Geology Reference
In-Depth Information
formations indicates that process of uplift and erosion were active in the Precambrian
time period.
Piecing observations together
Geologists apply the theory of uniformitarianism (which I describe in Chapter 3) and hy-
pothesize that granite-gneiss complexes and greenstone belts are the result of crustal
plates moving apart from each other and then moving back together repeatedly. Most
greenstone belts are located in linear, parallel arrangements, separated by granite-gneiss
complexes. One hypothesis suggests that the parallel arrangement is the result of activ-
ity along a plate boundary that experienced cycles of rifting, convergence, subduction,
and volcanism. The associated granite-gneiss complex would be a result of intrusive ig-
neous rocks (granite) and rocks transformed by compression and heat (gneiss) as the
plates moved together.
Multiple repetitions of this sequence of events may seem impossible, but over a period
of
billions
of years, they are entirely possible.
Feeling hot, hot, hot: Evidence for extreme tem-
peratures
Scientists have found evidence in the ancient rocks suggesting that the internal temper-
ature of the earth was much hotter than today and has been slowly cooling off over the
last 4 billion years. When the earth first formed, the coalescing of all that matter gener-
ated a large amount of heat that became trapped inside the earth. In the process of cool-
ing, the heat from inside the earth escaped to the surface and radiated out, away from
the planet into space. This process still occurs today, but the levels of heat are much
lower than they were billions of years ago. Much of the heat generated by earth today is
believed to be the result of radioactive decay of elements in earth's core.
In the Archean greenstone formations, geologists find really dark, dense, iron-rich vol-
canic rocks called
komatiites.
Komatiites have high amounts of iron in them, indicating
that the magma (molten rock) from which they were formed must have been hot enough
to melt iron-rich minerals. This means the magma was much hotter than the magmas
that erupt as lava on the surface of the earth today. Komatiites must have cooled from
molten rock that was at least 1,600° C at the surface of the earth. For comparison, con-
sider that today the highest temperature recorded for a surface flow of lava is 1,350° C.
With temperatures this high just below the surface of the earth (where magma forms),
the mantle of the earth must have been even hotter. Scientists conclude that higher
mantle temperatures might have led to faster mantle convection (see Chapter 10) and
