I mentioned in Monday's Sand post that this was "book month" here at NOVA Geoblog. That means it's now time for a quick book review of CATACLYSM: When Human Stories Meet Earth's Faults, by Douglas W. Huigen...

When I was writing my Benchmarks piece for EARTH magazine about the Hebgen Lake earthquake and the Madison River landslide, I spoke on the phone to Doug Huigen, who was then just finishing a multiyear project learning about the geology of the Hebgen Lake area, and interviewing survivors of the event. He was very genial and shared some great information when we spoke.

Later in the year, my summer Rockies field course brought me out to the site of the landslide itself. Here's me and my students at the Earthquake Lake Visitors' Center, talking about the structure of the mountain behind us, and why it failed almost fifty years previously:


After I was done pontificating, we went inside and watched the compelling movie they show there, and then I noticed that Doug's book was for sale on the counter. I bought a copy.

Months later, I finally found the time to read it. For some reason, though, I've found it difficult to finish up with my "book review" blog posts. I started this one in late October, for instance. I'm hoping that by declaring February to be "book month," I can motivate myself to crank through these reviews.

Cataclysm is a nice introduction to the events of August 1959, viewed both through the people on the ground experiencing the earthquake and landslide, and through the perspective of modern-day geological insight. Huigen spoke to a great many survivors of the event, and relates their stories with compassion and an ear for colloquial language. The book is subdivided into three main sections: (1) stories of people during the event, (2) a bunch of photographs and graphics showing the area, the people, and the geology, and (3) a description of the geology underlying the earthquake and landslide. The story is very compelling, and I think it's worth reading this book if you're going to be visiting the Hebgen Lake landslide site.

The book is self-published by Huigen, so there's some issues with typos and formatting of photo annotations, but I guess that could also be seen as part of its charm. It's an excellent repository of a lot of information, and I learned some new things by reading it. I was particularly pleased with the image Huigen has on the inside of the front cover: a sketch of the major geological features in the area. The inside of the back cover is a gorgeous geologic map of the same terrain, but Huigen didn't include the map's explanation, so you have no idea what the various rock units actually are (unless you're already familiar with the area).

Bottom line: not the most amazing piece of literature in the universe, but an important compilation of data about the Hebgen Lake earthquake and landslide: data both of the geologic variety and the 'oral history' variety. I'm glad I read it.

Labels: books, earthquakes, mass wasting, montana

After our explorations of the Los Perros Glacier, its moraine, and the bedrock it has scraped so deliciously clean, we headed on up the trail, towards the highest point on Torres del Paine's Grand Circuit: Paso John Gardner. Here's a look back at the valley we've been hiking up from Refugio Dickson... Note the Los Perros moraine and the edge of the lake:


First graded bed of the day. I photographed this one for the lovely scours into the underlying muddy (dark) layer:


Another turbidite clast. Is that a clastic dike on the left?


I thought this was really cool, too. It's a vein: a fracture filled in with a mineral deposit. I really like here how you can see little shreddy flakes of the mudrock (dark) peeling back and flexing in the fracture's void space (prior to being locked in place by mineral deposits):

I interpret this to indicate that the fracture opened in a transtensional fashion, with the top to the right.

A ravine revealed this blind thrust:


Can't see it? Here's an annotated version. The thrust fault below morphs into a fold further up:


A sand-dominated series of graded beds:


Annotated below. Some of the turbidites I saw were a meter thick!

...and what's up with those rotty-appearing rusty spheres? (like the one left of my boot) I saw them several places... hematite concretions? (???)

Brace yourself. Here is possibly the most spectacular boulder I've ever seen:


Annotated version below. This boulder shows a series of graded beds (sand = light colored; mud = dark colored). The direction of gradation shows us that the boulder is upside-down relative to original depositional orientation. A couple of small flame structures reinforce this interpretation. It has been gently folded into a broad anticline (remember, it's upside-down!) and there appear to be some small "parasitic folds" superimposed on the broader fold (at boulder-bottom; depositional-top). Additionally, the turbidites are cross-cut by a small fault which has offset the layers. If I could choose just one boulder to be airlifted from Patagonia to the front of the Science Building at NOVA, this would be the one I would choose.


We keep hiking. We cross several snowfields and other bouldery alluvial aprons, interspersed with fingers of forest reaching up towards the hills. Looking up at the peaks, we can see turbidite layers intensely folded. Check out the straight-limbed anticline (left) and syncline (center) on this mountainside:


Looking up ahead -- there at the left center (between the two peaks) is John Gardner Pass:


We cross through a few more stretches of forest. This one really struck me: "Creep much?"

Besides the freeze-thaw soil-shoving action of creep, I think another factor for the J-shaped (or even L-shaped) tree trunks in this forest is the thick blanket of snow they get each winter: this tamps down the whole forest in a downhill direction.

Look! On the left! Another glacier!


...Shift the perspective a bit, and something else pops out. Once the hammy glacier is off-screen, you can see the wallflower in the background: A mountain composed of pink granite rather than black turbidites.


We keep climbing. Higher up, another opportunity for gazing down the valley we have climbed. The Los Perros Glacier moraine and lake are readily distinguishable even from this distance:


Up in the snow, we trudge higher and higher, and eventually reach the Pass. But for that, and for what we saw on the other side... I'm going to make you wait for Part III.

Labels: chile, glaciation, mass wasting, patagonia, primary structures, south america, structure, travel

Today, picking up where we left off yesterday, some images from the hike upwards from Jenny Lake to Hanging Canyon...

Joel and Ken take a breather:


The approach to the final lip of Hanging Canyon:


A view down over Jenny Lake and Jackson Hole:

Jenny Lake is dammed by an end moraine (which is characterized by pine trees growing on it here, making for a nice dark stripe around the lake).

We could also see across Jackson Hole to the Gros Ventre valley, where the Gros Ventre lanslide scar was readily visible:


...And lastly, the view to the north, over Jackson Lake (with String Lake in the middle distance):


More tomorrow about what we found once we got up into Hanging Canyon itself... (Hint: it's white and cold and fun to ski on...)

Labels: glacial landforms, mass wasting, national parks, nova, travel, wyoming

50 years and 2 days ago, the Hebgen Lake Fault slipped, and triggered the Madison River Landslide. Here's the article I wrote about it for EARTH magazine.

By the way, someone already pointed out to me that Orion's a winter constellation... d'oh!

Labels: earthquakes, mass wasting, montana

Wowzers.
Hat tip to Dave Petley.

Labels: hurricanes, mass wasting, taiwan

The Berkeley Seismological Laboratory (which initiated an RSS feed a few months back -- well worth subscribing to) has a post up today showing the seismic signals generated by last weekend's Yosemite rockfall. Check it out!

Labels: blogs, earthquakes, mass wasting

In Environmental Geology lab last week, we were playing with dirt... and sand... and gravel... and other granular materials, piling them up to see the angle of repose.

One of my students, Kristen P., brought in little "Monopoly" houses so that her experiments carried a bit more significance...




I thought this was very clever -- it made you "care" more about the angle of repose when someone's "home" was at stake... Good work Kristen!

Labels: humor, mass wasting, nova, teaching

In prepping for a mass wasting lecture this week in Environmental Geology, I checked out YouTube's "avalanche" offerings. Found a couple of cool videos:

Cheesy music on this one...

French skiers chatting on this one...

Labels: mass wasting, snow

Here's a pretty cool outcrop I found as we were leaving Cotopaxi National Park in Ecuador (in early January). I've got two small photos taken laterally on different parts of the outcrop (exposed by a stream), and then I follow those with two close-up crops, showing the details. I've posted the full-size versions of the first two photos on Flickr, so you can click through if you want more details. The zoomed-in shots are displayed here at the same size you'll find on Flickr.





What's going on here? It looks like we've got a series of thinner, relatively fine-grained layers below, topped off with a massive, poorly-sorted layer. The lower layers are all ash- and lapilli-sized grains, each stratum pretty well sorted. The upper layer consists of all kinds of different-sized chunks, including some boulders, "floating" in a really fine-grained matrix. Check it out:





I interpret this as a series of volcanic ash-(& lapilli-)falls that were then buried beneath a lahar, a volcanic mudflow. The lahar's slurry-like consistency was capable of transporting really large clasts, and when it slowed down, it set up like nature's concrete.

I think this is pretty spectacular stuff.

Labels: ecuador, igneous, mass wasting, south america, stratigraphy, travel, volcano

Ed Adam's "cold lab" (which I toured this past summer as part of my "Examining Life in Extreme Environments" class at Montana State University) gets mention in an article in today's New York Times. They also profile some of Adams' experiments setting off avalanches at Bridger Bowl, in the Bridger Range north of Bozeman. Worth a read. Some cool photos, too.

Labels: ice, mass wasting, montana, msse, snow

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

I've got a few more stories to tell from Hawai'i... Today I'd like to share the tale of a backpacking trip that my friend Lily and I took along the northern coast of the big island. From the road's end at the Pololu Overlook, we descended into the Pololu Valley, across its excellent beach, then up the adjacent ridge to the east, down into the next valley, up another ridge (and further east), and then down into the third valley, where we camped.

The route is shown on this Google "My Maps" map:


Here's a look eastward into that final valley:


Descending into the final valley:


The view from our campsite:


The substrate of our campsite: a poorly-lithified conglomerate:


The thing that stands out in my mind most about this excursion was a landslide scar that had cut off the trail at one point. This landslide occured in the middle valley (between Pololu and our campsite valley). The landslide scar is nice and visible in the lower-left of this Google Maps image:


It happened in 2006, triggered by the big earthquake that struck the big island that year. It was one of several landslides that were set off by that shaking. (Wikipedia has a nice "live-action" photo of another cliff collapsing up the coast at Waipio.)

Here's the landslide scar viewed from the east, looking west (on our hike back towards Pololu):


Another shot from the same perspective shows the run-out of debris below the source:


The tricky thing about this was that we had to get past this landslide, since it wiped out the trail. On our way in, we somewhat stupidly climbed down the face of the landslide itself, gingerly picking our way down the steep slope, so we didn't trigger any further mass wasting. Here, for instance, is a poorly-put-together composite photo showing Lily descending into the valley:


(On the way out, we found some ropes in the vegetation next to the slide, and hauled ourselves up those rather than getting on the slide surface again.) But on the way in, when we got to the bottom, we weren't sure where the trail was, and plunged through some dense bamboo forest. I felt like I was in LOST, where the characters are perpetually fighting their way through similar vegetation:


Eventually we found the trail, and continued along. Because of the landslide blocking access, this part of the trail hasn't been used as much for the past two years. Lots of pandanus leaves had been shed off and blanketed some parts of the trail. Hiking across these dried pandanus leaves was a noisy affair:


On the eastern side of the ridge between "Landslide Valley" and "Campsite Valley," we saw this two-inch-wide crack opening up along the trail, parallel to the ridge/valley trend. The edge of the ridge was about twenty feet away towards the east (direction my boot toe is pointing). Certainly something like this portends a future episode of mass wasting...

Labels: hawaii, landslide, mass wasting, sediment, travel

One of the most interesting spots I visited this summer was the Madison River landslide area, between Hebgen Lake and Ennis, Montana. Here's a photograph of the landslide scar:


Google Map of the area:


My "Northern Rocky Mountain Geology" class (through the MSSE program) visited the area this summer. Here's the class (all science educators of one sort or another) walking up to the viewing platform:


What happened here? On August 17, 1958, a large earthquake ~10 miles to the east occurred. Known as the Hebgen Lake earthquake, it was a magnitude 7.5 on the Richter Scale, and shook much of the northern Rocky Mountain area. The earthquake's effects were most deadly where the Madison River drops down out of the mountains and into a graben to the west. There, schistose bedrock with a plane of foliation that dipped steeply into the valley was jarred loose. Sliding along the foliation's plane of weakness, and unthinkably massive amount of rock ( estimated at 70 to 80 million tons) went downhill, crushing a forest service campground and damming the Madison River. The momentum carried the rocky debris up the other side of the valley, where the Visitor Center is located today. There are some huuuuuuuge boulders there, as big as a house. 28 people lost their lives in the landslide (and related smaller-scale rockfalls further up the valley).

The Madison River began to back up behind the new dam, and it formed a "quake lake" called Quake Lake (sometimes called "Earthquake Lake," as in the Google Map above). The U.S. Army Corps of Engineers was worried that the dam would fail, draining Quake Lake rapidly and causing a catastrophic flood downstream. (They were cognizant that this had happened at the Gros Ventre landslide several decades earlier in nearby Wyoming.) So they bulldozed open a spillway, and got the lake level down to where they felt it didn't pose a huge flood risk for Ennis and other downstream communities.

But they didn't get the water back down to pre-landslide levels. Today, you can see a drowned forest along the shores of Quake Lake:


We also visited a couple of exposures of the Hebgen Fault scarp. Here's one at a campsite in the Gallatin National Forest. You can see the big dirty slope in the background: that's the actual fault scarp. Total offset here is something like 2.5 meters.


Another view of the fault scarp, looking along its trace.


Clever wayside sign, mimicing the offset in the land with offset in the sign:

Labels: landslide, mass wasting, montana, msse

A few more photos from the Buffalo trip last week... All of these were taken by Victoria, my Honors student.

Here's some malachite in the sandstone of the Whirlpool Formation: the field trip leader suggested this was due to brine flow through these rocks during the Alleghanian ("Alleghenian") Orogeny:

Herringbone structure ("reverse cross bedding") in the Gasport Formation, overlying the DeCew Formation, which appears flat-lying and calm in this photo, but just below this shows disrupted bedding suggestive of seismic activity:

I showcase a sample too big to lug back to the van (ripple marks):

Watch where you stand! In the Niagara Gorge, we see some evidence that the Gorge is widening through mass wasting processes. Here's a small gap / scarp opening up as a block of rock to the right slumps down into the Gorge:

Lastly, on the trip home, we had an obligatory getting-stuck-in-the-mud moment:

Eventually, we got unstuck and headed back down the road!

Labels: geologists, mass wasting, meetings, nova, primary structures, sediment, teaching, travel

Labels: japan, mass wasting

Picking up from yesterday's post about my hike along Windy Run in Arlington, Virginia...

Just downstream from the waterfall (and crossing the trail) is a recent rockslide. Between D.C. and Great Falls (12 miles upstream), the Potomac River flows through a canyon called the Potomac Gorge. It's hundreds of feet deep overall, and consists of a series of nested straths (bedrock "terraces"), each shaped roughly like (half) a canoe. (At the tip of each canoe is a waterfall leading up to the next strath). Where the vertical distance between straths is great, as it is at Windy Run, mass wasting events serve to break down the cliffs and reduce the crisp profile of the straths.


This rockslide happened in 2005, and the area of "raw" rock up at the top of the cliff reveals the source area for the rock debris below. I wish I had taken a photo of this three years ago when it was really fresh -- it would be an excellent place to do repeat photography to show how the talus pile and cliff face change over time. Upstream are several examples of older talus aprons that have been overgrown by plants and buried in soil. Already, you can see that a few Ailanthus trees (single, upright pole-looking things) have taken root in this fresh landscape.


Once you get down from the Windy Run trail to the Potomac Heritage trail, here's the view of the river, looking upstream. Virginia's on the left; D.C. on the right. A slight "shelf" can be seen on the Virginia side where a notch has been cut to host the George Washington Parkway.


As I hiked along, I found this dead mole. It's a big fat sucker, and it must be quite fresh: probably a casualty from the previous 24 hours. Lens cap is 5 cm in diameter.


More critter evidence: here's a couple of small tree trunks that were decapitated by a beaver. Again, this is recent -- note the fresh curls of wood shavings at the base of the trunk.


But enough with these living entities: let's look at some rocks. This is the metagraywacke rock that makes up most of the Piedmont in our area. This rock is metamorphosed to various degrees up and down the Potomac River, in some places all the way to gneiss and migmatite. In some places, it's schisty, but in others primary sedimentary structures are still preserved. Upstream by Great Falls, for instance, we find graded bedding in isolated less-metamorphosed, less-deformed areas. Down along this stretch of the river, it preserves a diversity of sedimentary clasts, as shown in this image:

Here, you're seeing the graywacke matrix mixed in with a bunch of dark chunks. Today, these dark chunks are mostly biotite, but that's metamorphic. Originally, they were probably mud clasts. Little pebbles of granite and vein quartz are mixed in too. It's worth noting that not only are they metamorphosed, but they're also stretched out in the same direction: foliated and lineated. Many are squashed into X>Y>Z ellipsoidal shapes (where the letters refer to the lengths of the different axes of the ellipsoid), like a mango seed. Lens cap is 5 cm in diameter.

Let's pause for a moment and bring people up to speed if you haven't previously spent any time thinking about Appalachian geology. These rocks are part of the Appalachian mountain belt, which runs from Newfoundland to Georgia (by one definition) or from Texas to Scandanavia (by a more inclusive definition). The Appalachian mountain belt consists of three provinces: from west to east: the Valley and Ridge, the Blue Ridge, and the Piedmont. Two of these are topographically mountainous today: the Valley and Ridge and the Blue Ridge, as their ridgey names imply. But the Piedmont certainly counts as part of the ensemble, and if you compare it to the other two, you'll find that it experienced the most metamorphism, the most deformation, and is intruded in many places with syn-orogenic granites (which neither of "the Ridges" can claim, at least not for Paleozoic orogenies). The Blue Ridge and the Valley and Ridge are deformed, yes, and even lightly metamorphosed, but the Piedmont is really where the action is: this is the center of the ancient Appalachian mountain range. These rocks experienced some serious continental convergence.

So what was the Piedmont before it was the Piedmont? An ocean basin. Before the Atlantic, before Pangea, there was an ocean basin off the "east" coast (it was really the south coast at that point, but no matter...). We call this dead ocean the Iapetus Ocean. The Iapetus was closed via subduction throughout the Paleozoic, and it closed for good when Africa rammed into North America, metamorphosing these rocks and raising the Appalachians. As subduction narrowed the Iapetus, sediments atop the oceanic crust were scraped off in a big jumbled pile called an accretionary wedge. (It is for this mixed-up melange that the infamous geo-blog carnival is named.) You want to see an accretionary wedge being scraped up today? Dive down to the Peru-Chile Trench, off the west coast of South America. You want to see a fresh one at the surface? Visit California's coast ranges, which are a Mesozoic accretionary wedge, raised above sea level. You want to see what an accretionary wedge looks like after it's been tectonically squeezed between two continents? Come to the Piedmont!

Our metamorphosed accretionary wedge consists of a bunch of the sediments that were deposited in the Iapetus Ocean, including what was originally graywacke (a mix of sand & mud). Occasionally, you find a sedimentary clast that's a bit more intriguing, like this one (white arrow):

What intrigues me about this little sedimentary cobble is the fact that it's foliated, which indicates metamorphism and differential pressure, but its foliation does not line up with Appalachian foliation. This cobble was foliated before it was deposited in the accretionary wedge. Therefore, it was derived from some area that had previously experienced mountain building & regional metamorphism (presumably a continent). That ancestral orogenic episode produced a source rock from which this cobble was derived. Then that cobble was deposited by sedimentary processes somewhere and (possibly later) incorporated into the accretionary wedge, which then was metamorphosed (& foliated) itself. Lens cap is 5 cm in diameter.

Here's another one, which shows its foliation a bit better:

When I see something like this, I start to wonder, where did this cobble come from? What was its sedimentary provenance? Is this a North American cobble that attained its foliation in the Grenville Orogeny (~1 Ga)? Is this an African cobble that got squeezed in some pre-Pangea Gondwanan orogeny? Is it derived from a nameless microcontinent that was formerly marooned in Iapetus oceanic crust (a la Madagascar) and is now accreted to some continent as an exotic terrane? Do the answers to these questions change how we think about the (1) closure of the Iapetus, (b) Appalachian Orogeny, (c) assembly of Pangea?

Elsewhere in the Potomac Gorge, there are other clasts in the accretionary wedge complex that encourage similar thoughts (for instance, you can check out the photos at the top of this page). Another question raised by these clasts is this: Does their position amidst such relatively fine grained sediments (the mud and sand of the graywacke) represent original deposition? Or is that simply tectonically-induced "shuffling" in the blender-like environment of the accretionary wedge? The rocks in an accretionary wedge are not stratigraphically coherent, but sometimes they have little areas that are. If these clasts are in their original depositional position relative to the graywacke matrix, what does that tell us? Are these landslide deposits? Or are these "Snowball Earth"-related glacial dropstones? Without the original sedimentary bedding (destroyed via orogenic metamorphosis & deformation), it's impossible to answer these questions, but it sure would be nice to know.

Lastly, I'll note that everything I've talked about so far (metagraywacke, mysterious clasts, quartz veins, granite intrusions, and regional foliation) are all cut by a series of joints, brittle fractures in the rock. These joints are arranged in a series of joint sets which intersect one another, resulting in the "blocky" nature to bedrock exposures in the Potomac Gorge (example). Here, along one Gorge-bounding cliff, I saw that the joints had begun to accomodate some sliding of the blocks of rock on either side. Technically, they aren't joints any longer, but faults, instead. Total offset is only a few inches, but it shows up well in a photo like this. Note the similar sense of motion on the more distant fault "scarp." A housekey (with pink ribbon attached!) is jammed into the closer fault to give a sense of scale.


All in all, an hour strolling along Windy Run provides some terrific opportunities for reflection on the checkered geologic past of the Piedmont and the Appalachians, and the continuing geomorphic evolution of the Potomac Gorge landscape. I enjoyed my little stroll. It was with reluctance that I turned around and headed back to the house to grade exams...

Labels: appalachians, faults, geology, mass wasting, plate tectonics, virginia

Mass wasting is the downslope movement of Earth materials (rock, soil, debris) under the influence of gravity. Landslildes, rockfalls, slump, and creep are all examples of mass wasting. Today in Indonesia's island of Java, there were a series of landslides triggered by lots of rain. Rain often acts as a catalyst for mass wasting, for several reasons. First, rain is heavy. Once soil gets waterlogged, it just plain weighs more. Heavier masses are more likely to slide than petite ones. Second, water expands soils, pushing outwards from pore spaces. This expansion factor can cause slopes like the hillsides in this AP photograph to increase their gradient every so slightly -- sometimes beyond a critical angle called the "angle of repose". When a slope is below the angle of repose, it stays put ("reposes"). Above the angle of repose, it slides. Lastly, and possibly most importantly, water acts as a lubricant, reducing frictional inertia and allowing soil particles to slide past one another. I call this the "Slip N Slide" effect -- consider the difference between going down a waterslide with water and one that's dry. The water "greases the skids" and facilitates movement. Indonesia is particularly susceptible to landslides because of volanically-steepened slopes and heavy tropical rains. Sometimes, its landslides are triggered by seismic shaking. More on today's landslides can be seen on the BBC's website.

Labels: indonesia, landslide, mass wasting