Geology Reference
In-Depth Information
Lamination vs. foliation: Similar outcomes from
different processes
Geologists describe characteristics of rocks with the intention of understanding the pro-
cesses that formed those characteristics. For this reason, an observed characteristic,
such as “layers,” will have different terms indicating what kind of process resulted in
those layers. Here's an example of what I mean:
A rock with layers may be described as
laminated.
Laminations are thin layers
formed by the accumulation of tiny particles that settle through standing water
(such as at the bottom of a lake or pond). This layered (laminated) rock is a sedi-
mentary rock.
A rock with layers may also be described as
foliated.
Foliations are thin layers or
sheets of minerals that are created by intense amounts of pressure and heat deep
within the earth's crust. This layered (foliated) rock is a metamorphic rock.
The layered characteristic of these rocks may seem similar at first glance but
is actually the result of very different processes that occur under very different
conditions on the earth. Closer inspection of the rocks (perhaps with a micro-
scope) will reveal that the layers made of particles are different in appearance
than the layers made of mineral sheets.
Gabbro vs. basalt: Different outcomes from simil-
ar processes
Another defining characteristic of rocks is their
composition,
or what minerals they are
made of. However, rocks with the same mineral composition may have different names.
Why? Geologists want to categorize rocks according to both composition
and
formation
process. An example is the distinction between the rocks called
gabbro
and
basalt.
Both gabbro and basalt are dark-colored rocks with the same mineral composition. They
both are formed by the cooling of liquid rock (magma or lava) into a solid. Gabbro is
formed when the liquid rock cools underground, slowly, over a long period of time.
Basalt is formed when liquid rock cools very quickly, at or near the surface of the earth
where it is exposed to air or water.
