The Thread Where We Talk About Igneous Rocks

This is the place for talking about or having philisophical debeates about Igneous rocks.

It is not at all clear why, but minerals at the top of Bowen's Reaction Series tend to be dark in color (e.g. pyroxene and amphibole), and minerals at the bottom tend to be light in color (e.g. Na plagioclase and quartz). After all, what could temperature of formation have to do with mineral color?
And it is also not clear why, but magma compositions tend to segregate out, mafic magmas at the top of Bowen's Reaction Series, intermediate magmas in the middle, and felsic at the bottom. Result? Mafic magmas produce dark colored rocks made of dark minerals (such as basalt), intermediate magmas intermediate colored rocks (e.g. diorite) and felsic magmas light colored rocks (e.g. granite).
Because of these fortuitous conditions it is natural to classifiy igneous rock on color and texture. As a first approximation, a classification based on color and texture is ok, but can lead to great mistakes and ultimately a color/texture classification is inadequate. Thus, other classifications exist, per below.

Go To Color/Texture Key (not "clickable" yet) - pdf version (is clickable).


--------------------------------------------------------------------------------

Modal Igneous Classification
A modal classification classifies igneous rocks on the relative abundance of five minerals they may contain:
(1) Quartz,
(2) Alkali feldspars (orthoclase, but including albite [sodium plagioclase] if anorthite [calcium plagioclase] content does not exceed 5%),
(3) Plagioclase,
(4) Feldspathoids (silica poor minerals; no samples available)
(5) Mafic minerals (such as pyroxene and amphibole).

A modal classification includes the common names like granite, basalt, diorite, etc. The classification is based on arbitrarily defined boundaries between classes on a ternary diagram. In general the modal names correspond with names derived from a color/texture classification, it is just the decision on what to call a rock is based on mineral content rather than color. Such a classification works readily in a key.
Go to Composition/Texture Key (not "clickable" yet) - pdf version (clickable)

But the classification is also commonly displayed as a "Mineral Percent Abundance" chart.
Go to Mineral Percent Abundance Chart (completely clickable with lots of pictures.)

Observe that to identify rocks in this classification it is necessary to make several crucial observations and decisions. The first is how much quartz does the rock have; >20% and the specimen can be only one of three rocks, alkaligranite, plagiogranite, or granodiorite (see key).
A second decision is whether the rock is dominated by feldspars (see key).
A third decision is whether the rock is about a 50/50 mixture of mafics and feldspar (see key).
And finally a decision on whether the rock is mostly mafics see key).

These decisions divide the rocks into four broad categories which are then easier to remember. It is definitely better to have a systematic strategy for observing and idenifying the rocks than just going at it randomly.


--------------------------------------------------------------------------------

SUITES
A Normative Igneous Classification
The normative classification groups together igneous rocks we normally think of as unrelated, such as basalt, diorite, and granite. For this reason normative rock classification is not as easy as in a color/texture or composition/texture system. These rocks do have different mineral assemblages, but may be very similar in their chemistry, reflecting an origin from a common parent magma via fractionation.
On the other hand, a normative classification works extraordinarily well when igneous rocks are examined in terms of plate tectonic processes, the place we ultimately want to go. It is the level at which geologists must study igneous rocks. In compensation, however, for those not interestd in the details, it is possible for a normative classification to in fact be

 

Pummace rules.

 

[image=http://www.calstatela.edu/faculty/acolvil/igneous/pumice.jpg]

 

Igneous rocks suck! Sedimentary rocks r00l!!

 

Metamorphic rocks own your face!!

QGM

 

Obsidian rock is one of my very very ver favorite rocks!!!!!

[image=http://www.transistor.org/personal/photos/klamath2001/obsidian.jpg]
(I have been here and seen that stuff!!!)

[image=http://www.wicca.com/celtic/stones/images/obsidian-pol.jpg]It is so pretty polished!!!

[image=http://www.oregongeology.com/learnmore/jpegs/orig/obsidian.jpg]
And unpolished!! So cool. I have a number of pieces of Obsidian, both polished and unplosihed.

 

Obsidian is a good name to for use in super hero names.

Like, OBSIDIAN AVENGER for example. Is that not the coolest name?

 

Rocks classified as igneous are 37% more likely to commit a major felony than those from other classifications.

 

I have a friend who's a geologist, and he can talk about rocks for HOURS.

The rock collection he has at home is really cool, though. Lots of neat fossils and things to look at.

I like shiny things, so igneous rocks are my favorite.

 

wOOt! I've been waiting for one of these threads for years! Thank you Robotic-Bagel! Thank you!

 

I have a friend who's a geologist, and he can talk about rocks for HOURS.

If a person talks about rocks for hours and nobody is awake to hear it, does he make a sound?

 

Detrital Sedimentary rocks > you.
[image=http://core.ecu.edu/geology/harper/Sedimentary/S12.gif]
[image=http://wps.prenhall.com/wps/media/objects/346/354316/sandstone_tasa.gif]

 

At first glance I thought the title said Ingenious Rocks. I was looking forward to in-depth discussions about rocks with brilliant ideas.

Like, OBSIDIAN AVENGER for example

Or "Iron Fist of the Obsidian Sith"! [/obligatory]

 

There cannot be a bad rock, because all rocks rock.

If there were no rocks, there would be no buildings.

And then where would we all be?

Outside, that's where.

 

Darth_Ignant rocks!

Hell yeah!

 

obsidian is my favorite

 

I live under an Igneous rock.

 

I remember a ST: Enterprise episode where they kept yelling SHALE! all the time!

 

I am 40% titanium. I might have trace amounts of obsidian for prettiness, I think.

beep beep beep