Rock Identification Lab
Name:
Period: 1 2 3 4 5 6 7 8 9
Background Information
Rocks can be divided into three main types: igneous, sedimentary and metamorphic.
To identify rocks and classify them into these three categories, some basic
questions should be asked. These are as follows:
1) What is the rock made of?
A rock can be described as a complicated word that is made of several letters of
the alphabet. Just like a word can be made of many of the letters of the alphabet,
rocks can contain various material such as minerals, fragments of fossils and other
organic material like wood or bones, fragments of other broken down rocks, etc.
A mineral is a naturally occurring solid substance that has a specific chemistry.
The name of a rock can depend on the abundance of light color minerals such as
feldspar vs. dark color minerals such as biotite mica. Fossil materials often look
like broken-up remains of shells, bones, teeth, wood and any other remains of plants
and animals. Rocks can also contain pieces of other older rocks.
2) How are the materials in a rock arranged?
The word texture refers to the arrangement of material in rocks. Some of the
commonly used texture names for rocks include: chemically precipitated, frothy or
vesicular, glassy, fine or coarse grained, layered, banded, clastic or non-clastic.
SEDIMENTARY
Non-clastic - Imagine a glass full of seawater. If the glass is left undisturbed
for many days, eventually the water evaporates out of the glass leaving behind some
crystals of salt.
Some rocks are precipitated from water by inorganic or organic processes. The
dissolved substances are derived from the chemical weathering of preexisting
rocks or composed of plant/animal (shell) remains. These rocks contain no visible
rock fragments.
Clastic – Rocks composed of broken particles (mineral or rock fragments). These
particles can be gravel, sand, or silt sizes.
IGNEOUS
Phaneritic (coarse-grained) - Rocks that contain large mineral grains that are
visible with the naked eye. The grains are >1mm in size.
Aphanitic (fine-grained) – Rocks that contain small mineral grains that can only
be seen by a hand lens. The grains are <1mm in size.
Porphyritic – Rocks with a mixture of large and small mineral grains.
Frothy or Vesicular - When lava cools rapidly, gases within the lava get released
quickly and leave behind spaces or vesicles.
Glassy - When lava cools rapidly, minerals don't have a chance to form crystals.
The rocks that form often looks like glass. Such “glassy” rocks are known to be
non-crystalline.
METAMORPHIC
Layered - Sometimes rocks have visible evidence of layers. This is common when a
rock is made of flat mineral like biotite mica or various clays.
Foliated - In some rocks layers can be stronger and thicker, and visibly divided
into different color mineral. This is common in rocks that have formed under
pressure, where minerals are forced to become aligned and separate into different
bands.
Non-foliated – Rocks which exhibit no signs of mineral alignment or banding.
ROCK GROUPS
Igneous rocks are rocks that form from fast or slow cooling of lava or magma.
These rocks can be light or dark colored depending on their mineral content. Dark
colored rocks high in magnesium and iron are called mafic. Light colored rocks high
in silica are referred to as felsic.
The rate at which the magma cooled can indicate how the rock was formed. Large
crystals (phaneritic texture) in the rock indicate a slow cooling process, while small
crystals (aphanitic texture) indicate a fast cooling process. If lava cooled very
rapidly, they can also have glassy, frothy or vesicular textures.
Sedimentary rocks are rocks that form from accumulation of broken down
material such as minerals, fossils, etc. Sedimentary rocks can be made of fine or
coarse grains that are glued or cemented together from fragments of quartz,
feldspar, biotite, shells, remains of plants and animals, or through chemically
dissolved substances.
Metamorphic rocks are rocks that have formed when igneous or sedimentary
rocks become exposed to heat and/or buried under high pressure within the Earth.
Metamorphic rocks that have been heated form coarse crystals. Metamorphic
rocks that form under high pressure show evidence of layering where the aligned
minerals produce a shine, or banding where minerals are separated.
Pre-Lab Questions
1. Distinguish the difference between a rock and a mineral (define each and briefly
discuss the difference).
2. What specific characteristics define a rock’s texture?
3. List the 3 main rock groups and discuss the processes that form them.
Sample
1
2
3
4
5
6
7
8
9
10
Texture
How are the
materials in the rock
arranged?
Color
Of
Rock
Fine Grained,
Coarse Grained,
Frothy,
Conglomerate
Mixture of Sizes,
Angular, Glassy
Foliated
Or
Non-Foliated
planes/bands or
same throughout
Mineral/Particle
Composition
What is the rock made
of?
Fossils, Felsic minerals,
Mafic minerals, Shells,
Sand, Organic
Remains, Clay,
Chemical Precipitate
Rock Type
Igneous,
Sedimentary,
Metamorphic
Rock Formation
Based on the rock
type you chose, how
did the rock form?
Compact/Cement
Heat/Pressure
Cooling
Chemical
Precipitations
Prediction of
the Rock
Identity
Now go and identify the following rocks in the
table based on your observations and your notes.
Granite
Sandstone
Pumice
Fossil Limestone
Shale
Marble
Obsidian
Gneiss
Basalt
Slate