Planetary morphologic processes - Introduction to Planetary Geomorphology - page 44

Figure 3.17. Maar craters result from steam
explosions as magma rises through water-
saturated host rock, exempli ed by the 1.3 km in
diameter Crater Elegante, in the Pinnacate lava
field of northern Mexico.

material from the uppermost layers is thrown farthest from
the crater. Thus, if one were to sample ejecta by making a
traverse from its outermost extent to the crater rim, the
samples would re ect progressively deeper (and hence
older) rock layers. In this respect, impact craters represent
a drill-hole with the “ drill core ” laid out on the surface as
ejecta.
The final stage includes various post-cratering modifi- -
cations not directly attributable to shock waves
( Fig. 3.23 ). These include slumping of the crater walls,
isostatic adjustments of the floor and rim, and erosion and
in ll of the crater. This stage may continue over long
periods of time until the crater is eventually obliterated
by gradation and other processes.

Figure 3.18. Pseudocraters, such as these seen at Mývatn in northern
Iceland, develop in lava flows that pass over swampy terrain,
resulting in local mild explosions that form small cinder and spatter
cones with summit depressions.

3.4.2 Impact craters on Earth

The morning of 30 June 1908 witnessed an unusual
event in Siberia near the Tunguska River. A series of
blasts knocked local reindeer herders off their feet,
broke windows, and hurled one man from his porch.
Seismometers recorded the events, while explosions
and a fiery cloud were seen some 400 km from the
event. No one knew what had happened, and it was
many months before an exploration party reached the
area. When they arrived at the location identi ed by
seismic records as ground zero, the party found that all
of the trees had been flattened into a pattern radial to the
blasts, except for a small clump of trees right at ground
zero, which were still standing upright but had all the
branches stripped away.
The Tunguska event posed a problem that remains
today. No crater was found, leading some wags to suggest

severely shock-metamorphosed that rock can be melted
and even vaporized.
The next stage involves excavation of the crater as
shock waves and attendant rarefaction (or decompression)
waves set target material into motion. The material exca-
vated from the crater, termed ejecta ( Fig. 3.25 ), is dis-
tributed radially as a blanket of fragmented debris
(continuous ejecta), which, in turn, grades into zones of
discontinuous ejecta and secondary craters, formed by
the impact of ejecta boulders. A particularly important
aspect of impact craters is the inversion of stratigraphy
in the crater rim. As shown in Fig. 3.22 , rock strata are
overturned as part of the ejection processes. Moreover, the