Page 137 - Laboratory Manual in Physical Geology

FLOWCHART FOR CLASSIFICATION OF ROCKS
AS IGNEOUS, SEDIMENTARY, OR METAMORPHIC

START The rock is
probably:
Layers are flat and soft or crumbly. sedimentary
Layers are made of mud, sand, or gravel. sedimentary
The rock
has Layers are made of crystals, which are about the same size in all layers. metamorphic
layering.
Layers are made of crystals, which are of different sizes in different layers. sedimentary
or igneous
Layers are folded or otherwise deformed. metamorphic

Looks like dry clay or mud sedimentary
Dense and hard, like window glass igneous
No visible Looks glassy
grains Brittle, lightweight, opaque black: i.e., anthracite coal metamorphic
are present
in the rock. Very soft, easily scratched with fingernail sedimentary
Very hard igneous
Reacts (effervesces) with drop of dilute HCl sedimentary

Whole or fragmented leaves, stems, shells, footprints sedimentary
Contains fossils The fossils are so deformed (sheared, folded, or of
changed composition) that they are barely recognizable metamorphic
Grains are mostly sand, silt, or gravel (a clastic texture). sedimentary
Contains rounded grains (like pebbles) (a clastic texture) sedimentary
Contains grains that are fragments of other rocks (a clastic texture) sedimentary
igneous or
Crystals are randomly oriented in very hard rock.
metamorphic
Large crystals sit in a very hard mass (groundmass, matrix) igneous
Grains are mostly mineral crystals (crystalline texture). Long crystals are foliated: lay parallel to one another. metamorphic
Visible grains with smaller crystals or no visible grains.
are present Crystals can be easily scratched and/or broken apart. sedimentary
in the rock. Crystals are flat (e.g., mica) and foliated: lay parallel to one metamorphic
another, giving the rock a layered or scaly appearance.

metamorphic
Crystals are calcite:
Equigranular: all about same size
they react (effervesce)
Have different sizes and/or occur in layers
with dilute HCl.
sedimentary
Crystals are mostly gypsum or halite.
Crystals are mostly olivine, feldspar, or pyroxene.
igneous
Crystals are mostly garnet, micas (muscovite, biotite), sedimentary
metamorphic
serpentine, graphite, galena, or sphalerite.
Rock looks metallic (silvery, brassy, or copper-like) and scaly or smooth. metamorphic
The rock has
additional Rock is vesicular: has bubble-shaped holes/cavities or looks frothy. igneous
features.
Rock is very hard and has ropy, streamlined flow structures. igneous

FIGURE 4.4 Flowchart for classification of rocks as igneous, sedimentary, or metamorphic.

118 ■ L ABOR ATORY 4