Classification
Distinguishing Feathers are the most universal and common
There are usually many ways to classify things. The best classification is
one that is useful for communication among interested persons, but also
one that is based on some FUNDAMENTAL ASPECTS of how the
process originated
Good Classification schemes help develop deeper understanding of the process
in the origin of igneous rocks.
Over the past 200 years, geologist have developed and debated numerous
classification schemes for igneous rocks.
Show students: Blackline Master Bedrock Geology 2.1: Classification of Igneous Rocks.
Igneous Rocks are mostly classified by crystal size and chemical composition with most
containing elements of oxygen and silicon.
Students will need to consider the colors and patterns that they see and how they are
different from or similar to the sedimentary rock samples
It can be difficult to distinguish some sedimentary rocks from igneous rocks
Igneous and sedimentary rocks are similar in one sense: both are
Composed of grains. Chemically, however, they are quite different, and
They are entirely different in origin. Because igneous rocks were molten
before becoming rocks. DO NOT EXPECT TO FIND DELICATE
FOSSILS IN AN IGNEOUS ROCK
Magmas
Remember Magma is found below the Earth’s surface and Lava is found above
the ground.
Igneous rocks form by cooling and solidification of magma. Magma is the general
term for melted rock.
1) All igneous rocks are made of interlocking crystals that cool and
crystallize out of magma.
2) The interlocking nature of crystals makes igneous rocks very
resistant to physical weathering and erosion.
Minerals are the building blocks of igneous rocks (and other rocks as well)
Although, there are literally thousands of kinds of minerals in the Earth’s
crust ONLY SIX, kinds of minerals are common in igneous rocks.
Silicate Minerals: These are the six minerals and they are: Quartz, Feldspars,
Micas, Pyroxenes, Amphibols, and Olivnes mostly consisting of silicon and
oxygen.
The color of an igneous rock is determined mainly by the chemical composition,
and therefore the mineral composition, of the rock.
**
ANY ROCK when heated to a sufficiently high temperature WILL MELT.
1. Rocks melt over a range of temperatures, this is because most rocks
consists of a number of minerals, and each mineral has its own melting
point.
2. Nearly all substances expand when they are heated. When rock is
melted into magma, its volume increases by about 10%. This makes the
magma less dense than surrounding rock. Like a hot air balloon that rises
through less dense surrounding air, magma rises toward the Earth’s
surface.
Some magma can best be thought of as a “crystal mush” with just enough
Liquid to allow the material to flow
Volatiles: are compounds that are dissolved in the magma but have such low
Boiling points that when the magma rises up to the Earth’s surface they
exsolve (come out of solution in gaseous form). The presence of abundant
volatiles is what makes some volcanic eruptions explosive, and therefore
extremely dangerous.
Volcanic Rocks and Plutonic Rocks
 The main distinction among igneous rocks has to do with WHERE they solidify.
o Rock types, whether intrusive or extrusive are associated with places
where LITHOSPERIC PLATES are moving together and magma is
formed.
o Magmas rich in silica do not flow very easy and usually cool before they
reach the Earth’s surface to form granite.


Magma either flows onto the surface as lava to form effusive EXTRUSIVE
volcanic rocks or are ejected explosively as pyroclastic material.
Magma that solidifies below the Earth’s surface forms plutonic or INTRUSIVE
igneous rocks.
Extrusive Rocks are very fine-grained crystals, due to rapid cooling on the Earth’s
service and are so small in size that it is sometimes hard to see the individual crystals.
The crystals cooled so fast that they did not have time to form big crystals.
Basalt, andesite, dacite, and rhyolite.
Intrusive Rocks have bigger or coarse-grinned crystals that can bee seen with the
naked eye. Magma cools slower under the surface of the Earth
Gabbro, diorite, and granite
Some Lava’s cool so rapidly that no crystals form (nucleate). Obsidian is a common
example of an igneous rock with no crystals.
The appearance of an igneous rock reveals whether or not it is formed below or at the
Earth’s surface.
Where Magmas Form
Basaltic magma is the most common magma on Earth. Most basaltic magma is
formed beneath mid-ocean ridges, where hot upper mantle rock rises slowly upward,
at speeds of centimeter per year, as part of the gigantic mantle convection cells
associated with plate tectonics
Basalt is the MOST COMMON rock on the Earth’s surface, because it is formed
where the lithospheric plates are spreading apart or where magma is rising through a
mantle hot spot.
Show students Blackline Master Bedrock 2.2: Cross Section of Mantle and Crust
Explosive Volcanism
Igneous rocks that are of the most concern to human society are the occurrence of
catastrophic explosive volcanism.
1) Fragmented magma is then ejected explosively out of a volcanic vent at speeds
of up to several hundred meters per second.
2) Ash is one of the main components of a volcanic eruption and the ash’s main
Form comes as PUMICE. We use pumice in the shower/bathtub.
Rock formed from volcanic ash is a sedimentary rock because it is formed
by the deposition of material. However, because it came directly from a
volcano it is usually considered to be an igneous rock. PUMICE FLOATS
ON WATER.
In a sense, pyroclastic volcanic rocks are sedimentary rocks: They are deposited on the
land surface as solid particles
Igneous Rocks on Geologic Maps
Intrusive igneous rocks are commonly in the from of sheet like bodies, called sills
if they are intruded along planes of layering in the surrounding rock, or dikes if they are
intruded across the layering of the surrounding rock.
Batolith a large intrusive igneous rock with irregular geometry. They are
composite bodies formed by multiple generations of intrusion over long periods of time.
Volcanic rocks are usually in broad, sheet like units when they are first deposited
Goals:
Understanding how rocks from will help students make inferences about the
geologic history of their area.
Students will be able to:
1. Identify several igneous rocks using a rock chart
2. Students will describe how the two main types of igneous rocks form
3. Students will understand that igneous rocks are classified based on how they are
formed.
4. Students will use a geologic map and legend to search for evidence of past
igneous rock formation.
5. Students will understand that classification helps scientists organize the natural
world into smaller, workable components.