Geology Reference
In-Depth Information
Figure 3.19. Classi
cation of common volcanic landforms (italicized names), showing the relationships to the quantity of magma, magma
composition, type of volcanic activity, and the nature of the vent.
Figure 3.21. These intersecting dikes in the Spanish Peaks area,
Colorado, form ridges that stand higher than the now partly eroded
rock (US Geological Survey CL-34
-
71).
diameter, releasing energy equivalent to 10
-
15 megatons
of TNT. The energy was released as a series of air-blasts,
reflecting multiple objects generated by the breakup of the
asteroid or comet, thus accounting for the multiple blasts
observed in the town of Kirensk, some 400 km from
ground zero.
Even to advocates of impact cratering, prior to 1930
fewer than ten impact structures were acknowledged on
Earth. In the early Space Age of the mid 1960s, the
number had risen to only about 33, but, after intensive
searches and establishment of criteria for the recognition
of impact craters (
Fig. 3.26
;
Table 3.3
), by the early part
of the twenty-first century nearly 180 craters and related
Figure 3.20. Green Mountain, Wyoming, is a dome formed by
intrusion of magma into sedimentary rocks (US Department of
Agriculture photograph BBU-29
-
78).
the preposterous idea that Earth was hit by a black hole! A
more probable explanation suggests that an asteroid or
comet broke apart in the atmosphere, forming a shotgun-
like blast that left neither a crater nor large remnants of the
object itself. Tiny bits of meteoritic material have been
recovered from some of the tree trunks, supporting this
hypothesis. Current best estimates suggest that the
Tunguska event resulted from an object 30
-
60 m in
