Geology Reference
In-Depth Information
This finding suggests that the moon did not form at the same time as the earth or from the same cloud
of gas that formed the earth. (I discuss the earth's formation in Chapter 18.) Rather, the composition of
the moon rocks suggests that the moon was created by materials from the
surface
of the earth, long
after the heavy metal core was covered by mantle and crust materials.
While this idea is still being hotly debated, scientists suggest that the materials that formed the moon
were removed from the earth by a giant impact. Another planet-type body, possibly as large as Mars,
may have crashed into earth, vaporizing some of its crustal and upper mantle materials, leaving them
to orbit the earth until they came together forming the sphere now known as the moon.
Wouldn't there be a giant crater as evidence of such an impact? At the time, yes, but this was 4.5 billion
years ago, and since then much about the earth and its surface has dramatically changed. The best
evidence scientists have is the composition of the moon rocks.
It's only skin deep: The earth's crust
The change from mantle rock to crustal rock in the lithosphere is labeled the
Moho dis-
continuity,
named after the scientist who discovered it, Andrija Mohorovicˇic´. This
boundary, illustrated in Figure 4-4, is where the composition of rocks changes from the
somewhat dense mantle rocks to the much lighter crustal rocks, which are composed
primarily of silica.
The layer of crust covering the earth comes in two types:
continental
and
oceanic
. These two types of crust vary in thickness and are composed of slightly
different materials:
Continental crust:
The crust that composes the continents is pretty thick. At its
thinnest sections, continental crust is about 12 miles thick; at its thickest sections
(where there are mountains), it is up to about 45 miles thick. The rocks that com-
pose the continental crust are primarily granites (see Chapter 7).
Oceanic crust:
This crust, which lies under the earth's oceans, is thin — only
about 5 miles thick. This type of crust is composed of dark, dense silica rocks
such as basalt and gabbro (see Chapter 7). Oceanic crust is relatively young, being
created even now from the eruption of molten rock along ridges in the sea floor
(see Chapter 10).
Drilling for the Moho
