Review: Metamorphic rocks, as their name, "meta" (changed), "morphic" (shape), implies, are rocks whose original rock structure and minerals have been changed, often dramatically. Unlike igneous rocks, metamorphic rocks were not melted. Students can tell you about Morphs, Power-Rangers that change their properties.

THE STORIES THAT METAMORPHIC ROCKS TELL:

Metamorphic Bedrock Story Part 10
Metamorphic rocks tell the story of how they were metamorphosed, of the temperatures and pressures that caused them to change.

Metamorphic Bedrock Story Part 2)
Metamorphic rocks often tell the story of what the rock was before it was metamorphosed.

NOTE: These are the two bedrock stories that a sedimentary rock tells, but in reversed order. The BIG story for a sedimentary rock is "What was it like when the sediments were deposited?" The BIG story for a metamorphic rock is "Gosh, what happened to you?"

Part 1 of the Metamorphic Bedrock Story.

HOW THE ROCK WAS METAMORPHOSED:

What is the rock telling you?
Observations you can make to determine the severity of metamorphism.

Key Observation # 1

• • Look at individual crystals and grains of your metamorphic rock through a hand-lens.
• • Look at the general pattern of your metamorphic rock at arm's length. Can you tell what the rock was before it was metamorphosed? Can you still see the outlines of pebbles and see the rock was a sedimentary rock before it was "cooked"?

Can you see vestiges of layering ... almost like a sedimentary rock but "smushed" so some of the layers drift into each other and apart? Is the rock so full of quartz it just had to be a sandstone before it was fused together?
If you can see these features ... you're rock was not nearly as "cooked" (metamorphosed) as a metamorphic rock with swirls of quartz and dark minerals?
How much have pressure and heat changed the characteristics of your rock? (Does it need a psychiatrist?)

Metamorphic rocks range from partially "cooked" low-grade metamorphism to almost-melted toothpaste-like hardened mush that was not-quite melted. Three factors affect the "grade" of metamorphism ... temperature, pressure, and fluids. Most metamorphism takes place with pressures from the overlying rocks AND pressures from forces of crustal plates colliding, sliding, pushing and grinding.

• • High-grade metamorphic rocks tell the stories of enormous pressures of continental collisions, of adventures at the root of mountain ranges as impressive as the Himalayas today, of forces that make rocks act like toothpaste and stretch like taffy. (Great stories! Think big ... this means these rocks were there! Of course that raises the question of how they got here ... and those are good questions ... I'd answer "Mountain building and then erosion to this level, how do you think that works?")
• • Low-grade metamorphic rocks tell less dramatic stories of burial by megatons of sediments above them and then great pressures and stresses and strains but not so great as the high-grade metamorphic rocks.

Key Observation #2:
Notice how hard-to-break your metamorphic rock is.
The rock can be very very difficult to break open except along cracks. (It can be quite dangerous when a chip flies off.)

In metamorphism, some minerals, especially quartz, mobilize and fuse themselves and rock components together. When you break open metamorphic rock with your rock hammer it will crack across pebbles rather than break around them. Some rocks will not even split along their sedimentary-related layering but will split along a differently-oriented patterns of rock fabric which developed as a result of the horrific pressures of metamorphism.

Key observation #3
What do you know about the bedrock outcrop this rock came from?
Try to figure out how extensive and severe the metamorphism is ... was it contact metamorphism from proximity to a hot body (unusual) or regional metamorphism from mountain building or other crustal changes.

There are two general ways that rocks get metamorphosed.

• • One is the usual way (regional metamorphism)
• • One is the unusual way (contact metamorphism).

Most metamorphic rocks that you'll be handed have come from units of "regionally-metamorphosed" bedrock. These broad volumes of rock were metamorphosed under a mountain range or in an area of the crust experiencing great pressure and / or high temperatures. However, there is another kind of metamorphism, contact metamorphism, which takes place right against the margins of igneous rocks. The rocks that surround an igneous, hot body, are not heated to such an extent that they melt, but they become altered by the intense heat, pressure, and fluids associated with the igneous body. These contact-metamorphosed rocks are in close proximity to an igneous body.

For example: The Wasatch Range, Little Cottonwood Canyon has both kinds of metamorphic rocks. The dark gray and brown rocks at the mouth of the canyon were regionally metamorphosed during the first chapters of Utah's geologic history. Granitic igneous bodies intruded the already-regionaUymetamorphosed rocks in the seventh chapter of Utah's geologic history. (For our usage of chapters of Utah's geologic history, see our Geologic History of Utah for Non-geologists, in preparation). The hot igneous body cooked some of the other rocks in the immediate area such as the gray and white streaked marbles at Alta.

Our hunch is that you will encounter relatively few contact-metamorphosed rocks in your lifetime. But if you lived in Davis County you'd see regionallymetamorphosed metamorphic rocks as part of your every-day mountain vistas (and virtually no contact-metamorphosed rocks). If you lived in New England, you might not see any rocks except regionally-metamorphic rocks for hundreds of miles around you, (and no contact-metamorphosed rocks).

Part 2 of the Metamorphic Bedrock Story.

WHAT WAS METAMORPHOSED?

Key observation #1
Look for clues ... the clues for low-grade metamorphic rocks often include signs of layering and shadows of sediments such as gravels or pebbles. These are hints of sedimentary rock features and the sedimentary-rock origin of your metamorphic rock.

Low- to moderate- grade metamorphism:

• When you metamorphose limestone you get marble.
• When you metamorphose mud-stone (shale) you get slate.
• When you metamorphose sandstone you get quartzite.
• When you metamorphose conglomerate you get meta-conglomerate.

BUT ... lots of metamorphic rocks are not low-grade metamorphosed rocks with obvious sedimentary origins. They can be mid- or high-grade metamorposed rock of igneous, metamorphic, or sedimentary origin. How do you tell what was metamorphosed into a high-grade swirly-looking metamorphic rock? This is one of the most challenging rock-identification fields because so much of the evidence has been obliterated. The chemical composition of the metamorphosed rock is about the same as that of pre-metamorphosed rock, but the minerals have changed into new minerals. The stories that the metamorphic rocks of Antelope Island, Davis County and extreme north Salt Lake County tell is very complex and difficult to unravel. The complexity of the story is one reason the rocks are named "The Farmington Canyon Complex".

Caution: Low-grade metamorphic rocks of sedimentary origin are the easiest fat figure out. Remember that igneous rocks and metamorphic rocks can be metamorphosed. How can an igneous rock that was once melted become metamorphosed? Will a high-grade metamorphic rock, exposed to lesser pressures and temperatures develop characteristics of a low-grade metamorphic rock? These are very good questions. The simple answer is yes. However, it takes professional training and special equipment to see clues of "lesser" metamorphism when the rock has been exposed to even "worse" pressures and temperatures.

COMMON METAMORPHIC ROCKS

Remember. .. the most important concept is to tell the story of the metamorphism of the metamorphiC rock. However ... names are helpful handles of communication and if your students want to learn a few names, here are the names of three relatively well-known metamorphic rocks found in Salt Lake County:

Gneiss ... examples: Antelope Island has marvelous exposures of gneiss (pronounced "nice" as in nice gneiss). Salt Lake County has exposed gneiss along the Davis County line near City Creek Canyon.

Bedrock Story 1: How the rock was metamorphosed.
Gneiss is a high-grade metamorphic rock, regionally metamorphosed. Gneiss tells the story of colliding continents and conditions at the root of tall mountain ranges.

Bedrock Story 2: What was metamorphosed.
What it was before? That's not easy to figure out... the characteristics of the pre-metamorphosed sedimentary, igneous, and metamorphic rock is too obliterated to tell. It might have been sedimentary, igneous or metamorphic bedrock before it was deeply buried, subjected to heat and pressure and changed beyond easy recognition.

Quartzite examples: gravels and boulders all around Salt Lake County.

Story 1: How the rock was metamorphosed.
Quartzite is a low-grade metamorphic rock, metamorphosed by either regional or contact metamorphism.

Story 2: What was metamorphosed.
What it was before? Sandstone. (Beware of potential confusion. Because quartz mobilizes so easily, (you can think of it as a cowardly mineral), sedimentary bedrock can have quartzite units of altered sandstones among layers of otherwise not-metamorphosed rocks.)

Marble:

Story 1: How the rock was metamorphosed.
Marble is a low-grade metamorphic rock, metamorphosed by either regional or contact metamorphism

Story 2: What was metamorphosed.
What it was before? Limestone.

Schist:Not so common in Salt Lake Counry. Common in New England.

Story 1: How the rock was metamorphosed.
Shist is a mid-grade metamorphic rock, usually the product of regional metamorphism.

Story 2: What was metamorphosed.
What it was before? The original sediment was clay-rich, a mud. It became sedimentary bedrock, a mudstone or shale. Then it was metamorphosed, at first as a low-grade metamorphic rock it was a slate. Then it was further metamorphosed into a mid-grade metamorphic rock, a schist. The rock escaped the more severe metamorphic processes which would have transformed it into a high-grade metamorphic rock, a gneiss.

THREE EASY STEPS TO TELL THE STORY OF A METAMORPHIC ROCK...

Imagine, you have just been handed a piece of metamorphic rock.

Bedrock Story Step 1)
The rock is a piece of sediment. What bedrock did it come from? You recognize the rock is a metamorphic rock because of its pattern which isn't really layered and isn't homogeneous. It has vestiges of pattern, shadowy shapes of what it once was.
"What an interesting rock ... look at these shadowy patterns. I bet this rock has a really interesting story of what it used to be. It's changed since then ... that's why it's called metamorphic."

You examine the rock with the help of the student.
You notice how hard the rock is, how its grains are fused together. You crack it open and it cracks right across former pebbles, or right across former layering.
"Notice how the fabric of the rock, its pattern has changed from what it was. It doesn't even break along the old layering."
"It's as if it had a new personality because it has experienced so much pressure .... This rock could use a rock-psychologist. It's a mess."

Bedrock Story Step 2)
How severely was the rock metamorphosed?
What was metamorphosed?
Based primarily on how evident the layering and shapes are, you decide whether you can tell whether the rock was:

• sedimentary rock. If so, you can talk about the sedimentary origins of the rock. And then how the rock was buried, got pressured and changed by forces due to crustal movements that forced the rock and all its neighbors into zones of greater pressure and temperature so the minerals, particularly quartz, fused together.

-or-

"It's like cooking ... you start with a raw meat-loaf, and you end up with a cooked meat loaf that looks pretty much the same ... but it isn't."

• You can't tell what the rock was before it was metamorphosed ... it's all swirled and all almost like an igneous rock but never was melted.

"Wow, this rock had an amazing history. We can't even tell what it was. It's just shadows of it's former self. It really got caught up in the crust, plates colliding and forcing the bedrock way into the crust. This plate tectonic stuffis so mind-boggling. I wonder what it was like back then. This whole region must have been into the crust slammed between continents or maybe along an ocean margin like the "ring of fire" around the Pacific Ocean. "

Bedrock Story Step 3)
Where is the rock going (on the rock cycle)?
Just about any rock a child gives you is being broken down into smaller bits of rock and chemicals by erosion and carried by water downhill.

The bits and pieces of sediment are headed toward the lowest part of the basin, Great Salt Lake, unless they get thwarted and buried as sediments ... or made into a wall ... or added to a rock collection. What will eventually happen to the wall, or the rock collection? Eventually what will happen to the sediments buried in Salt Lake Valley? And then what may happen?

STORIES ABOUT IGNEOUS ROCKS >>