Igneous Rocks--Text

Earth Science Essentials

by Russ Colson

  

One key idea from lecture :

Knowing the names of igneous rocks is not the main goal of scientific investigation.   Rather, geologists are interested in the story that igneous rocks tell us.   However, it is how we know how to read the story that is at the heart of doing science.   From the lecture, you know that we know how to read the story of igneous rocks because

 

 

 

 

Taking Notes:

You should take notes as you read through each of the texts in this course.   Yes, I know that you can cut and paste the information.   And you can Google stuff.   None of that really aids the learning process.   Taking notes does.   It forces you to process ideas presented to you and make those ideas your own.   Remember, this course is not about learning information (which is free out on the internet--why pay a university for that?), but rather it's about learning to read the Earth and reason like an earth scientist.   Did you take notes during the lecture?   If not, then you've done yourself a disservice.   It's not too late to start!   Take notes as you go through this text, making the ideas your own and exercising your own ability to reason through problems.

 

The Story Igneous Rocks Tell

As discussed in the lecture, the key story that igneous rocks tell us is how fast the rock cooled from its molten state.   Faster cooling happens at the Earth's surface (which is a very cold place if you happen to be a molten rock) while slower cooling occurs deeper in the Earth's crust where overlying rocks slows down the rate of heat loss.   The slower cooling allows more time for the elements that make up a particular mineral to diffuse through the melt to a growing crystal and thus allow for growth of larger crystals.  

 

Other factors can also influence crystal size, such as ease of getting crystal growth started (called nucleation), the abundance of elements in the melt that make up that particular mineral, or the intrinsic ease of adding new atoms to the growing crystal.   However, the rate of cooling often is the dominant factor in determining whether crystals are big enough to see with the naked eye (phaneritic) or are microscopic in size (aphanitic).

 

The pictures below show rock of similar composition, both from the Duluth MN area.   Both were found at the surface of the Earth.   Both were collected from a position stratigraphically under thousands of feet of frozen lava.   The following quiz tests your ability to read the stories of these two rocks.

 

 

 

 

Below are a couple of rocks that have a combination of microscopic crystals and crystals large enough to see.   This type of texture is called porphyritic.   Before continuing, what story do you think a rock with this texture might tell us?

 

 

 

 

 

The key idea is that the rock experienced two different cooling stages, one fast and one slow.   The slow had to occur first, deeper in the Earth's crust, allowing time for the larger crystals to grow within the magma.   Then the rock was erupted to the surface of the Earth, allowing the rock to cool quickly and the remainder of the molten material to freeze.

 

Classification Philosophy and Igneous Rock Names:

I mentioned in the lecture that naming rocks is not the main goal of science.   Even so, we have to name rocks in order to facilitate communication and to aid good observations in the field.   This means that our classification and naming should be such that we don't have to rename rocks (which would confuse past communications) and should not bias our observation-making by making us see only features we expect or think we should see.   Because of this constraint, good science classification schemes usually name things according to features that are clearly observable (descriptive classification) and not according to how with think the rock formed (genetic classification, from 'genesis' meaning origin). With a genetic classification, every time we learn something new and add to our understanding of a rock, we'd have to rename it.   What's more, a genetic classification focuses our mind not on the features actually present in the rock but on the features we expect to see based on how we think it formed.

 

Thus, naming a rock according to the features we see is usually better, allowing us time to figure out how it formed and with less bias.

 

For igneous rocks, our primary observable features are size of crystals and composition.   The chart below shows some common types of igneous rock.

 

	Bigger Crystals (Phaneritic)	Smaller Crystals (Aphanitic)
More SiO2	Granite	Rhyolite
	Diorite	Andesite
Less SiO2	Gabbro	Basalt

 

 

Here's a story puzzle for you:

 

The light from my electric torch swept over the smooth floor, pausing at the rough pile of dust and bone where the captain must have died.   A million years ago.   I turned the light to the rock in my hand that I'd brought with me from the quarry outside--the quarry where the mine operators found the ship under a hundred feet of stone.   White beams glistened off the cleavage planes of minerals in the rock.   Plagioclase, biotite, and quartz, each crystal a millimeter across, made a multicolored patchwork.   I was so engrossed I barely caught the head scientist's analysis.

 

"We think the ship came to Earth about a million years ago and either crashed into or mistakenly landed in a lava lake.   The crew must have died in minutes as their systems failed and their ship sank to the bottom."

 

I knew he had it wrong.   Something more puzzling than a crash landing in a lava lake was afoot.

 

How does Our Hero know that the chief scientist has it wrong, and what does he realize that makes the story even more puzzling?

 

 

 

Other Stories:

Although the size of crystals tells the main story of igneous rocks that we are interested in here, the composition of an igneous rock also tells a story.   When rocks partly melt or partly freeze, the solid part and the liquid part will have different compositions.   This allows different composition rocks to be generated from a single-composition progenitor.   We call this process fractionation.   It is fractionation that allows all the variety of rocks, gems, ores and the layers of the Earth to form from the same basic star dust.   We'll talk more about fractionation processes later on in this course.

 

The more primitive compositions, that is, those that are more like the Earth's mantle from which most igneous rocks originate, have less silica (SiO₂) and more magnesium and iron.   We call these compositions mafic.   These rocks are often dark colored.   A more evolved or fractionated rock will contain more silica and aluminum.   We call these compositions sialic (or felsic, for feldspar and silica).

 

Often, basalt, one of the more mafic compositions, occurs at mid-ocean ridges where lithospheric plates are diverging.   Andesite, an intermediate composition, often occurs at subduction zones where lithospheric plates are converging.   Granite, a more felsic composition, is more common in continental regions. On other planets, presence of lots of more evolved compositions suggests a long and active geological history.

 

Last updated 10/7/2014.   All text and pictures are the property of Russ Colson.