Review: igneous rocks were once melted rock (called magma). The melted rock crystallized and hardened into bedrock when it cooled.

THE STORIES THAT IGNEOUS ROCKS TELL:

Igneous Bedrock Story Part 1)
Igneous rocks tell us the story of how the melted rock became solid rock.

Igneous Bedrock Story Part 2)
Igneous rocks tell us stories of what was melted, where it came from, and how it was melted.

Part 1 of the Igneous Bedrock Story.

HOW THE MELTED ROCK BECAME SOLID ROCK

There are two ways melted rock becomes solid rock:

• Cooling inside the Earth,
• Cooling on the outside of the Earth.

Key observation # 1
Look at the crystals of your igneous rock. Can you see them without the help of your hand lens? or are they so tiny that you need the hand lens to distinguish individual crystals?

There are two general ways by which magma solidifies into igneous bedrock: 1) fast (as lava, extrusive igneous rock, on the outside of the Earth) and 2) slowly as an intrusive igneous body inside the Earth. Salt Lal{e County is unusual in that it has good examples of both.

The fast way ... extrusive igneous rocks (HINT: extrusive like "exit.") The melted rock makes it to the surface of the earth and explodes like Mount St. Helens or flows like lavas of Hawaii, or a combination of both. The crystals of the melt don't have time to grow. They cool almost instantly as ash from the volcano, or as lava flowing across land or into water.

The slow way ... intrusive igneous rocks (HINT: to intrude is to force yourself in.) The magma does not make it to the surface of the earth. It cools as a mass, a blob, below the earth's surface. Crystals have time to grow. Cooling may take many thousands of years. Some crystals have time to grow, in extraordinary cases, several feet.

Key observation #2
What is the relationship of the igneous bedrock to surrounding bedrock? CAUTION: Remember the difference between sediment and bedrock. Can you tell where your rock came from and the general vicinity of the bedrock it was eroded from? If so, look for relationships of the igneous bedrock with that of other bedrock.

With your Key observation #1 (crystal size) you may have sufficient evidence to determine whether your rock is intrusive or extrusive. Did it cool as a mass or slab encased by miles of rock -or- did it make it to Earth's surface and flow like lava or explode as ash? Can you see those relationships in the field?

Imagine a photograph of the hill-slopes and hill-tops such as Table Mountain of the Traverse Mountains in south western Salt Lake County. Where are the tan and dark brown igneous rocks with respect to other rocks? on top of them? below them? into them?

Caution: Adults may find it unsettling to realize how areas they are familiar with today have changed dramatically from how they looked in the past. Earth science looks at time differently than other sciences. A child looking at the igneous rocks of Little Cottonwood Canyon may not feel comfortable with the concept that several million years ago (older than their parents but lots younger than the dinosaurs) there was no canyon there. Blobs of cooling, melted rock, were cooling miles below the land surface. Millions of years of erosion have exposed the igneous body (the granite) and the gray rocks it intruded into (very old, already metamorphosed bedrock).

Part 2 of the Igneous Bedrock Story

WHAT WAS MELTED, WHERE IT CAME FROM, AND HOW IT WAS MELTED.

Key observation #1:
Color: how "dark" or "light" colored is your rock.
Caution ... sometimes a polished surface or a "fresh face" wi11look very different from a weathered surface. Geologists carry rock hammers in the field to crack rocks open and have both sets of information.

REMINDER: Crystal size gives clues about how fast the melted rock recrystallized.
COLOR: gives clues about the type of material that was melted. Different colored igneous rock means different chemistry of the magma, and different chemistry of the magma implies a different portion of the crust was melted.

Color of the igneous rock tells you about the chemistry of the magma.

The chemistry of the magma tells you about the crust that was melted to form the magma.

The crust underlying the oceans or areas that once were oceans consists of significantly more "dark minerals" than the quartz-rich crust of the continents. Some magmas contain material that came from below the crust in the uppermost mantle. These magmas also have proportionately more dark minerals and less quartz than melted continental crust. Dark-mineral-rich magmas with less quartz flow more rapidly than quartz rich magmas. Basalt, an extrusive dark-mineral-rich lava can flow like rivers of rock for miles. Quartz-rich magmas that reach the surface of the earth tend to be more explosive such as the magma of Mount St. Helens.

COMMON IGNEOUS ROCKS
Remember ... the most important teaching is to tell the story of the igneous rock and the melting and crystallization process. However ... names are helpful handles of communication and if your students want to learn a few names, here are the names of two very different igneous rocks: basalt and granite.

Basalt:
Where found: the lava rock of Hawaii or hill-topping lava rock of St. George. Salt Lake County does not have basalt outcrops.
Bedrock Story 1: How did the melted rock crystallize into solid rock? Note the very fine crystals. So the melt cooled rapidly. You conclude (correctly) that basalt is an extrusive igneous rock.
Bedrock Story 2: What bedrock was melted? Where did it come from? You observe how basalt is dark-colored with many little-dark-minerals and not much quartz. Basalt travels rapidly from deep in the crust tapping lower crustal magma, upper mantle magma, or melted oceanic crust (the bedrock that underlies the oceans). It flows like a fluid usually up cracks or narrow zones to the surface of the Earth and then spreads out as lava flows.

Granite:
Where found: Salt Lake County has magnificent exposures of granites in Little Cottonwood Canyon.
Bedrock Story 1: How did the melted rock crystallize into solid rock? You note the coarse, large crystals. The melt must have cooled slowly. You conclude (correctly) that granite is an intrusive igneous rock.
Bedrock Story 2: What bedrock was melted? Where did it come from? You observe granite's light color. It has abundant quartz and less than 10 - 20 percent little-dark-minerals. The crust of continents melts into magma rich in quartz. Once the crust is melted by high pressures and temperatures, it is less dense that the unmelted, surrounding rock and it slowly works its way upward. The granites of Little Cottonwood Canyon cooled 5 - 10 miles below the surface of the Earth in the upper part of the crust. Erosion has exposed the granites and the rock they intruded.

THREE EASY STEPS TO TELL THE STORY OF AN IGNEOUS ROCK ....

Imagine, you have just been handed a chunk of igneous rock.

Bedrock Story Step 1)
The rock is a piece of sediment. What bedrock did it come from? You recognize the rock came from igneous bedrock because of its homogeneous texture and its interlocking crystals.
"What an interesting rock ... look how all the minerals in it are all mixed together and you can see lots of individual crystals and how the crystals are grown together, sort of locked together, interlocking. These crystals became crystals as a liquid mush of melted, molten rock cooled and solidified. This rock once was all melted and very hot. Big crystals means it cooled slowly and the crystals had a chance to grow. Little crystals means it cooled fast. The rock is called an igneous rock. Igneous means fire, like ignite a fire or your car's ignition. All igneous rocks were once very hot, under great pressure, and melted."

You examine the rock with the help of the student.
You notice individual crystals and minerals particularly quartz, feldspar, and the little-black-minerals. How much void space is there in the rock? You have an opportunity to comment on individual minerals, their hardness and habits.
"Look at all the quartz crystals. They remind me of the glass of a window."
"See how these white ones look like walls and reflect light if you hold the rock just so.»
"See how fine, fine, fine the crystals are. They must have cooled very fast. "

Bedrock Story Step 2)
Is the igneous rock an intrusive or extrusive rock, and what was melted? You base your answer primarily on the size of the individual crystals:

• very fine = extrusive, fast cooling, or
• coarser = intrusive, slow cooling.

1. "This is the type of lava rock that comes out of the volcanoes of Hawaii. Look at the very fine crystals. It is hard to see them. Your rock tells the story liquid rock that was me/ted in the lower skin of the Earth, and traveled like a fluid up cracks and narrow passageways until it reached the surface of the earth and poured like a river of melted rock across the landscape that was here then. I wonder what it was like back then. "

-or-

2. "The big crystals of this rock mean it was melted rock that cooled slowly, see the big crystals. The light color of the rock means the melted rock came from an area like our Rocky Mountains not like the crust under the oceans. I can't imagine that it moved very fast through the earth. Really slow stuff It must have incorporated the rock above it toget closer to the surface. And these big crystals mean it never made it to the Earth's surface. It just sort offroze, slowly, crystal by crystal below the surface of the earth Wow ... that means that back then these rocks were not at the swface at all. They were under the surface with miles of rock on top of them Weird. I wonder what this area looked like back then. I bet there wasn't even anyone to figure out there was a hot blob of melted rock cooling below. It's changed a lot."

Step 3

Where is the rock going (on the rock cycle).
"Look how rounded and srrwoth this rock is. The stream knocks off little bit by little bit. The quartz just become sand. It's so tough against erosion. But these white crystals tum into clay. Maybe in a thousand years the little pieces will make it to Great Salt Lake.» Now you're well launched into the story of sediments of Salt Lake Valley.

Learn about Rock Teaching Techniques.>>