Please help me to answer the following questions, need accurate.
Geology 1202 Planet Earth
Assignment – Igneous Rocks
There are hundreds of different types of igneous rocks but we can condense this down to a few main categories using
two fundamental properties mineralogy and texture. But as you have seen in the lecture the formation of this group
of rocks is directly related to plate tectonics, and so once we identified these properties we can make a fairly
confident statement about where each rock formed. That is what you are going to attempt in this assignment.
Property One – Mineralogy
Recall that in Module 5 we looked elements and minerals. The main rock forming minerals are silicates and are
made from the eight most abundant elements in the Earth’s crust. There are only eight common rock-forming
silicates. These silicates can be lumped into two categories of minerals.
Ferromagnesian minerals. (ferro means iron; magnesian means magnesium) Ferromagnesian minerals contain
iron and magnesium and some other minor elements.
·
Olivine
·
Pyroxene (varieties of pyroxene are augite and hypersthene).
·
Amphibole (variations in several different mineral series)
·
Biotite (a form of mica).
Non-ferromagnesian minerals
These do not contain iron and magnesium.
·
Plagioclase Feldspar
·
Orthoclase Feldspar
·
Muscovite (a form of mica)
·
Quartz. (pure silica, silicon and oxygen sometimes with small amounts of impurities).
So how does this help unless you are a mineralogist with a microscope?
Fortunately the ferromagnesian minerals are dark and the non-ferrromagnesian ones are light in colour. That means
that we can begin to name igneous rocks on the basis of colour as illustrated in the figure below.
LIGHT
(FELSIC)
INCREASING FERROMAGNESIAN MINERALS
INTERMEDIATE
DARK
(MAFIC)
INCREASING DENSITY
GREEN
(ULTRAMAFIC)
The light coloured (and therefore felsic) rocks are on the left and moving to the right the amount of ferromagnesian
minerals present increases and so the colour of the rocks moves through the intermediate to the dark (mafic) and ot
green (ultramafic) on the right.
Note: colour mean the overall colour of the rock – it does not necessarily need to be uniform.
Notice that the density of the rocks also increases with the increase in ferromagnesian minerals – this is due to their
higher density (specific gravity).
Property Two – Texture
The overall size of the crystals in an igneous rock is referred to as texture and it reflects the rate at which the rocks
cooled. The cooling rate is dependent upon whether the rocks formed deep beneath the surface or on (or near) the
surface. There are basically three kinds of textures.
Phaneritic – rocks with coarse grain textures (by this we mean that the crystals are visible with the naked eye)
indicate a slow cooling rate. These rocks are plutonic (if you don’t remember this term review the lecture) and
typically cool below the surface of the Earth.
Aphanitic – rocks with fine grain textures (crystals are too small to see individually without a microscope) indicate a
fast cooling rate. If there is very rapid cooling and crystals don’t have time to form at all the rocks may be glassy.
These rocks are volcanic and cool above or near the surface of the Earth.
Porphyritic – two distinct sizes of crystals within the rock. The large crystals are called phenocrysts and the smaller
ground mass is called the matrix. Porphyritic rock indicates that cooling had occurred in steps – a period of slow
cooling followed by rapid cooling. They can be phaneritic with individual crystals in the matrix visible as well as the
phenocrysts, or aphanitic with the visible phenocrysts in a matrix in which the crystals are not visible.
Naming Igneous Rocks
So by looking at the colour (and therefore the mineralogy) and the texture we can start to assign names to igneous
rocks.
Light (felsic):
A light coloured igneous rock is composed almost entirely of orthoclase feldspar and quartz with a small amount of
ferromagnesian minerals and can be either granite or rhyolite.
If you can see the individual crystals it is a granite. This a phaneritic texture. You may hear it described as “salt
and pepper” texture. The salt part is the orthoclase and quartz; the pepper is biotite mica.
If you can’t see the crystals then it is called rhyolite and has an aphanitic texture.
Intermediate:
Intermediate rocks are mostly grey or beige in colour. The main minerals are white plagioclase feldspar with with up
to 50 percent of black ferromagnesian minerals. There is virtually no quartz (this is how it differs from granite)
If you can see the crystals and it looks a lot like granite then it is called diorite.
The fine texture version is called andesite, Sometimes andesites will consist of fine crystals with some larger dark
slender crystals of horneblende and so the texture would be described as propyhritic.
Dark (mafic):
These rocks are generally black or very dark. About half or more of the minerals are ferromagnesian and the rest are
plagioclase feldspar and almost no quartz.
If you can see crystals and it is overall black it is called gabbro.
The fine texture version is called basalt.
Green (ultramafic):
If an igneous rock is green and you can see crystals then it is called ultramafic. (there is no fine texture equivalent)
These rocks are almost entirely ferromagnesian minerals and can consist on only one mineral, peridotite or olivine.
Igneous rocks and plate tectonics
Igneous activity and the formation of igneous rock is directly related to plate tectonics.
At mid-ocean ridges the upwelling molten magma is derived from partial melting in the mantle and will crystallize
to form basalt. Therefore basalt is the dominant rock type of mid-ocean ridges, ocean islands, and ocean crust.
At subduction zones ocean crust, made of basalt, and marine sediments get subducted and partially melted. As this
magma moves up toward the surface it encounters the base of the Earth’s crust (mostly light felsic rocks) and
incorporates some of these rocks into the magma. So, at convergent plate boundaries, andesite and its plutonic
equivalent diorite are among the most common igneous rocks.
At continent-continent collision zones upwelling magma produces mostly light coloured granites and rhyolites.
There are a number of reasons why, relating to fractionization and partial melting, but all you need to remember is
that the darker and heavier minerals get left behind as the magma rises. This is why most continental interiors are
composed of granite and rhyolite.
The mantle is believed to be rich in the ferrromagnesian minerals and so is the source for all ultramafic rock.
Continent-Continent
Collision Zone
(continental craton)
Felsic rocks
(light coloured)
Mid-ocean Ridge
Subduction Zone
(volcanic island arc)
Intermediate rocks
mafic rocks
(dark coloured)
intermediate
mafic
felsic
Mantle
Ultramafic rocks
Distribution of igneous rocks in the Earth’s lithosphere
Felsic
Intermediate
Mafic
Ultramafic
Rhyolite
Dacite
Andesite
Basalt
None
Phaneritic
Granite
Granodiorite
Diorite
Gabbro
Peridorite
Percentage of
minerals by volume
Aphanitic
80
60
Quartz
Orthoclase
(Feldspar)
Pyroxene
40
20
Plagioclase
(Feldspar)
Muscovite
Biotite
Amphibole
0
Igneous Rock Classification Chart
Olivine
What do I do?
First look at the images of igneous rocks at the end of this document. I have also put a link to digital versions in the
Weekly Module. You will be able to zoom in and out on those images.
Then fill in the table on the next page. To avoid confusion I have given you the list of terms (below the table) you are
to use for each column. You should find that if you do this in the order I have listed below you will have a better
chance of giving conflicting information. Think about how your answer to each column affects the answer in the next
column.
If you use any other terms I will mark you wrong.
1. describe the mineralogy – on the basis of the colour.
2. identify the texture.
3. indicate if it formed above or below the surface. (extrusive = volcanic; intrusive = plutonic)
4. name the rock (use the Igneous Rock Classification Chart to help you)
There are seven images in total and I have given you seven common rock names; with one done that only leaves you
six. Only use each rock name once.
Finally identify the tectonic setting where each rock may have formed by placing the sample letter in the correct
white circle on the figure on the next page. For each tectonic zone there are two locations indicated by the black lines
from the circles – one extrusive and on intrusive. Make sure you place the rock in the correct locations.
What do I submit?
Fill in your name and student number in the given locations on the page.
Photograph your completed table and the figure and submit the image – check that it is readable. You may have to
take two photos. You can put them into a MS Word file if you wish.
Do not submit a digital version of the table – it will not be accepted!
Assignment Igneous Rocks
GEOL1202
Name:
Student Number:
SAMPLE
COLOUR
TEXTURE
INTRUSIVE/EXTRUSIVE
NAME
green
Phaneritic
Plutonic
perioditite
A
B
C
D
E
F
G
These are the terms for each column
dark (mafic)
intermediate
light (felsic)
green (ultramafic)
Continent-Continent
Collision Zone
(continental craton)
Felsic rocks
(light coloured)
felsic
Aphanitic
Phaneritic
Volcanic
Plutonic
Porphyritic (can be
combined with the other
two terms)
Mid-ocean Ridge
mafic rocks
(dark coloured)
mafic
Mantle
Ultramafic rocks
Dorite
Gabbro
Granite
Perioditite
Rhyolite
Andesite
Basalt
Subduction Zone
(volcanic island arc)
Intermediate rocks
intermediate
Igneous Rock Samples
A
30 cm
F
1 cm
G
1 cm