Page 328 - ABCTE Study Guide_Neat

rivers and streams. These kinetically active areas spread the weathered materials to lower points of
elevation and form such areas as fertile river deltas, rich in sediments that build up over time.

Formation processes of igneous, metamorphic, and sedimentary rocks

The rock cycle can be thought to begin when rocks are liquid, under intense heat and pressure. When this
liquid reaches the earth’s surface the rocks cool and crystallize forming what is called igneous rocks either
through volcanic activity or sea floor spreading. As they are exposed on volcanic mounts they are
weathered by erosion and transported by rivers and streams to become sediments such as mud and
sand. As they are compacted and cemented together they can form sedimentary rocks. These rocks can
then be subducted and buried to become liquid magma once again or with intense heat and pressure can
be metamorphosed to become metamorphic rock found under mountains, for example. Metamorphic rock
from mountains can also be eroded and transported to be deposited as sediments or subducted as well to
become magma once again. These processes take many millions of years.

Geologic Time and Dating Rock Strata

For a very long time, geologists examined rock and sediment layers on the sides of hills, in deep canyons,
or on the sides of mountains all over the world. Upon inspection, it is easy to assign a relative order to the
geologic events that laid down these layers. The sandstone on the bottom came first, then later the layer
of limestone, and much later the clay layer, and so on. The basic concepts of a geologic time scale began
late in the 17th century when scientists correctly deduced that each rock strata represented a different
time period.

During the 18th century, scientists realized that they could not read rock strata as simple layers because
geological processes deep in the earth complicate the layer order. Strata once on the ocean floor millions
of years ago could now be on the top of a mountain. It can be distorted, eroded, or tilted long after it was
deposited. Rock strata deposited at the same time in different parts of the world can vary in composition
greatly. Clearly, marking distant geologic times by types of rock strata in layers only gives part of the
picture.