Igneous Rocks (L7) Flashcards
Simply put, what are ROCKS?
rocks aer solid aggregates of minerals and mineral-like materials
Where does the term Igneous come from?
Igneous is from the Latin ignis meaning fire, the same root as the word “ignite”
How is Magma created?
from the melting of rock in the mantle or crust, or both
What does magma contain?
mostly liquid, but can contain gases such as water vapour and carbon dioxide, and mineral crystals
How does magma become lava?
when it reaches the surface and loses its gases (changing in composition)
As a general rule, temperature and pressure ____
increases with depth in Earth’s lithosphere
What are the 2 ways rocks can melt?
Decompression Melting and Subduction
Decompression Melting
is the reduction of pressure on, which allows the mateial of the asthenosphere part of the mantle to (whichis already very hot, near liquid) to completely molten into magma
Where does Decompression melting occur?
in places where plates of lithosphere are moving apart
Subduction
the formation of magma through one lithospheric plate being pushed under another (in the oceanic crust) and is heated and releases water in the form of vapour and the vapour hydrates the rock in the mantle
this lowers the melting point and the wet rocks become magma
Wet rocks tend to melt at lower temp than dry rocks. T or F
T
What happens in both cases of magma formation?
the magma is less than the rock surrounding it so = rise towards the surface
When magma rises, where can it collect?
Large pockets called “magma chambers”
What 2 things can happen to the magma that arose?
It can remain trapped in a magma chamber, or can escape to the surface and erupt from volcanoes
What happens when magma ascends towards the surface and starts to cool?
it allows minerals to crystalize from chemical components of the magma melt
What happens when magma remains trapped in a magma chamber?
it cools very slowly, allowing the crystals to grow to large sizes
What are Plutonic Igneous rocks?
rocks that form from the cooling of magma below the surface
what are plutonic igneous rocks AKA? and Why?
Intrsuive Igneous Rocks, b/c they are formed from magma intruded into pre-existing rocks of the crust
What is a Batholith?
a very large mass of igneous rock formed from cooling of a major magma chamber
A Dyke?
a tabular-shaped igneous body oriented at a significant angle to layers of pre-existing rocks
A Sill?
a tabular-shaped igneous body intruded parallel to layers of pre-existing rocks
Laccolith
blister like bodies which aer the overlying layers where magma is pushed through in between
Volcanic Neck?
Igneous material that once occupied the vent of a volcano
Why are crystals in most rocks shaped imperfectly? and what is this texture called
because intrsuive igenous rocks cool from magma below Earth’s surface and mineral crystals have ample time to grow
phaneritic texture
What is the extrued liquid material that escapes a magma chamber and erupts from a volcanic centre?
Lava
Why is lava extruded?
extruded from a volcanic vent due to the expansion of volatilies as they escape to the surface in response to a decrease in pressure
Why is lava not the exact same as magma?
because lava has lost much of its gas content
What is the name of the texture where the rocks are too small to be ovserved with the unaided eye?
Aphanitic Texture
the texture of igenous rocks that show large mineral crystals inbedded in a “groundmass” of much smaller crystals
porphyritic texture
how is a porphyritic texture produced?
produced when the magma initially cools very slowly, but later moves closer to the surface, cooling more quickly and producing smaller crystals
a 2-step cooling history
How is a glassy texture created?
when lava solidifes too quickly for any significant crystalization to occur
What is an example of a glassy texture?
Obsidian
What is a pyroclastic texture?
rocks that have dust particles of molten material made from volcano eruptions within them
What are most igneous rocks principally made of?
silicate materials
What do all silicate materials contain?
the substance silica
What is the makeup of Silica?
a complex ion made of 1 silicon atom bonded to 4 oxygen atoms
what is silicon atom’s overall charge?
-4
What are Ferromagnesian silicate minerals?
the most important silicate minerals used in the identification of igneous rocks
basicallly iron and/or magnesium ions ionically bonded with silica units
Planes of weakness are represented by ___
planes of ionic bonding
What are all ferromagnesian silicate minerals made of?
iron, magnesium, and silica, with only minor additional metals
What difference is found between the ferromagnesian silicate minerals?
the main diff lies in their structure
Why do biotite mica break cleanly into thin sheets?
because the intermolecular bonds that hold the sheets together are weaker than the ionic bonds
What does Quartz consists of?
only silica
How are all of the bonds in quartz equally strong?
because of the complete sharing of oxygen by adjacent silicon atoms
what kind of cleavage and fracture do Quartz generally show?
quartz doesn’t have cleavage and generally shows a conchoidal fracture
What is the most common mineral group
Feldspar
What are the 2 most common types of feldspar?
Plagioclase and Potassium Feldspar
Plagioclase? Contain? Color?
contains ions of sodium and calcium. Color ranges usually from white to bluish grey
Potassium Feldspar? Contains? Color?
contains ions of potassium, and the color ranges from creamy white to salmon pink
Why aren’t all igneous rocks made of just one mineral with one composition?
the reason is that diff minerals crystalize at diff temperatures
Bowen’s Reaction Series
The sequence of crystalization when magma cools, minerals crystalize in a distinct sequence
how odes paglioclase feldspar crystalize?
In the continuous series.
where the mineral composition gradually changes
What is the shorthand way of describing composition?
Based on the color or tone of an igneous rock
Mafic igneous rocks
igneous rocks with high concentrations of dark-colored minerals
Felsic Rocks
Igneous rocks containing high concentrations of light-colored minerals with only minor amounts of dark minerals
Intermediate Rocks
rocks with intermediate composition and a color in-between mafic and felsic
What does the rock name tell us and why is it important?
it tells us something about the rock’s composition and texture
Gabbro
Phaneritic texture and Mafic Composition
Diorite
Phaneritic Texture and Intermediate composition
Granite
Phaneritic texture and Felsic Composition
Basalt
Aphanitic texture and mafic composition
Andesite
Aphanitic texture and intermediate composition
Rhyolite
Aphanitic texture and Felsic Composition
Gabbro is a ___ igneous rock (decompression melting)
Intrusive
Basalt is a ___ igneous rock (decompression melting)
extrusive
Granite is a ___ rock (hydration melting)
Intrusive
Rhyolite is a ___ rock (hydration melting)
Extrusive
Diorite is a ___ rock (hydration melting)
Intrusive
Andesite is a ___ rock (hydration melting)
extrusive
aside from aesthetic value, what is the main reason igneous rocks are used
durability
What are the 3 components that play a role in the durability of an igneous rock being helpful
Naturally resistant to chemical damage
The Silicate minerals that make up most igneous rocks have high hardness values
composed of tightly interlocking crystals and is very tough
3 disadvantages of using igneous rocks?
expensive
difficult to cut and polish
heavier (transportation costs)
