After yesterday's post on a new feature I found on the USGS earthquakes site, reader Tony Edger asks, "After exploring the USGS website and elsewhere without much success, I am hoping you might steer me to a description of how to read a seismicity cross section. " He was referring to these images:

So here's how this works: the top image is a map. It gives you a "bird's eye" perspective on earthquake locations at the subduction zone near Samoa. It shows you the epicenters (location on the earth's surface above a quake's actual location, called its "focus" or "hypocenter") of many earthquakes, along with Tuesday's big quake, shown with a star. The thick red line is the position of the trench, a bathymetric expression of the subduction zone. The epicenters are color-coded for their depth. Orange and yellow are shallow; green and blue are medium depth; and purple and red are the deepest. Notice that they make a sort of "rainbow" pattern, with the shallowest quakes in the east, and the deepest quakes in the west. This is "looking down" on the subducting slab: it's like we're able to "see" the subducting slab as it descends into the mantle.

The lower image is the cross-section. It gives you a "gopher's eye" perspective on the same data. A cross section is drawn along the line A-A' on the map. This is conceptually slicing the Earth open along that line, then removing half, and looking sideways at the remaining half. Note that the A-A' line is now along the top of the figure, representing the surface of the earth. Along the horizontal axis is horizontal distance, measured in kilometers. Along the vertical axis is depth, also measured in kilometers. The two axes are not drawn to exactly the same scale, but pretty close. In other words, 100 km of horizontal distance is approximately equal to 100 km of vertical distance (depth). The same data are plotted, or at least the subset of the map's data which happen to fall on that particular line, A-A'.

With this new perspective, a side-view, what do we see? Well, there's the star, which shows the depth of the quake that triggered all this discussion, and a whole bunch of other (historical) earthquakes. Now, instead of the epicenter being plotted, we're getting a more robust sense of the hypocenter (or focus). Note that the earthquakes are being generated in a big swath, starting at the surface in the northeast, and dipping down deeper and deeper to the southwest. This line of seismic activity reflects the jerking passage of the subducted slab of oceanic lithosphere. As it plunges down, it generates lots of shaking. This zone of seismicity was first described (independently) by two scientists, Kiyoo Wadati and Hugo Benioff: in their honor, it is referred to as the Wadati-Benioff zone. (Wikipedia has more) Their realization is our gain: we can "see" the subducted plate diving at an angle of 30 to 40 degrees. That's what's so cool about this:

Something that no human will ever directly observe is "visible" to us because we can pinpoint the three-dimensional location of thousands of earthquakes. These bumps and jolts reveal the position of the bumper and jolter: the subducting plate!

Labels: analogies, art, earthquakes, plate tectonics, websites

As an analogy for how most minerals never get to attain their full habit (hemmed in by surrounding space constraints), perhaps even better than the boxy watermelons I mentioned last December are Buddha-shaped pears!

Labels: analogies, humor, minerals

Went for a hike on the good old Billy Goat Trail last Sunday and saw this beautiful outcrop. I love it how every time I walk that trail, I see something new and blog-worthy. Here you see the metagraywacke of the Mather Gorge Formation getting squished and squeezed under conditions of partial melting. Granitic magma (light-colored rock) is leaking out, while the foliated mafic residue (schist chips) are getting strung out and boudinaged under conditions of mountain-building. This granite yeilds late Ordovician isotopic ages (Taconian Orogeny, ~460 Ma).

Seeing an outcrop like this reminds me of making cheese: squeezing the liquid whey (felsic magma) out from the solid curds (higher-melting-temperature solid minerals like those comprising the 'schist chip' boudins). As orogenic forces squeeze from the sides, granite oozes out the top.

I love that there are outcrops where this process is caught in freeze-frame: not all the granite escaped from its migmatitic source rock here; instead the process stopped before it was complete, and through the luck of uplift and exposure by the probing erosion of the Potomac, we get a glimpse of a fundamental process in making the Earth look the way it does. A single outcrop shows rocks that were oceanic sediments, then became metamorphic schist, and now are were transitioning to igneous granite! That's pretty wild. We have caught the rock cycle red-handed.

Labels: analogies, granite, igneous, maryland, metamorphism, piedmont, primary structures

(Parts 1, 2, 3, 4, & 5 of this series...)

As we were climbing up a steep snowfield, we saw something that made us rush up to the top:


Interpretive sketch:

At first, we thought this was a big isoclinal synform that was cross-cut by a ptygmatically*-folded granite dike, but closer inspection at the "axis" of the "fold" revealed that it was instead just the trailing edge of a big boudin. It pinched down and then swelled again in the downward direction, hidden in this photo by the snowpack. Not quite as cool... but still pretty cool. And I can never say no to ptygmatic* folding, regardless of the setting.

This is also kind of cool:

What you're looking at here is a gneiss, with alternating layers of coarse-grained mafic and felsic minerals. The view of the photo is orthogonal to the plane of foliation, but the boulder has been weathered so that in some places the uppermost mafic layers has been worn away. There's one spot where you can "see through" the mafic layer into the underlying felsic layer (upper right) and another spot where there's a little isolated scrap of the mafic layer where the surrounding material has been weathered away. This reminded me of a larger-scale phenomenon where the same thing happens to thrust sheets: an erosional hole through a thrust sheet into the rock beneath is a tectonic "window" or "fenster" (German for window). An erosional remnant of a thrust sheet is a "klippe." The Grandfather Mountain Window in North Carolina is an example of a fenster. Chief Mountain in Glacier National Park, Montana, is an example of a klippe. So this little boulder gives us a nice physical analogue for regional-scale tectonic/erosional features.

Ahh... what cool stuff to see and think about. But the sun was setting, and we had to head back to camp and the rest of our team... Tomorrow: the story of the long hike home.

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* Really, more of a "cuspate-lobate" fold, without the parallel limbs that make for a truely ptygmatic fold.

Labels: analogies, art, montana, national parks, north carolina, nova, structure, travel, wyoming

In October of last year, I presented a list of my favorite analogies for geological processes. Effjot followed up with a visualization of one that was presented in the comments.

Today, I'd like to add to that list with three more evocative analogies.

Hydrothermal disseminated deposits are sweat stains.
Certain types of ore bodies are thought to be "sweated out" from magma chambers as they intrude to shallow enough levels in the crust. The shallow depths have low pressures, and that encourages the magma to devolatilize. The resulting hydrothermal fluids pick up lots of consitituents like sulfur and metals and stream away from the pluton. As they cool off, the dissolved constituents become supersaturated and begin to precipitate out as mineral deposits. These hydrothermal disseminated deposits end up in the pore spaces of surrounding rocks, or filling in cracks. This is kind of like how your body sweats out a solution of dissolved salts in water. When the water evaporates, the salts precipitate out wherever they find the space:

Sills are a funny kind of peanut butter sandwich.
A dike is an igneous intrusion which cuts across local stratification of the host rocks. Sills, in contrast, exploit the weaknesses between strata and inject their magma parallel to bedding. I think of this as being like using peanut-butter-in-a-tube to make a peanut butter sandwich without separating two pieces of bread. Like this three part series:




Exotic terranes are roadkill.
I show the following sequence of images to my Physical Geology students when discussing how exotic terranes accumulate on the leading edge of a drifting continent:








... and I think you get the idea. That one kind of speaks for itself...

How about you? Got any good analogies for relaying geological concepts?

Labels: analogies, art, humor, ore, teaching

This semester, I employed a new tool in teaching structural geology. Built by NOVA's uber-clever engineering guru Rob Woodke, this is the Butter Buster. The idea came from Structural Geology of Rocks and Regions by Davis & Reynolds, the text I use for teaching structure, and was recommended as a crowd pleaser by Aaron Martin, the structural geologist at the University of Maryland.

So what's the deal? The deal is that materials like rocks behave differently if they are cold or if they are warm. (They also behave differently if they are under high or low pressure, and if strain is applied quickly or slowly, etc., but here our independent variable was temperature).

We can demonstrate this difference by creating an analogy between rocks and a more familar substance, butter. The butter buster creates a fault/shear zone of adjustable width, and displaces the two ends of the butter in opposite directions. If it's cold, it breaks. If it's warm, it flows. Ta-da!

Check it out...

Cold:


Room temperature:


Warm:

Labels: analogies, books, structure, teaching

After seeing the feldspar megacrysts in Maryland's Ellicott City Granodiorite two days ago, I wanted to share some even more impressive megacrysts, those found on the periphery of the Cathedral Peak Granodiorite pluton ['CPGD'] in California's Sierra Nevada mountains.

Here's a typical look at the CPGD close to its contact with metasedimentary & metavolcanic host rocks. It's chock-full of 3-7 cm crystals of potassium feldspar, set in a more typical-looking granodioritic matrix of sub-0.5 cm crystals:

This is a nice example of an intrusive porphyry. Not all porphyritic textures result from two phase cooling: The way the story usually goes is that the magma starting underground at a realtively slow rate, then the magma (solid crystals + remaining liquid) gets tapped and erupts, with the rest cooling at a faster rate on the surface. This one clearly shows a phaneritic (coarse-grained) texture throughout; it's just that some crystals grew bigger than others. I'm not an igneous petrologist, so I won't claim to understand why. Enlighten me if you know.

Here is a close-up of one feldspar crystal shows lines of mafic inclusions (earlier-crystallizing minerals like amphibole which were caught up in the advancing front of feldspar crystallization, and trapped in the larger feldspar crystal):

My mind wants to see this as a spiral pattern, like a snowball garnet, and hence to interpret this as a feldspar crystal rotating as it grew, but that's surely wishful thinking. Especially seeing as how there's no foliation to get wrapped up in the 'rotating' porphyroblast. But... I've never seen another igneous crystal that shows this same pattern. Anyone else? Trick of the light?

Now here's something really wild:

Recall that when I took these photographs in 2003, I was out in the Sierras looking at the Sierra Crest Shear Zone, a 1-2 kilometer wide zone of smooshed rocks adjacent to the eastern boundary of the Sierra Nevada Batholith. So mainly I was interested in these older "host rocks" which were metavolcanic and metasedimentary, but I was also interested in how they related to the batholith as a whole. In places, I could see clear evidence that the plutons of the batholith were sheared, too, and in other places they appeared to have intruded post-deformation. This photo shows that the Cathedral Peak Granodiorite came along after the bulk of the deformation had happened.

How do we know? (1) It's not especially foliated itself. (2) Here, magma oozed between the foliation layers in the metasedimentary rocks immediately adjacent to the pluton. These layers flexed to allow the magma to intrude; I think of curtains billowing underwater. Then, as the pluton inflated (or as regional deformation continued to squeeze these rocks; or both), a compressive stress was exerted on these mingled layers of foliated rocks and magma. The liquid magma squished out of the way, but the solid megacrysts were trapped, and the foliation flexed and wrapped around them.

Twisted food analogy: Say I make a peanut butter and raisin sandwich. (Seriously, they're good!) I have a piece of bread, and I smear it with a mix of creamy peanut butter and chunky raisins (the giant ones from Trader Joe's). I place another piece of bread on top. Then, because I value my geology more than my manners, I lean over like I'm going to perform CPR, and exert pressure perpendicular to the plane of the bread. The peanut butter, being ductile, squishes out the sides, while the raisins are trapped, and the bread deforms around them.

Such, such are the thoughts of the hungry field geologist...

Labels: analogies, california, granite, metamorphism, minerals, primary structures

One of my Honors students, Hope W. (author of yesterday's discussion of the Chalk Point Power Plant), kept a tally on Facebook of quips and phrases from this semester's Environmental Geology class. Now that the semester is over, I offer them to the public at large, despite the utter lack of context. Enjoy!


"Imagine how the lava feels."

"Earthlings are made of Earth."

"What do meteorologists study? Hint-- NOT meteors."

"It [the oceanic crust] is like a giant sheet of tissue paper."

[Referring to the continental crust, in comparison to the oceanic crust] "It's light and fluffy, like a souffle."

"We don't know the actual specifics."

"When you go up, you're not going North - you're going away from the Earth."

[Dramatizing the extraction of paleomagnetic data from rocks] "Continent, where was the pole for you 600 million years ago?"

"Oceanic crust is like James Dean and continental crust is like Dick Clark."

"Here's what we know about tectonic plates: some of them are big... some of them are itty-bitty."

"You can't forget Djibouti."

[Referring to the 1811-1812 New Madrid earthquakes] "There was just no one west of that to report how much shaking there was. Or at least no one who spoke English and felt like talking."

"Take my word for it man! I'm a scientist... No, that's not how it works."

"I have a nice layer of peanut butter on my arm."

"The same thing happens with rocks... it just takes longer."

"As continents move along they pick up junk."

"L.A. will end up in the armpit of Alaska."

[Referring to Redoubt] "Drama-queen of a volcano."

[Comparing geologic hazards] "If you use up all your water, then you die and you don't have to experience the earthquake."

"If bamboo collapses and falls on you it doesn't hurt anywhere near as much as brick."

Let me know in the comments if any of these requires an explanation...

Labels: analogies, humor, nova, teaching

I shot these two videos this weekend from the Billy Goat Trail. They both show the surface of the Potomac River, decorated with little blobs of foam. As the river flows, the blobs of foam record the flow and deform in distinct patterns. I am reminded of the processes that must have occurred in the very rocks I was standing on to take these videos. (See the previous posts on boudinage, folding, and texture in migmatites.) You can see foliation developing, shear zones, folding, and even boudinage. The blobs of foam are acting like more competent geological units (feldspar or garnet porphyroclasts, for instance), while the intervening water is less competent (easily flows out of the way, like quartz or calcite under sufficient pressure).



This one really shows boudinage well. Track the big blob that gets "fed" into the shear zone a few seconds into the video. As deformation proceeds, it separates into three augen-shaped chunks that then move apart along the plane of foliation (which is itself deformed).




A note of caution: these foam blobs are not perfect analogies for the flow of rocks at depth. The dynamics you're observing in these videos are playing out on a two-dimensional surface where water meets air. Because its density is intermediate between the water and the air, the foam stays at this surface, though the water in between the blobs is free to circulate downward into the river if conditions demand it. In real rocks, the deformation would be a three-dimensional phenomenon, and hence a bit more complicated.

Labels: analogies, rivers, structure

Kilauea Iki is the name given to a lava lake that formed in Hawai'i Volcanoes National Park in 1959. It erupted from Pu'u Pua'i, the mound you see in the middle distance of this photograph:

The lava pooled in a pre-existing crater below to a maximum depth of about 400 feet, and has been solidifying ever since. Researchers have drilled though the cooling crust of Kilauea Iki to determine how fast the lava cools. By 1981, a good 200 feet of solid rock had formed at the top of the lava lake.

Here's a view into Kilauea Iki from a different angle, with me rotated about 90 degrees along the crater rim relative to the first photograph:



As you look down there, you'll see that Kilauea Iki does not display a nice smooth surface. Instead, it's fractured, and those fractures have a familiar shape: polygonal and relatively regularly-spaced. They look kinda like the tops of ginormous columns...


When you get down inside, it's pretty flat. You really get the feeling you're walking on a giant layer of soup scum:


...But it's not completely flat. There are cracks and crevices, buckles and upwarps:


Dynamics playing out in this mega-scum layer atop a roiling lava lake are thought to be human-scale analogues of the motion and dynamics of tectonic plates. Here, for instance, two "plates" of cooled lava have drifted towards one another. This meso-scale "convergent boundary" has raised up a mountain range fit for Lilliputians:


Elsewhere, "plates" of lava scum have drifted apart, opening up a "rift" between them. Here, I lie down to bridge the rift:


These cracks are utilized by plants because they offer a shaded nook where moisture isn't immediately evaporated by the sun:


Lastly, I thought I'd point out some neat mass wasting and structural geology I saw there. Here's a shot looking roughly westward across Kilauea Iki, towards the cinder cone of Pu'u Pua'i:

I know it's kind of washed out, but in this photo, you can see a big solidified lava flow that came over the lip of the crater, and then solidified, and then partially collapsed downward.

This sequence resulted in the big talus pile you can see at center-right, but there are remnants of the original sheet (or "tongue") of basalt there.





















Zooming in and cranking up the contrast, let's label a few things:
Up at the top, we can see some fault scarps that have developed as the massive tongue of basalt pulled downward.

A major scarp marks the edge of the cliff, and then below it you see a big slab of basalt with an edge that's just barely in the sunshine, and a bunch of more fragmented pieces below that (marked "breakdown"). Another big slab is seen alongside the breakdown.

What really caught my eye, though, was the en echelon array of pull-apart fractures seen in between the arrows. Here, the stress of the main tongue of basalt sliding downhill sheared this slab of rock, causing it to develop fractures at a ~40 degree angle to the shearing direction. These pull-aparts therefore represent a big surface-condition analogue for tension gashes that can form in subterranean rocks experiencing shear stress.

Labels: analogies, basalt, hawaii, landslide, mass wasting, plate tectonics, structure, travel

Quick quiz!

What does this...


...have to do with this?


A mineral's habit is the shape that a crystal of that mineral will attain if it gets the chance. When most people hear the word "crystal," the image that comes to mind is of a mineral crystal that has attained its full habit. However, most crystals aren't that pretty. If there aren't enough elemental ingredients, or if there isn't enough time to grow nice and big, or if there are other crystals in the way, then you won't get a nice, sexy crystal. Instead, the mineral crystal's internal structure will fill in whatever space it can, and that will determine its shape. The lower image shows a cartoon of a thin section of rock. In it, you can see a mineral with a "hexagonal" habit, but this actual crystal's shape is jagged and irregular, as dictated by the space available to grow. Most mineral crystals are like this: stunted and "misshapen" as a result of their circumstances.

And that brings us back to the upper image... the square watermelons. As everyone knows, watermelons are approximately ellipsoidal in shape, if given the chance to grow into their full "habit." However, that ellipsoidal shape is tough to cram into a small fridge; it occupies a larger space than its bulk actually takes up. There's a lot of wasted fridge space in the areas adjacent to it. In Japan, a solution has been developed: grow the melons in boxes, so that they are forced to take on a square or rectangular shape. Then, when mature, Japanese consumers can put the square melons in the fridge, confident that no space is being wasted: the melon is taking up almost all of the fridge volume given over to its storage!

Like most minerals, the Japanese watermelons are constrained by their circumstances to grow into shapes that they wouldn't attain on their own.
____________________________
Image sources:
Japanese watermelons - Oddee.com
Thin section cartoon - me, redrawn from a figure in Marshak, 2006.

Labels: analogies, art, japan, minerals, teaching

Here's how I explain the carving of horns as erosional features of glacier geomorphology:








Once you've scooped into the pint of ice cream and out (away from the frozen core towards the thawed exterior), you end up leaving a pinnacle in the middle with curved facets ("cirques"):


... Kinda like this:

Labels: analogies, geology, glacial landforms, teaching

This image came to my attention the other day via Lutz's Geoberg blog. It's one of the high-res images provided by the newly-launched satellite, the GeoEye-1, which is supplying new images to Google*. The image shows a marginal lake associated with an alpine glacier in Kenai Fjords National Park, Alaska (just south of Seward):


The top of the above image is not north; it's southwest. Mentally rotate it, and you can see that the resolution is a lot better than the current level on Google Earth and Google Maps:


The thing that struck me about the new GeoEye image, aside from its beauty, is the distinct pattern of iceberg sizes in the lake: freshly calved off the glacier, the biggest icebergs are close to their source, while further away the icebergs are smaller. This pattern struck me as being analogous to sediment. Fresh from its source, sedimentary particles are at their largest size, and the further away they travel, the more weathering they experience. This weathering (in particular of the physical variety) tends to break them down into smaller pieces. Adjacent to an orogenic belt, for instance, you tend to find deposition of sedimentary particles shed off the uplifting mountains. As a general rule, these are of the largest sizes and the greatest volume closest to the source, and then particle size and stratum thickness both diminish with increasing distance from the orogen.

For a North American example, consider the Catskill Clastic Wedge, a tick pile of sediments shed off the late Devonian Acadian Orogeny along the east coast. Here's a cross-sectional view** (pre-Alleghany Orogeny deformation) of the wedge, running from the Bay of Fundy west to Michigan:


Same pattern! Coarse stuff, and more volume of stuff, close to the source. Finer stuff, and less volume of stuff, further from the source. Just like the iceberg, except the weathering of the icebergs is mainly thermal, while the weathering of the sediments is physical, accompanied by depositional sorting by the transporting currents of water.

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* An original version of this post misidentified Google as the owners of the GeoEye-1, as opposed to the company called GeoEye, which sells images to Google. Thanks to Bruce Haley for the correction. (updated 8:14AM eastern time on Dec. 9, 2008)
** Image redrawn (by me) from an original in Prothero & Dott (2003).

Labels: alaska, analogies, art, canada, glacial landforms, glaciation, maine, michigan, mountains, new york, satellite imagery, sediment

Second in my "ductile flow in everyday objects" series... Ultimately the goal of posting these photos is to develop a repository of teaching images for familiar substances which flow when conditions of temperature and pressure are sufficient.

Here's a plastic cat-food dish (originally square) which deformed in a ductile fashion after going through the heat-dry cycle on a kitchen dishwasher:


Note how the dish has "sagged" around one of the dish rack's supporting bars, like a damp cloth draped over a stick.

Now that it has cooled, it can be removed and show how much it has deviated from its original shape (how much it has strained):

Labels: analogies, structure, teaching

Rocks flow when conditions are right. At the introductory level, many students exhibit an initial tendency to resist the idea of something they "know" is hard and brittle acting in any other way. Faulting, they get. Shear zones... not so much. I find analogies useful in communicating the behavior of rocks at depth, like mylonites. Often I invoke wax, which can be cold & brittle, hot & ductile, or molten.

But I reckon it's instructive to have other clear indications of ductile flow: everyday objects that have flowed under stress.

Today, I offer the first in what I hope will eventually build into a longer series: everyday examples of ductile flow. We begin with a cassette tape left in a hot car (viewed through the back window, which is why the photo is so lousy):

Even the relative moderate stress of leaning on the seat cushion was sufficient to bend this cassette tape, provided it had attained the right temperature (which it's easy to do in the Virginia summer time in a closed automobile).

Anyone else have examples of everyday examples of ductile flow?

Labels: analogies, structure, teaching

Tuesday, I asked for my fellow geo-bloggers' favorite analogies, with a promise that I would share mine in 48 hours. The time of revelation is nigh... Here are a few of my favorite "geo-nalogies":

The continental crust is high-proof liquor
I see partial melting as a kind of distillation. Just as "sour mash" can be distilled to concentrate the alcohol it contains (separating it from the water it's dispersed in), so too can partial melting act as a "distillation" of the silicate earth. The minerals with the lowest melting temperatures will melt, leaving behind a solid residue enriched in Fe, Mg, Mn, and Ca, and yielding a magma that is enriched in Si, K, Na, and O. With its~granitic composition, the continental crust is 80-proof Jack Daniels. Where did it come from? It's distilled from the sour mash we call "the mantle":

Rocks are cookies
I love a good chunky cookie. Save your Oreos and Lorna Doones for yourself. What I really like is one of those cookies with chocolate chips, oatmeal flakes, raisins, macadamia nuts, and those sinfully good butterscotch chips. What I like about these cookies is not so much how they taste, but how I can tell the difference between the individual ingredients and the cookie they comprise. I use this analogy early on in Physical Geology to illuminate the difference between minerals and the rocks that the minerals comprise:

Continents are old sofas
Like many of us, I had an old sofa in college. The sofa was ripped, had been scratched by a cat, and had coffee spilled on it. It was draped in several layers of blanket in an attempt to cover up the lousy state of the upholstery. Someone added a pillow to the sofa at some point. When I was working for the C&O Canal National Historical Park (translating their geologic history into non-geology-speak), it struck me that the North American continent* was kind of like that old sofa. It had been scratched by glaciers instead of cats, and lava had been spilled on it kind of like that errant French Roast. It had rift valleys, but unlike the sofa's, North America's rifts didn't have springs poking out. New material had been added in the form of exotic terranes, kind of like that pillow got added to the sofa. And the blankets draping parts of the continent were made of sediment instead of fabric... but essentially the two were alike:

*Yes, I know that's the outline of the contiguous 48 United States, not North America the continent. So shoot me.

Tectonic plates are UFOs
In cross-section, a tectonic plate could be seen to have a profile kind of like a flying saucer. The thick part in the middle is the continental crust, but then it has a thin fringe encircling it (the oceanic crust). You can hardly blame a visiting Martian for feeling kind of attracted to it:

The Washington Monument shows geologic time
I didn't come up with this one... But read it somewhere (McPhee, maybe?) that I have since forgotten. Anyhow, the basic idea is that the Washington Monument's obelisk here in Washington, DC can show the difference between the Precambrian portion of geologic time (most of the monument, 88% of Earth history) and the Phanerozoic eon (post-Cambrian, 12% of Earth history). The little pyramid-shaped bit on top is the Phanerozoic. The thickness of a single sheet of paper draped on top of the tippy-top would represent the entire span of human history:

Okay, that's all I've got for today. What have YOU got?

Labels: analogies, geologic time, geology, granite, minerals, teaching