Characteristics of the Environment - Field Sampling: Principles and Practices in Environmental Analysis - page 30

Metamorphic rock is rock material that has undergone a change in appearance as a
result of melting caused by high temperatures and pressure. Sometimes this also causes a
change in the mineral composition of the rock. Both sedimentary and igneous rocks can
undergo this process. Common metamorphic rocks are slate, marble, and quartzite.
Water can often move through rock, even when it appears solid. Softer rocks will have
pores, and when near the surface, even the hardest rock in the Earth's crust will have
cracks in it. In both cases pores and cracks allow the movement of water and
contaminants downward. There are many springs in the hills on the islands of the
Philippines. Rainwater enters the soil and the underlying fractured rock layers on the
upper slopes of the hills. This water moves downhill under the ground through the
fractured rocks until it finds an outlet. At this point a spring forms, the outflows of which
can be large enough to support large waterfalls year round. At other places wells can be
dug into these broken rock layers and a good, steady supply of water obtained.
About a fourth of the Earth's surface is underlain by limestone. Two features of
limestone are caves and sinkholes. When rainwater enters limestone, soft or easily
dissolved portions dissolve, leaving caves, which are sometimes extensive. Such
landscapes are called karst, a good example of which is Mammoth Caves in the state of
Kentucky in the United States. Limestone caves are common throughout the world,
however. When the size of the dissolved area becomes very large the cavity cannot
support the roof and the cave collapses inwards. This produces a sinkhole that may or
may not have a hole connecting to the cave network (see Figure 2.2) [14].
Water can pass directly from the surface to the underground karst network, which may
extend for hundreds of miles, and will have an underground river associated with it.
Contamination entering such networks can travel long distances and contaminate large
areas. In addition, animals living in the caves, such as bats, can contribute to this
contamination. This is particularly dangerous because many water wells may tap into its
water system.
2.2. SOIL FORMATION
It is easy to assume that soil is simply decomposed rock lying above a consolidated rock
layer. Soil is much more than simply ground-up rock, however. Immediate evidence of
this is that ground rock would be gray, while soil is brown to black on the surface with
red hues in the subsoil. There are soils with very light colored surface horizons and little
color in the subsurface horizons, but even these soils are not gray.
When rock is exposed on the Earth's surface it is subject to degradation. Rain falling
on the decomposing rock dissolves salts as it infiltrates. These salts are eluviated with
percolating water and moved lower in the regolith. With a moderate passage of time and
rainfall soil salts are lost and less soluble components remain, forming soil. Illuviated less
soluble materials result in the development of horizons. With sufficient time and rainfall
salts and other easily dissolved components are completely leached out of the solum.
Most of these salts eventually are deposited in the oceans.
Oceans are not the only reservoirs of salts. When the water falling on mountains is
caught in a basin without an outlet to the sea, a salt sea such as the Great Salt Lake in