GG101 Mass Wasting

GEOLOGY/GEOPHYSICS 101 Program 20

Mass Wasting
Hello, and welcome to today's program which is program 20 on "Mass Wasting".

Well, today we begin the study of erosion and the role it plays in shaping the landscape. Erosion is one of destructive of geologic forces, and we remember this balance between constructive forces and destructive forces. Downhill movement of Earth materials by gravity is the single most important process and cause of erosion. This movement is sometimes fast and catastrophic, but mostly it's slow and imperceptible on a daily basis, and like many slow imperceptible processes, magnified over large amounts of geologic time can create very large results.

In fact, of all erosional processes, the greatest amount of material is moved over long periods of time by very slow movements. Now, these types of movements are not as dramatic as volcanic eruptions and earthquakes, but they're still extremely important in shaping the Earth's surface. They act both alone and in conjunction with other erosional processes, especially streams and lakes, and mass wasting processes also operate without human intervention, but human activities can affect it, so before we move on with the lesson, let me remind you of the assignment for today.

We're in Chapter 13, pages 285 to 301, and, as usual,

Okay, so mass wasting now refers to the general downhill movement of Earth material under the influence of gravity so let's examine this downhill movement under the influence of gravity. We've already noted that the movement can be sometimes fast or slow, but it can also involve a single rock or a single grain of clay, or it may involve a whole mountainside, so the size and time scale of these mass moving processes is really quite broad. It's aided by water and also by earthquakes.

Earthquakes, of course, can shake things loose and cause them to start sliding. We'll come back to water in a second. We know that gravity is directed toward the center of the Earth; in fact, our definition of the word "down" comes from the direction that gravity points, so gravity in general can only move materials when it's able to overcome their resistance to motion.

A piece of solid rock, for example, is prevented from moving from its internal strength, but the weathering process makes this material less strong and prepares it for transport. Water helps this process in several ways: First of all, we know that water is an active agent in chemical weathering, but even once the material is weathered, the water can act as a lubricant, so the layers of rock may slide one on top of the other as if they were oiled.

Water also interferes with the cohesion of the particles that make up the soil and rock. "Cohesion" simply means that the particles tend to stick together like a ball of clay. Water also causes clay minerals like montmorillonite to swell up, and when it does this it can sometimes provide just enough of a push that the material can slide downhill.

Okay, we're also aware that although gravity acts straight down that if you have a sloping surface, there will be some component of that downward force of gravity acting along the slope, and the steeper the slope the stronger the component is along the surface, so if there's a vertical surface like a cliff face, then gravity acts directly; if there's a very gentle slope, then the effects of gravity may be somewhat diluted by the slope.

Okay, so gravity, I think, you can see is important in moving material, not only alone in terms of rocks simply falling, and sliding, and rolling downhill, but also in conjunction with stream and wave erosion, and also with ice and wind. Mass wasting is one aspect of the erosional process which is destructive, and it's especially important here in Hawaii for several reasons.

I can give you some examples: We observe that stream valleys like Manoa Valley here on Oahu are many times wider than the streams that flow in them. How is it that a tiny little stream can cut a valley two miles wide and a thousand feet high? How about sea cliffs? We have here in Hawaii many example of cliffs two or more thousand feet above sea level. How can ocean waves that are only a few feet high erode a cliff several thousand feet above sea level? The answer to both of these questions have to do with mass wasting.

Basically, mass wasting processes operating under gravity move material into places where it can be reached and moved further by running water. In the case of the sea cliff, for example, the waves can undercut the cliff forming a notch or a cave, and eventually the material above it can't support the weight any more, and so it falls into the water leaving a nice smooth faced cliff and a pile of rubble at the bottom, which is rapidly then removed by waves. We will take up these processes in later lessons when we study stream erosion and the processes of ocean and wave erosion, so what we see here is that gravity modifies the landscape in the course of destroying it, so that although these erosional processes are actually destructive processes, we could also look at them as modification processes where the landscape goes from one form to another by losing material through gravity.

These movements may be en masse, or they may be piecemeal. "Piecemeal" means individual rocks. "En masse" means the whole mountainside.

We see that the end result of this is that the walls of valleys are enlarged, and the slope is lessened. We go from nearly vertical walled valleys to "v" shaped valleys to a flattened "u" shape. The rate at which these processes operate varies dramatically, from extremely rapid to extremely slow.

Let me give you some examples. Extremely rapid: a landslide or a rock fall or a rock avalanche may reach speeds of a hundred kilometers per hour at 60 miles per hour; on the other hand, soil creep, which is a slow imperceptible process, is usually measured in terms of millimeters or tenths of millimeters per year, so the speed scale from 60 miles per hour or 100 kilometers per hour to a tenth of a millimeter per year is really quite a range of different sizes,

so what about the word "mass"? Wasting we can understand because wasting refers to the destructive process, but what about "mass wasting"? It turns out that we call this "mass" wasting because the fragments generally tend to stay together during transport and move as a unit. They move "en mass." When this happens, transportation and deposition may be inseparable when the material moves en masse.

There are several basic types of mass wasting. I'll just give you some words, and we'll come back or you can look for these in the video: "rock falls," "landslides" "mud flows," "soil avalanche," "soil creep."

Notice that in each of these terms there's a word like "rock," which indicates the material, and "fall," which indicates what the material does. It's not too complicated to figure this out. In a rock fall, what happens? Rocks fall. In a landslide, what happens? The land slides. I don't think I need to go on with each one of the terms. It's fairly simple to see where these terms come about.

Okay, the type of mass wasting. Exactly which one of these processes takes place depends on several factors.You may want to refer to Table 13.2 on page 289 in the text.

so the classifications of these various types, which you might want to refer to table 13.1 on page 288. These classifications are based upon the type of material like "rock" and the speed of movement like "fall." There are many names given. It's not important to know all of them, but you might think of words like "creep," "flow," "slide," "avalanche," and "fall." These are all verbs, which somehow characterize the speed.

Names like "earth," "debris," "rock," and "mud" are general terms to describe material. Okay, so generally these terms are composed of one of those two words.

There are some special terms which may be used to indicate a particular feature formed by mass wasting. For example, "slump." What does "slump" mean? "Slump" means to sort of settle down under your own weight.

Okay, "solifluction " is a particular type of mass wasting that takes place as material is frozen at the surface and a permafrost layer and basically moves downhill in a series of steps as the ice freezes and falls.

Another term you'll come across is "talus". "Talus" is also called "scree ." It's simply an apron of rock fragments which accumulates at the bottom of a cliff as the individual rock pieces break off and tumble down the cliff,

so in the video look for these terms and classifications. Watch for the features which result from these processes but don't worry too much about memorizing all of the possible names and all of the possible combinations of terms, so with all this in mind, let's watch the video.

Funding for this program was provided by the Annenberg C.P.B. Project.

The date is November 13, 1985. At 9:08 that night, the Nevado Del Ruiz Volcano in Colombia, South America erupts. The intensity of the heat melts the glacier atop the volcano sending a torrent of water rushing down the mountain. Soil, trees, and boulders are ripped from the ground. The mass of debris picks up speed bearing down on the nearby town of Armero at 30 miles per hour. Then, shortly before midnight the unsuspecting residents are awakened by the roar of a wall of mud about to bury anything in its path. By morning the desperate panic and confusion of the night had given way to the chilling reality of just what has happened.

Of the 25,000 who lived here when the day began, only 3,000 remained alive. In a matter of seconds, the proud community of Armero has been wiped from the face of the Earth. The tragedy at Armero is an extreme example of the geologic process known as "mass wasting," the downslope movement of Earth materials under the influence of gravity.

Mass wasting can take many forms, from a deadly mudflow rushing down a mountain slope, such as we saw at Armero to destructive landslides such as this one, which in a few second delivered tons of debris, including these boulders onto the road below, to more subtle forms in which the downslope movement is almost imperceptible.

Slopes are the most common of land forms, so mass wasting operates over virtually the entire surface of the Earth.

Think of mass wasting as a continuous chain of processes that move Earth materials from the tops of the highest mountains down to the deep ocean floor. The two driving forces behind mass wasting are tectonic activity, the uplift and mountain building that continuously maintains the slopes, and gravity, which tends to pull the slopes down even out the landscape.

Mass wasting is a natural process that continuously shapes the landscape, and it occurs without human involvement; however, it's important for us to recognize that our decision, such as where we build our roads and structures, and the ways in which we alter natural landscapes can affect this process and sometimes trigger mass wasting events.

Land has always been a valuable commodity. Some want it simply as a place to live; others are more interested in developing it for profit, but regardless of motive, anyone about to undertake a building project needs at least a fundamental understanding of how mass wasting operates.

A number of factors contribute to mass wasting processes. One of the most important is the angle of a given slope. Some slopes are naturally steep, such as those along glacial valleys, riverbanks, or at the shore where waves cut back the cliffs, but in many areas the primary force responsible for steeping the slopes is human activity. When bulldozers cut into hills to create home sites or roads, their stability is sometimes greatly reduced. Over steepening is not the only problem, however.

The heavy weight of a building itself can diminish the stability of an underlying slope as can the presence of water. As water seeps into the ground, it begins to fill up pore spaces. The surface tension of water actually helps to hold fragments of rock and soil together, but if a lot of water is present, it completely fills the pores and forces the fragments apart, thus weakening the slope.

Whether it comes from a sprinkler, a septic system, or a rainstorm, an excessive amount of water in the soil can trigger a mass wasting event.

Processes of mass wasting are often transitional in nature. What begins as one form of mass wasting can ultimately evolve into another. For this reason, these processes are sometimes difficult to classify with absolute precision. The types of mass wasting vary according to several factors: the composition of a material, the manner in which it moves, and the speed of movement. In some cases, the movement is slow, that it's extremely difficult to detect, but the absence of obvious movement doesn't necessarily mean that an area is unaffected by mass wasting. At first glance, for example, this site appears tranquil and undisturbed, but a closer look reveals the presence of "creep," a sluggish form of mass wasting that sometimes moves as slowly as one centimeter per year.

Creep operates every day everywhere, no matter how gentle the slope. Creep occurs when the soil and uppermost part of the bedrock within a slope moves slowly downhill under the influence of gravity and water. The wetting and drying of the uppermost ground material results in alternate expansion and contraction with gravity pulling the contracting Earth imperceptibly down slope. This can happen even when the angle of the slope itself is very small, almost to the point of being horizontal.

Creep is especially active in regions where moist ground seasonally freezes and thaws such as here in Alaska. Each freeze thaw cycle moves soil particles downhill in minute increments, and despite the fact that no one has ever died from creep, it does more long term economic damage than all other forms of mass wasting combined. This can take the form of buckling railroad tracks or cracked building foundations, as well as broken underground water and sewer lines.

If we think of mass wasting as a kind of continuum with creep representing the slowest form of movement, the next fastest would be "slump." Frequently, slump occurs on hillsides that have a thick cover of loose rocky debris. This takes place most commonly after heavy rains saturate the ground. What starts out as a block of slumping earth, often spreads out as an earth flow downslope.

Depending on the amount of water, this type of mass wasting can be active over a period of hours, days, or months. In some earth flows, intermittent slow movement continues for years with large quantities of soft or easily weathered bedrock on the move. One of the most dramatic examples of slump can be found at Point Ferman, California.

Wave erosion here has cut away at the moist soft rock beds underlying the coastal bluffs, and this has created an oversteepened condition. In this area dubbed the "sunken city" by residents, what began as a slump eventually led to a faster rate of slipping and much more extensive mass wasting activity. Geologist Perry Elig has a long standing interest in the geologic history of Point Ferman. This is the Point Ferman landslide. It started moving in 1929. It moved only a small amount, and people continued to live here, but then in 1940 it started moving very extensively, and as you can see here, it's broken into numerous small blocks; in fact, most of the movement was in 1940 and then stopped, and then the area that we see over here that's all sunken has continued to move since that time.

Now this is the old road, the Coast Highway. We're standing on Paseo del Mar right here, the remnants of it, and you can the slabs out there that represent the offset portion of Paseo del Mar. If somebody wanted to put enough money into it, they could remove and recompact the slide material and found it on firm material below the beach level. It might well be something that could be done economically if this were to be a hotel site or something like that. If an area has been designated as a possible building site, it's especially critical that potential mass wasting problems be identified before the decision is made to start building.

Unfortunately, in a spectacular mountainous or coastal locations most prone to mass wasting, these problems often go undiscovered or ignored. Frequently, developers and future residents are so taken by an area's scenic beauty of investment potential that they never stop to consider the consequences of building. In few places is this more true than in an area of California's Palos Verdes Peninsula called "Portuguese Bend." It's wide open spaces and magnificent views would make this an ideal place to live were it not for the problem of landslides, which are mass wasting events that move faster than even creep or slump.

Here in the 1950s a slump occurred which later spread out downslope as an earthflow, but that was just the beginning. In more recent times, the situation has grown much more serious as larger masses of earth have shifted. Perry Ehlig acknowledges that Portuguese Bend was once the site of ancient slide activity long before there were any people around, but he contends that modern slide problems of the area are a direct result of road building. A great deal of fill dirt was placed on the upslope edge of the slide where the slide base has a very steep slope, and this loaded the slide and caused it to start to move. The movement propagated seaward, and it eventually developed into the large landslide that occurs throughout this area right now.

Initial attempts to control the slide activity centered around the insertion of numerous three foot wide concrete and steel nails at the base of the slide. When these efforts proved unsuccessful, the focus then changed to strategies for living with the slide, which in some places had shifted homes as much as 200 meters from their original positions. One of the more visible approaches was putting utilities, such as gas and sewer lines above ground in order to prevent rupture. Unfortunately, most of the homes of the area had already become virtually uninhabitable.

Of the 156 houses built here, most have been torn apart by the shifting ground or relocated to other areas. While some residents say they remain in the area because of its aesthetic appeal, the fact is that many of those who live in Portuguese Bend have no hope of selling their homes. Steve Mirich's house was built in the early 1950s, and the trouble began shortly thereafter. There used to be roughly three times as many houses through this valley, and some houses met their demise very fast, were torqued apart by the land movement quite fast. Some people sold out. Insurance wrecked down other houses, and a few people bought houses that were still remaining up right, lived in them cheaply until the house was ready to fall apart until about 18 years ago.

Someone came up with the idea to take the house and lift it up off the ground and build a steel framework underneath it to hold it together, and then all we do is once in a while, and it's only been once since I've been here that we've actually leveled the house off and adjusted the blocks that rest in certain areas and brought it back down to somewhat levelness. Other residents in the area have developed equally inventing methods of coping with the restless ground.

Bob McJones has placed his home atop three large containers, the kind used on cargo ships. the three containers were welded into a triangle. The containers are strong enough that they will support the weight of the house comfortably, and the house is now resting on the ground in only three places. It's like a tripod. The result of this is that if one of those three supports settles or it goes up due to landslide action, it's only a few minutes work to raise or lower that corner of the house, and the house is back level. These are at best only temporary solutions. Attempts to permanently stabilize the landslide itself continue.

Perry Ehlig regularly monitors wells that have been installed to drain water from the slide area. Thus far, lowering the level of the water table is an effort that seems to have paid off. Measurements made along the edge of the slide indicate that the movement has slowed to a fraction of its former speed. Elig hopes to completely stop the upper portion of the landslide within the next few years. The lower edge, however, is a different and somewhat more complicated matter.

The problem here is that wave erosion undermines the base of the landslide, which weakens its upslope area further. Without protection coastal erosion will continue to be a factor contributing to movement.

Rock filled cages called "gabions" have been tested along part of the shoreline successfully reducing the wave erosion and helping to stabilize the slide. With the continued mitigation efforts of residents and geologists, it's conceivable that Portuguese Bend could one day be entirely stabilized. As destructive as the slide activity at Portuguese Bend has been, it's something that has happened gradually over a period of time.

This is Wrightwood, California. In contrast to the gradual slide activity of Portuguese Bend, the mud and debris flows that damaged Wrightwood in 1941, left their mark with terrifying swiftness. Today this community lives with the ongoing fear that the next season's storm may bring on a reprise of the 1941 disaster. Mud and debris flows which move faster than creep, slump, and some landslides are the product of loose unstable material mixed with water. Their consistency ranges from stiff mud to a souplike fluid. Because of their viscosity and high density, mud and debris flows are a form of mass wasting that can tear homes from their foundations and carry lighter objects like automobiles for substantial distances.

The debris that comprises these flows can range in size from gains of silt and sand to boulders. Once the debris has accumulated, all that's needed is an intense rainstorm or snow melt to saturate the area with water. At that point, gravity can move the debris, and as flows overwhelm low lying areas, the mass wasting process is set in motion.

This flood control channel was built to protect the community of Wrightwood from the mudflows that come off the nearby mountains after particularly wet winters.

Wade Wells is a U. S. Forest Service Hydrologist concerned with the control of mud flows. These channels have proven themselves to be very effective in protecting most communities. It's only in the worst years that we have a problem with the channels being overtopped and houses being covered with mud or destroyed and knocked off their foundations, things like that. Channels like this one, however, do require a great deal of costly maintenance. During every year and after every rainy season we come in with bulldozers, skip loaders, and trucks, and we carry out, we take the material and carry it away to a suitable disposal site.

One reason the Wrightwood area is vulnerable to mudflows is the proximity of the San Andreas fault, which has deeply shattered the bedrock composing the mountainside above the community. The flows that damaged Wrightwood in 1941 came from about the eastern third of that scar. It's widened to the west since that time until it's reached its present size. This indicates that the whole area is active, and the fact that there's nothing growing up there in the scar area means that it moves so much, so often, and so rapidly, that it'll probably be that way for several years, several hundreds of years hence.

Numerous preventive measures have been put in place since the 1941 mudflows, but even with the vigilant use of flood control channels and other mitigation efforts, there still isn't a way to completely neutralize the raw power of such an event. This sign was put in place following a series of rock slides. Like mud flows rock and debris slides are mass wasting events that take place in mountainous areas, but they move at an even faster rate of speed.

Rockslides like this one are a major problem on highways built through mountainous terrain where the oversteepened slope of the roadcuts are especially susceptible to slide activity. Some rockslides travel only a few meters to the base of the slope, while in country with high relief a rockslide may travel hundreds or even thousands of meters before reaching the valley floor, and if the movement becomes extremely rapid, the rockslide may break up and become a rock avalanche.

In some cases, the potential for mass wasting is so great that building probably should not be done at all; in other areas, it isn't necessary to suspend all building, but developers must identify and prevent potential mass wasting problems at every phase of the project from the initial planning right through final construction.

It was with just that in mind that a major real estate developer hired Ann Meaker and her colleagues at Schaeffer Dixon and Associates to assess the geologic stability of a 60 acre parcel of land. The developer planned to build 150 homes worth between $300,000 and $500,000 each. Ordinary, if a particular site is found to be unacceptable for building, a developer can always look elsewhere, but in this case that is not an option because the developer has already bought the land, so as Ann Meaker begins her work, the financial stakes are unusually high.

When we come out to a site, we'll typically map the area first on foot to look for features such as these landslides to match to a map of outcrops that we see to get an idea of what direction the bedding is dipping, the structure in the area. We'll be looking for any evidence of faulting, any evidence of superficial creep where the surface soils have slowly moved over time causing slumping or an irregular surface.

During the course of their investigation, Meaker and her associates find evidence of mass wasting. Their subsurface work reveals irregularities in the upper section of the material; in other words "creep." they also find evidence of slump. Material that was once in place has clearly moved downslope creating a shallow depression in the original location and leading to a thickening of ground in the downslope area. What worries the developer is that if the mass wasting problems go beyond creep and slump, the mitigation costs could be too high to complete the building project.

The main concern at this point is the possibility of major landslide activity. Creep and slump can be easily mitigated by grading the slope provided, of course, that they're discovered prior to building, but there are no such guarantees with landslides, which tend to be much larger and more deeply rooted, so, once they complete their mapping work, Meaker and her crew begin the process of drilling.

The geologist being lowered into the hole examines the structure of the soil. This helps determine the history of the landslide activity in the area. And as we drill the hole in that area, we will be sampling the material to determine how strong it is. We will also be describing it in terms of its sole characteristics, such as grain size, moisture, color, a variety of types of characteristics, which will be useful to the engineers in the office when they determine whether or not the material will be suitable to stand in the slope or to be reused as fill somewhere else on the site.

If it turns out that there are landslides present, a decision will have to be made as to whether or not mitigation is possible both from a geologic and a financial standpoint. Normally a landslide can be mitigated either by being removed or through the construction of some form of support, but if poor samples and other data obtained during drilling reveal that landslide activity here has been more extensive than anticipated or extends beyond the property line, mitigation efforts might prove impossible.

When we leave the field we'll take the information that we've gained and plot that information both in map form and cross section form. Based on those cross sections we will have our engineering staff evaluate the stability of proposed cut slopes, any removals which will be necessary of land slide areas, such as these. Having analyzed their data, Meaker and her colleagues will present their final recommendations to the client.

It turns out that the news is both good and bad. The bad news is that there is evidence of landsliding that crosses over to the adjacent property, and because the developer has no legal right to mitigate problems that are not on his land, some of the homes in the tract cannot be built. The good news is that with careful grading, the right drainage, and use of vegetation, the rest of the homes can be built, and residents of this housing tract will probably not have to worry about any mass wasting problems, but that is not always the case.

Some people either don't know the facts about mass wasting or simply choose to ignore them and build anyway, and as we've seen, mass wasting can be a problem of major proportions no matter what form it takes.

Creep, for example, the continuous downslope movement of soil often proceeds too slowly to be detected until the damage has been done.

Slump and earth flows often move large amounts of soft or easily weathered bedrock in slow intermittent fashion over a long period of time. Damage in coastal areas is frequently very serious.

In Portuguese Bend just west of Point Ferman what began as a slump evolved into a faster moving landsliding event that is an even more serious form of mass wasting. As residents in the area have learned, the impact of landslides on property can be devastating. Capable of speeds far greater than creep or slump and even some landslides are mud and debris flows. These flows are often the result of heavy rains saturating the ground in mountainous canyons over time. Because of their viscosity and high density, mud and debris flows can move large objects and be extremely destructive. Finally, at the very rapid end of the mass wasting scale are rockslides and avalanches. In these forms of mass wasting, rocks may travel hundreds or even thousands of meters before reaching a valley floor, and on the way down, they sometimes attain speeds of several hundred kilometers per hour.

People tend to view episodes of mass wasting as isolated unexpected disasters, and yet these events are actually individual frames, a continuously running picture of landscape evolution. Mass wasting is a natural process, and yet all too frequently people play a role in triggering these events sometimes undermining slope stability by making it too steep or by adding excess water, and so while we are certainly not able to prevent mass wasting altogether, we can at least plan intelligently and greatly limit the loss of not only property, but ultimately of human life.

While this video shows many varieties of mass wasting and their effects but don't try and memorize all of the details. Rather, try to understand that these classifications like all of our classifications simply tries to organize all of the possible ways in which material can move downhill.

Whether it's small pieces, or big pieces or rocks, or soil, or whether it's fast or slow, it's just a way of trying to understand and organize these processes. There are a few things I want to add and some things I want to elaborate on about the video. We might note that material is also removed from the slope in both solution and suspension; that is, some materials carried away from the slope as dissolved material, and some is simply carried down the slope as material picked up by the water.

When we study streams, we'll learn more about these processes, but it's also important to note here that many times at least early on in development of a stream, or early in a rainstorm, or at the top of a valley the water doesn't necessarily collect into channels right away, but there's a phenomenon called "sheet wash" in which the water simply flows over the surface for a small distance before it actually collects into little rivulets and rills.

Another thing to note is that one of the processes of mass wasting is called "rain splash." It's simply where water strikes the slope and picks up a piece of material and moves it downstream. It occurs mostly on unvegetated or partially vegetated slopes. Raindrops may be up to six millimeters in diameter. They can be like this, and they hit the ground at an average of nine meters per second.

Well, what all these numbers mean is that the impact of the rain is strong enough to move particles up to a centimeter in diameter. A centimeter is about half an inch. The splash of these particles as the rain hits it and moves it downhill results in a slight net movement downhill. Not much. A centimeter at a time. But that's quite a bit of movement. This may loosen even larger particles and prepare the way for larger downslope movements to take place later on.

One more distinction to make here. Geologists use two different words to describe material which accumulates or is deposited by mass wasting as opposed to that deposited by streams. The word "colluvium" means accumulations of material from mass wasting.

The reason we make these distinctions is that when we find material of a particular type, if we can identify the process that moved it, it can help us to not only interpret the geologic history of an area but also to make predictions about what's likely to happen in the future, so in colluvium with accumulations from mass wasting, the products of different processes, in other words, slumps, and slides, and rockfalls, and avalanches are usually intermixed.

The particles are poorly sorted, and it's generally impractical to identify and trace the history of every piece of material that's in there.

Alluvium, on the other hand, is accumulation from stream depositions. Alluvial deposits are those which are deposited by stream action, and they have certain characteristics of size, and shape, and sorting, and composition, and so forth, which we'll study when we get to that section of stream erosion.

We might also note some basic shapes of hill slopes. Now, when you think about all the possibilities for slopes of hills, you might think that the shape of a particular valley or a particular slope is, a variety, I should say, of shapes is infinite. It's true that they are, but slopes of all different kinds can be thought to be made up of different elements. Basically, a slope can be either convex, concave, or straight, and "convex," of course, means opened downward. "Concave" means opened upward, and "straight" means straight.

When you think about a hill slope, you can join these various elements together in different ways and different ratios, so the upper slopes of a hill, for example, are generally convex where the top of the hill of the cliff is a convex curve. Lower slopes of the hills are generally concave; that is from the slope to the level part generally is a concave shape. There may be different humps and hummocks on the way down that have various mixtures of convex and concave shapes. We can even have straight slopes.

Some of the volcanic slopes of the Hawaiian Islands, for example, are nearly straight slopes. Talus slopes are generally fairly straight and level. The reason I mention the shapes is because there are some influences or some things that influence the shape of the slopes, and when we try to understand how a landscape, a particular landscape evolves, we might want to know what kinds of things can influence this.

Well, one thing that can influence the slope shape is simply the type of rock. As we'll learn later on in more detail, the more resistant the rock is, the steeper the slope tends to develop; in other words, rock that weathers very easily or is very soft tends to form gentle slopes; whereas, resistant rock tends to form steep slopes.

Geologic structures can also affect the steepness of a slope. For example, here in Hawaii the lava flows are made up of alternating layers of various resistances to erosion and to weathering. A dense mass of aa center, for example, with aa clinker on top followed by a pahoehoe flow. The dense massive part of the aa flow tends to be more resistant, and so we wind up having nearly vertical valleys with resistant layers acting as impediments to the movement and to the development of the slopes.

Okay, climate is another factor which can influence the slope. Where the rainfall and temperature are higher, there's a greater amount of chemical weathering, and so that means that there's a greater amount of soil thickness, and, of course, the presence or absence of soil is a major factor in the type of mass wasting that takes place.

Obviously, where there's soil you'll tend to have mudslides, and landslides, and soil avalanches. Where there's not soil, you tend to have rockfalls and rock avalanches. Vegetation, of course, also plays a major role, and the thicker the soil, the more likely there is for vegetation to be there.

Time also plays an important role in determining the actual shape of the slopes. Slopes like other geologic features are dynamic active features that change with time. Sometimes fast; sometimes slow, but always changing. Cliffs or very steep hillsides retreat at a fairly well-known rate.

By "retreating" meaning each year they move a certain amount. The rates that have been measured show that the cliffs and hillsides retreat anywhere from three hundredths of a millimeter to three millimeters per year. That's really slow, folks. At three millimeters per year you're not going to notice that Manoa Valley's much wider this year than it was last year, but over time, of course, these do add up. Not only are valleys widened, but the angles of the slopes and the elevations in mountainous areas are also reduced with time at about the same rate at very equivalent kinds of rates.

Well, I want to turn some attention to mass wasting processes here in Hawaii. The factors that influence mass wasting of various types are pretty much the same everywhere, but here in Hawaii we have rather special conditions that need consideration. We do have alternating layers of rock. We have basaltic rock. We have a variety of climates and climate zones. We also have a variety of temperature changes and temperature ranges.

In Hawaii, dry versus wet climate is probably the most significant factor. The mass wasting processes that operate in dry climates versus wet climates are somewhat different, and part of this depends upon the predominance of chemical versus physical weathering, and from the program on weathering soils we should understand that where the temperature is high, and there's lot of humidity, chemical weathering tends to predominate, and where the temperature's low and there's not much humidity, physical weathering tends to predominate.

Now, in reality, here in Hawaii where we have many microclimates and many different climates fairly close together, we actually find a complete gradation from one region to another, but it helps to consider these two extremes separately so we can understand how the transitions work. Let's consider mass wasting in arid regions first, in dry regions. A good example on Oahu here is the Leeward Waianae range.

On some of the other islands, the Kona Coast of the Big Island of Hawaii, the Lahaina side of or the Kihei side of west Maui, the Na Pali side of Kauai, for example. In these arid climates the main processes tend to be simply rockfalls and sliding of rock fragments. Rainwash is also a significant factor as is soil creep. Let's see if we can understand why those are the main features. Let's look at the characteristics as you would expect of the material that's available for mass wasting in arid regions.

Much of the material in transport is relatively fresh or only slightly weathered. It's only slightly weathered because there's not much chemical weathering going on, so the material is relatively fresh. The fragments are also usually blockier because mechanical or physical weathering processes tend to create blocky rather than rounded features. The soil cover is generally far less continuous; that means it doesn't necessarily reach from ridge to ridge or from ridge down the valley walls, and the soil's relatively thin. This is also due to the fact that there's little chemical weathering. The valley walls are typically low cliffs on resistant layers, and again the cliffs themselves usually involve the massive centers of aa flows, so a profile of a cliff: aa, flow; aa, flow, so on down with ledges maybe separating them in between, the average slope 20 to 30 degrees. The slope on the less resistant beds, those which form the flatter part of the terraces, are often littered with rock fragments, which are washed down or have been tumbled down from higher altitudes.

This temporary colluvium. "Colluvium " remember is material moved by mass wasting represents a pause in the downslope movement. It's sitting there on these flat ledges or terraces waiting for some episode of rain, earthquake, or simply instability to move them downslope. In arid areas here in Hawaii, when we get rain, the rainfall is usually brief, infrequent, and heavy.

I mean it doesn't rain very often, but when it does, it rains really hard. A few years have been exceptionally heavy; 1989, for example, was a very wet year. During these wet years, more mass wasting may be accomplished in one rainstorm than happens in 20 years of no rain.

Okay, in these cases, sheets or rills wash away the finer fragments leaving behind the large fragments of rock leaving those exposed. Okay, let's turn our attention to mass wasting in wet regions, and let's see if we can't find some contrast between wet and dry. In wet regions, the mass wasting processes are at least as active as they are in dry regions. We generally tend to think of erosion as taking place mainly by stream erosion in places where it's wet but keep in mind that mass wasting simply sets up material and puts it in place for the streams to grab it, so at least as active in wet regions as in dry regions.

The difference is that the processes that predominate in arid climates are relatively unimportant in wet climates. There are few individual fragments like pieces of rock that are seen to simply roll or slide downhill. Soil creep may be significant if there's a weak or thin soil, though water from sheet wash is generally very clear, meaning that it doesn't have much suspended material, which means that it's not capturing very much silt or clay. The reason why: The silt or clay is often anchored by plant roots. Keep in mind that the more wet the climate, the more likely there are to be plants, so because of this the hillsides are steeper than typically found in dry climates.

The valley walls may be up to 50 degrees and the bedrock. In Hawaii these walls are typically covered by soil which is held to the bedrock by plant roots. Occasionally, in fact, quite often, we find soil avalanches or landslides taking place on the hillsides, and as you drive around, especially in the valleys here on Oahu or in some of the steeper valleys along the Hamakua Coast of the Big Island, or some of the valleys on the western side or even northern side of Kauai, you might notice these landslide scars, which are simply vast regions where the vegetation is missing.

Now, these happen often. They don't often happen in large sizes, but they happen often enough that we can see how the valley wall grows this way. Basically, what happens is the soil cover simply pulls loose from the bedrock and slides. This material may travel all the way to the valley floor, or it may be stopped in transit. If it makes it to the valley floor, it adds to the colluvial apron at the bottom, and eventually a stream will come along during flood or during its normal course and take this away.

Slides of this type, soil avalanches, may leave bright scars on the hillside for months. A good example is a slide which occurred in Olokele Canyon on Kauai in October, 1981. You can see the difference in these before and after photographs. The slide face here was about 300 meters wide and about 800 meters high. That's a thousand feet wide by 2,400 feet high, a slide of tremendous proportions.

This particular slide was caused by a combination of high rainfall and underground water seepage. Features like this or processes like this are responsible for much of the valley development here in Hawaii, and we'll come back and look at some of this when we get into the program on "Slopes."

One other thing to note here. Soil creep is not a dramatic process but especially important. It's a slow, imperceptible, continuous process where the movement may be only a centimeter or so per year, but it is a very important process, and it's especially important in housing and commercial developments.

Here where houses are built on the side of the hill, even very slow movements may cause the house to break. It may cause telephone poles to bend very slowly. It may cause fences to move. We've had several examples of this right here on Oahu. In Woodlawn in Manoa Valley a whole series of houses were sliding down the hill. Back in the 1960s in Aina Haina and Niu Valley here on Oahu, 7 many of the houses were sliding downhill. At the now defunct Kailua Drive on the Windward side of Oahu, they had oversteepened the slope, and for many years after the drive in opened, every time it rained, parts of the cliffs would collapse and block the entrances and so forth.

Okay, so you see human activities can increase the hazards of mass wasting, usually due to a process called "oversteepening." Like all geologic processes the slope that develops by mass wasting processes, erosion, or whatever is an equilibrium process. The equilibrium slope that develops on a certain material depends on many factors.

What I'm saying here is if you cut into a hillside, changes will take place on that hillside to try to reestablish some natural slope, slope, angle; slope, size. The equilibrium slope, the one which will develop depends on many factors. the amount and type of vegetation. The loading by houses and other structures may aggravate the movement. Changing the slope increases the frequency of mass movement. Landslide movements of all types to establish an equilibrium.

Road cuts. How many times have we seen landslides along road cuts? As you drive up the Pali Highway here on Oahu just after you leave downtown Honolulu look at a road cut, and you see chicken wire strapped around the edge of the cliff. The purpose of this is to prevent rockfalls and other things from sliding out onto the highway. Okay, housing commercial developments almost always cut into the land. Well, I think I'll leave that at this point for this lesson.

Let me remind you that the next lesson is actually a lesson on sedimentary rocks, and sedimentary rocks are sort of involved in the erosional deposition process, so we'll take up sedimentary rocks the next time; that's Lesson 17, Chapter 14, so you should

I'll see you next time.