the impact site after spreading. The crater itself grows very large rapidly during this stage and material at the lip of the crater folds and creates a rim. This stage takes longer than the compression stage. 3. Modification Stage: Loose debris from the impact tend to slide down the crater walls and terraces form along the crater’s sides due to some loosened material slipping into sheets. In some cases, a central peak forms due to target material splashing back upwards from the point of impact. This last stage of cratering takes approximately the same amount of time as the excavation stage but further modifications on the impact crater may occur due to erosion, lava flow, later impacts, and tectonic activity.3
3. The dimensions of the Barringer Crater (diameter and depth)
Diameter: The Barringer crater has a diameter of about 1 mile. This can be converted to about 1.61 kilometres
Depth: The crater’s depth can be measured to about 570 feet deep or 173.74 meters (2 d.p.).4
4. What determines the size of an impact crater?
The size, mass, speed, and angle of the impactor determine the size of the impact crater. Small, slow-moving impactors have low impact energy and cause small craters. Large, fast-moving impactors release a lot of energy and therefore form large craters. Therefore, the faster and larger the impactor, the larger the crater (or vice versa). Very large impacts can even cause secondary craters, as ejected material falls back to the ground which form new, smaller craters.5 6
5. The energy changes that happen when impact craters are formed
When the formation of an impact crater begins, a large amount of kinetic energy is released from the impactor which is transferred between the two bodies because energy is contained when two bodies strike each other. The energy from the impactor is then converted into heat and kinetic energy in the target. The heat energy melts and vaporises some of the target rock whereas the kinetic energy is used for the fracturing and displacement of the rock until the impact crater is eventually
