4.B.2 The Nature of Igneous Rocks

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From the course by University of Illinois at Urbana-Champaign

Planet Earth...and You!

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University of Illinois at Urbana-Champaign

Planet Earth...and You!

26 ratings

Earthquakes, volcanoes, mountain building, ice ages, landslides, floods, life evolution, plate motions—all of these phenomena have interacted over the vast expanses of deep time to sculpt the dynamic planet that we live on today. Planet Earth presents an overview of several aspects of our home, from a geological perspective. We begin with earthquakes—what they are, what causes them, what effects they have, and what we can do about them. We will emphasize that plate tectonics—the grand unifying theory of geology—explains how the map of our planet's surface has changed radically over geologic time, and why present-day geologic activity—including a variety of devastating natural disasters such as earthquakes—occur where they do. We consider volcanoes, types of eruptions, and typical rocks found there. Finally, we will delve into the processes that produce the energy and mineral resources that modern society depends on, to help understand the context of the environment and sustainability challenges that we will face in the future.

From the lesson

Week 4: Rocks and Mineral Resources

As part of the Week 4 Assignment, you will take a close look at your daily surroundings to identify Earth resources. The video lectures for the week examine various aspects of finding, extracting, and using resources such as metals and stones. For the lab, you will utilize Google Earth to examine several mining sites around the world. In the discussion, you will weigh the pros and cons of mining operations, as many communities have had to do already. This week also includes peer grading discussions, as explained on the How Graded Discussions Work page. Finally, we provide an optional assignment for those who would like to identify some common minerals.

4.B.1 Introducing Classes of Rocks5:42

4.B.2 The Nature of Igneous Rocks9:25

4.B.3 Different Types of Sedimentary Rocks4:46

4.B.4 Formation of Clastic Sedimentary Rocks6:53

4.B.5 Primary Sedimentary Structures and Sedimentary Basins6:26

4.B.6 Carbonate Sedimentary Rocks6:11

Meet the Instructors

Dr. Stephen Marshak
Professor and Director of the School of Earth, Society, and Environment
Department of Geology
Dr. Eileen Herrstrom
Lecturer
Geology

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We've looked at cross-sections of volcanos.

And we've seen that deep below a volcano there's often a magma chamber,

a space in which there's quite a bit of molten rock.

Probably not like a big open pool, but

probably a place where molten rock has made its way into the many cracks and

open spaces between the grains of pre-existing rock.

Eventually, that molten rock may find its way up a chimney or a crack up to

the Earth's surface, where it eventually erupts as lava or ash at a volcano.

Now with that in mind,

let's distinguish between two different realms of the igneous world.

The intrusive realm is what's underground.

It consists of rocks that form by the intrusion of magma into spaces underground

where it solidifies, so those rocks never reach the surface before they've formed.

In contrast, there's also the extrusive realm, and

these consist of igneous rocks that form either from lava that flows at

the surface, or ash that settles at the surface, or flows at the surface.

In other words, these are composed of rocks that were extruded,

or thrown out by a volcano.

The grain size, in other words, the texture of an igneous rock,

is controlled simplistically by the rate of cooling.

Specifically, if a melt or

a molten rock cools very quickly, it forms only small grains.

Whereas if it cools more slowly, the grains have time to grow larger and

the rock will contain larger or coarser grains.

So if you remember the different kinds of magma that form

that differ from one another in terms of their chemical composition,

specifically the ratio of silica to iron and magnesium.

Remember there was ultramafic magma, mafic magma,

intermediate magma and felsic magma.

Well, now let's take those magmas and cool them.

If those magmas cool fast, we get fine grain igneous rock,

they cool slowly they get coarse grain igneous rock simplistically.

We have different names for the different rocks that reflect the differences in

composition and the difference in grain size.

For example a fine grained mafic rock is called the basalt,

coarse grain mafic rock or gabbro.

Similarly a fine grained intermediate rock is an andesite and

a coarse grain one is a diorite.

And a fine grained felsic rock is a rhyolite

while it's coarse grained equivalent is called a granite.

It's not strictly the case all the time, but

in general the finer grain rocks form in volcanic settings or

in very shallow intrusions, intrusions that occur in the uppermost part of

the crust where the ground is cold, and so therefore the intrusions cool fast.

Whereas the coarser-grained rocks form deeper in the crust,

in a deeper intrusive environment.

Remember at a volcano there is also the eruption of ash and other debris.

There many names for different kinds of deposits that result from that,

we'll just one mention right now.

Geologists use the term tuff to refer to a pyroclastic deposit

composed primarily of igneous ash.

I also mentioned that in some cases, rocks are formed of glass.

Geologists used the term obsidian, for a mass of volcanic glass.

Now that we understand the various different kinds of igneous rock and have

some idea of the range of compositions and textures that igneous rocks can have,

let's turn our attention back to the realms in which igneous rocks form.

Specifically, recall that we can distinguish between intrusive rocks,

which are rocks that form by cooling and solidification underground,

from extrusive rocks, which are rocks that form either by the expulsion of ash

into the atmosphere or by the cooling of lava on the surface of the Earth.

Let's look at some examples of these different realms.

We're going to start our tour in Joshua Tree National Monument.

Here, over 100 million years ago the area was an active volcanic arc.

And underneath this arc large volumes of magma were being produced.

Not all of this magma was formed at the same time.

Modern research suggests in fact, that the magma was intruded as a series of layers,

one below the other over a period, of perhaps,

a few million years at depth in the Earth.

Gradually however, this vast massive rock cooled and

formed a large body of granitic rock which we call a pluton.

A pluton in effect is an almost blob like or to some extent irregularly shaped

mass of considerable size of granite that forms beneath the surface of the Earth.

Presumably at the time that this granite was forming there were volcanos perhaps

several kilometers higher up on the surface of the Earth above.

These volcanoes, and quite a bit of rock,

above the plutons, has been eroded away by the process of exhumation,

in order to expose the granites that we now see today in the park.

These intrusions of magma,

took place several kilometers below the surface of the earth.

Where the magma had time to cool very slowly, and thus, there was an opportunity

for the fairly large crystals that are characteristic of granite to form.

If we zoom in on some of this granite, we can see that it's a coarse grain texture.

The grains are big enough to see with the naked eye.

Now you might wonder what is that little dark patch.

That's called a xenolith,

that's a block of preexisting rock that was brought up with the magma and

then got frozen into place when the rest of the rock became solid.

So that was a deep intrusion, now let's look at a shallow intrusion.

When we say a shallow intrusion, we're referring to

an intrusion that occurs in the upper most crust, where the surrounding country rock,

or the wall rock, that the igneous magma is intruding into is relatively cool.

Typically, in this realm, rocks will follow planes of weakness and

propagate along cracks or between bedding planes.

Typically, we refer to somewhat vertical wall like intrusions

that may cut across the preexisting layering of rock as dikes.

Alternatively, in places where tabular intrusions inject parallel to preexisting

layering, and are somewhat horizontal, we refer to those bodies as sills.

Here's an example of basaltic dike.

A dike is a kind of igneous intrusion that is tabular shaped.

It's kind of shaped like a wall.

And it cuts across the preexisting rock, basically pushing its way up

along a crack, sometimes creating the crack as it propagates.

In some cases, igneous dikes cut across preexisting sedimentary layers,

in some cases, they cut across preexisting igneous rocks.

This example shows a basaltic dike that's cutting across a granite.

Here is an example of an igneous sill that's exposed on a cliff,

the sill is intruding between layers of sedimentary rocks.

It stands out as a cliff because it's much more resistant to erosion

than the surrounding sedimentary rocks that lie above and below it.

One thing about sills and dikes that is important to keep in mind,

because these are intruded as shallow levels in the crust,

they cool rather quickly, and therefore, are relatively fine grained rocks.

That's why the examples that we've shown are composed of the salt

rather than of gabro.

We've just looked at examples of intrusive igneous rocks.

Now let's come up to the surface of the Earth and

look at examples of the realms in which extrusive igneous rocks occur.

We will start our examination of occurrences of

extrusive igneous rocks by looking at pyroclastic debris.

Here, at Ubehebe Crater, a locality in Death Valley, California, we can see

these layers of pyroclastic debris deposited on top of an earlier lava flow.

The lava flow is the dark rock,

the pyroclastic debris has distinct layering within it.

What we're seeing here is an example of a tuff or

an ash deposit from a volcano in Hawaii.

Remember that the eruption of molten liquid rock that hasn't been blasted into

fragments that solidified to form little glass shards, will produce lava flows.

Here's an example of a lava flow forming on Hawaii.

The red part is still molten.

The black part has already cooled and partially solidified, and

the rock in the distance is completely solid through and through.

An earlier basaltic lava flow the same area covered a highway.

So, you know that the basaltic flow is younger than the highway.

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