Download Igneous Rocks

Rock Cycle and Rocks Lab
Rocks are aggregates of one or many minerals.
Three types of rocks:
A. Igneous
B. Sedimentary
C. Metamorphic
Rock cycle is a conceptual model of how all rocks can be formed, transformed, destroyed, and
reformed. The rock cycle is driven by earth processes such as atmospheric cycles, oceanic
circulation and especially plate tectonics
A. Igneous Rocks
Igneous rocks form when molten rock (rock liquefied by intense heat and pressure) cools to a solid state.
Intrusive igneous rocks crystallize or solidify within the earth’s crust. Magma is molten rock (liquid or liquid/crystal
“mush”) that exists below earth’s surface (when cooled it forms rocks such as granite, diorite or gabbro).
Extrusive igneous rocks are erupted in a molten state on the earth’s surface and then cool and solidify. Volcanic
processes associated with this are natural hazards or disasters.
Lava is molten rock flowing out of fissures or vents at volcanic centers (when cooled they form rocks such as basalt,
rhyolite, or obsidian).
Pyroclastic deposits are accumulations of fragmented material (e.g. ash, bombs, tuffs and volcanic breccias)
ejected during volcanic eruptions.
Texture is a description of a rock’s constituent parts in terms of their sizes, shapes and arrangement.
Rule of Thumb: The size of mineral crystals in an igneous rock may indicate the rate at which the lava or magma
cooled to form a rock. Crystal size can also be affected by the amount of gases or the availability of the
chemicals in the molten rock that are required to form the crystals.
Larger crystals generally indicate intrusive igneous rocks.
Smaller crystals generally indicate faster cooling associated with extrusive igneous rocks.
Types of igneous rock textures:
1. Aphanitic: fine-grained, less than 1 mm, grains not seen with unaided eye
2. Phaneritic: “coarse grained”; visible crystals;
1 to 10mm
3. Pegmatitic: “very coarse grained”; > 1 cm
4. Porphyritic: composed of both large and fine-grained crystals, and the large crystals are called phenocrysts,
and the background is the matrix
5. Vesicular: rocks that have vesicles, resembling a sponge (e.g. scoria and pumice)
6. Pyroclastic: fragmented, angular grains ejected during eruption (e.g. volcanic breccia)
7. Glassy: when lava cools quickly, there is not enough time for large mineral crystals to form (e.g. obsidian)
Igneous Rock Mineral Compositions (Assemblages)
1. Felsic: generally the lighter-colored igneous rocks; enriched in silica and aluminum bearing minerals (e.g. quartz and
potassium feldspar).
2. Mafic: generally the darker-colored igneous rocks; enriched in magnesium and iron bearing minerals (e.g. olivine and
pyroxene).
Fig. 1: Bowen’s Reaction Series. The diagram suggests the sequence in which minerals crystallize
from an average magma in the asthenosphere when it is slowly cooled. Note the relationship
between temperature, crystallization of specific minerals, type of magma, and rocks formed when the
magma is cooling. (This is approximate because pressure also affects mineral composition and
stability). Viewing the diagram in reverse (bottom to top) also suggests the sequence in which
minerals will melt to form magma when rocks are heated. Once these rocks are exposed at the
surface, the stability of minerals is greatest for those that formed at lower temperatures, therefore,
olivine, calcic plagioclase pyroxene, will weather first whereas quartz is one of the most stable
minerals at the surface under weathering conditions.
Fig. 2: Mineral composition and color index of igneous rocks.
Table 1: List of the rock types supplied for his lab:
1 Granite
5 ignimbrite
6 diorite
8 gabbro
10 peridotite
2 rhyolite
3 obsidian
4 pegmatite
7 andesite
9 basalt
Questions and assignments:
1. Take the samples # 1, 2, 3, and 4. Inspect them carefully.
What is the mineral content of samples 1,2, and 4? Figures 1 & 2 will be useful for this
question! (sample 3 is glassy, thus it is not composed of any minerals!)
1 granite
2 rhyolite
4 pegmatite
2. Now try the same with samples 8, 9, and 10. Enter your results in the table below.
8 gabbro
9 basalt
10 peridotite
3. What about samples 6 and 7? Which minerals would you expect in them?
How melts are formed
Melts on earth are being generated at very specific sites in the crust and there they are of a very
distinctive composition. Until the advent of the concept of global tectonics, this was a problematic
topic. Global tectonics offers an excellent concept how melts are being generated. In a nutshell, at
mid-ocean ridges, mafic melt wells up and emerges on the sea floor as the ocean plates are
spreading. This melt forms basaltic or gabbroic rock, depending on the depth below the sea floor.
This also means, that the sea floor is expanding and if this is happening, someplace else, crustal
material has to make space for this! This is happening at subduction zones where oceanic crust is
thrust deep into the mantle where it melts again. Wet sea floor sediments will release the water
content and this water under pressure aids melting. It also ends up in the melt and if this melt gets to
the surface, it is released as steam during an eruption. Notice that the continents don’t appear to get
subducted! Continents are mainly rocks that tend to be composed of more felsic rocks (such as
granites, diorites etc.). Among the places where this melt will appear again is at active continental
margins or at island arcs. But this melt has undergone some evolution and is now less mafic. The
original basaltic melt is now andesitic. This is named after the Andes Mountains in South America
where such an active margin exists. Continent-continent collisions do not lead to subduction but the
crust will thicken at those sites, mountains will build and metamorphism of the sediment packages
that are now deeper and under higher pressure and temperature will occur. Melting will also occur
and produce more felsic melts.
1. Mid-ocean Ridges
5. Back-arc Basins
2. Intracontinental Rifts
6. Ocean Island Basalts
3. Island Arcs
7. Miscellaneous IntraContinental
Activity
u kimberlites, carbonatites,
anorthosites...
4. Active Continental
Margins
Fig. 3: The locations on the earth’ crust where melt is generated.
Fig. 4: melt being generated at a subduction zone. Here aslab of ocean floor subducts
underneath a conitinent.
Fig, 5: Continent-content collision. No subduction occurs but the crust is thickening and
metamorphism occurs as well as melting of continental rocks leading to a more gfelsic melt.
4.
Why do continents appear to float on top and don’t subduct? To find out, let us try an
experiment: Last week, we determined the density of minerals by weighing them in air and in
water and thus determining the density. This week, we will use a sample of granite and one of
basalt and measure the density. If you are not certain how we did this, the last page gives you
the steps once more. What explanation can you come up with to explain the behavior of the
continental plates with respect to the oceanic plates?
Fill in the densities you determined into the table .
Basalt
Granite
Density measured
5. Does your answer of question # 4 fit well with the results of questions 1 and 2 as well as what you
learned last week about mineral properties? Why? The table of the mineral properties is included once
more below.
6. A final experiment: use sample 11 and a container with water. This is a sample of pumice, a volcanic
rock. Does it float or sink? Why? (Hint: What do you notice when you inspect it more closely?)
Density,
g/cm3
Cleavage/
fracture
Hardness
(Mohs)
Color
Streak
Luster
Quartz
2.65
No cleavage,
irregular
fracture
7
white
nonmetallic
Potassium
feldspar
2.54-2.62
2
perfect
cleavage
planes at near
90°
6
colorless,
white,
brown,
pink.
purple
white, pink,
gray, green
white
nonmetallic
Plagioclase
feldspar
2.62-2.76
2
perfect
cleavage
planes at near
90°
6
white, gray,
colorless
white
nonmetallic
Biotite
2.8-3.2
1
perfect
cleavage
2.5-3
2.76-2.88
2.5-3
Amphibole
(hornblende)
3.0-3.4
nonmetallic
3.2-3.4
white
to
off-white
nonmetallic
Olivine
3.27-4.37
white
nonmetallic
Garnet
3.5-4.3
white
Calcite
2.7
3
rhombohedral
cleavages
3
dark green,
black, dark
brown
black, dark
green,
brown
yellowgreen
to
olive-green
red,
redbrown,
yellow,
black,
green
Colorless,
white, gray,
pink,
yellowish
white
to
off-white
Pyroxene
(augite)
1
perfect
cleavage
2
perfect
cleavages at
60 and 120°
2
perfect
cleavages at
near 90°
no cleavage,
irregular
fracture
no cleavage,
irregular
fracture
white
to
off-white
to tan
white
nonmetallic
Muscovite
brown,
black, dark
green
light color
Pyrite
5.0
No cleavage
6-6.5
1
perfect
cleavage
1 cleavage in
coarse
metallic
hematite
2
No cleavage
6
Gypsum
Hematite
up to 5.25
Magnetite
5.1-5.2
5-6
5-6
6.5-7
7-7.7
1.5-6
Other
Sample #
diagnostic
properties
1
common
with
roughly
parallel
streaks
very thin,
perfectly
parallel
lines on
some
surfaces.
very easy to
cleave
2
very easy to
cleave
cleavage
angles and
dark color
cleavage
angles and
dark color
color, lack
of cleavage
5
nonmetallic
lack
of
cleavage,
hardness
9
white
nonmetallic
10
brassy
yellow
dark gray
metallic
colorless,
white, gray
red-brown
to
gray,
depending
on
coarseness
Silvery
gray
to
black
white
nonmetallic
red-brown
metallic or
nonmetallic
Hardness,
cleavage,
acid
response:
fizzes
Luster,
color,
hardness
Hardness,
cleavage
Streak color
Dark gray
metallic or
nonmetallic
magnetic!
14
nonmetallic
3
4
6
7
8
11
12
13
Density determination:
To accomplish this, fill the beaker about ¾ full with water and set it aside. Now turn on the balance and make
certain it reads ‘0.00’ and is in the ‘gram mode’, it should display ‘g’ on the upper right of the readout panel. If
it does not display ‘g’, hit the mode button until it does. Place the sample on it and obtain its mass. This value
is in grams. Next, place the beaker filled with ¾ of its volume in water. Hit the ‘tare’ button, the readout
should display ‘0.00’. Use the supplied string, make a slipknot and attach the sample to it. Now suspend the
sample in the water, making certain it is completely submerged. Do not let it touch the bottom or the sides of
the beaker. Record the number on the readout. This is the mass of the water displaced by the sample! Since
the density of water is 1.00 g/cm3, the volume displaced is the same numerical value but it is expressed in cm3.
Divide the mass of the sample by the volume to obtain the density of the sample in g/cm3.
1: turn on balance and tare to show ‘0’.
2. Weigh sample
3. Place beaker with water
on balance and tare to indicate ‘0’.
4. Suspend sample in water
to weigh displaced water