The high level of endogenic geological activity makes the terrestrial record of impact difficult to read. Terrestrial processes, such as erosion, rapidly modify craterforms and ultimately remove the evidence of impact. In their largely uneroded states, terrestrial impact structures have the basic so-called simple and complex forms observed on other planetary bodies, but few of them have morphometric parameters, such as apparent and true depth and stratigraphic uplift, that can be defined. Erosion severely affects such parameters, and can even result in a positive topographic form due to differential erosion. The principal criterion for the recognition of terrestrial impact structures is, therefore, not their form, but the occurrence of shock-metamorphic effects. These are well-documented and are described briefly. In parautochthonous target lithologies, they are limited to the central portion of the original crater floor, and attenuate radially and with depth. Shock effects also occur in allochthonous lithologies, such as breccias and impact-melt rocks. In addition to a characteristic geological signature, terrestrial impact structures have characteristic geophysical signatures. The most common is a Bouguer gravity low, which extends out to the rim. The low is due to impact-induced brecciation and fracturing . It increases in value with increasing size, reaching a limiting value of ~300 µm s-2 (~300 g.u.). In large impact structures, it can be accompanied by a central relative high. The magnetic signature can be more varied but generally corresponds to a subdued low. Local intense magnetic anomalies can occur in the centres of large structures, D >40 km; these magnetic highs have various sources, but many are due to post-impact hydrothermal alteration. Impact can also lead to a reduction in seismic velocities and resistivity of the target rocks. The geophysical, geological, and morphological characteristics at terrestrial impact structures are summarised in tabular form as an aid to the recognition of additional structures.
