EES 1001: Lab #12 Name:______________________________________________ Sec._________
NATURAL HAZARDS
Natural hazards are extreme eventsthat, because of their location, severity, and frequency, have the potential to affect humans, their structures, or their activities adversely. There are numerous natural hazards of geological origin and we will examine four of them here: mass wasting, earthquakes, volcanoes, and karst topography.
All of these processes have been operating throughout Earth’s history, but they have become hazardous only because they negatively affect us as human beings. There would be no natural disasters if it were not for humans. Without humans these are only natural events.
SECTION 1 Mass Wasting
Mass wasting is the geomorphic process by which soil, sand, regolith, and rock move downslope, typically as a mass on unstable slopes.This is an umbrella term that includes a variety of movement types on land, includinglandslides. Several factors determine the type of mass wasting event that will occur, some of which are described in the table below.
| TYPE of MOVEMENT | |||||
| MATERIAL | MECHANISM | CONDITIONS | |||
| Bedrock | Soil | ||||
| Fall | Rock Fall | Earth Fall (fine) Debris Fall (coarse) | Material falls, tumbles, or rolls downslope after separating along discontinuities such as fractures, joints, & bedding planes | Steep slopes; hard fractured rock; mechanical weathering,; interstitial water | |
| Slide | Rotational (few units) | Rock Slump | Earth Slump (fine) Debris Slump (coarse) | Surface of rupture is curved concavely upward; blocks of failed material can rotate as they fail and can at times be seen to tilt backwards towards the slope | Deep or shallow concave slip surface |
| Translational (many units) | Rock Block Slide | Earth Block Slide (fine) Debris Block Slide (coarse) | Mass moves along a roughly planar surface with little rotation or backward tilting sliding out over the original ground surface | Shallow, distinctive surface of weakness such as a fault, joint or bedding plane | |
| Flow | Rock Flow (deep creep) | Debris Flow | Rapid downslope movement of a water-saturated slurry of material | Recent intense water surface flow (rain or snowmelt); recent wildfires; steep slopes | |
| Creep | Surface rocks & soil | Imperceptibly slow, steady, downward movement of slope-forming soil or rock | Seasonal freezing and thawing of the land’s surface (temperate climates) | ||
| Topple | Large to very large chunks of in-tact mountainside rock | Forward rotation of unit(s) about a pivotal point, below or low in the unit, under actions of gravity | High interstitial fluid content; undercutting; differential weathering; stream erosion |
- Translational slide
Rotational slide
- Rockfall
- Debris flow
- Creep
Landslide classification adapted from Varnes (1958).
Examine the Highway Corridor Geologic Map and Highway Corridor Landslide Map from Big Sur, CA, along with the geologic unit key and answer the following questions.
- What type of rock is Ks? Based on the rock’s description and its distribution on the geological and landslide maps, does this unit appear to be susceptible or resistant to landslides? Why?
Ks unit is very susceptible to landslide. The location as per the key index of “green colouration” shows intact vegetation cover.
- Are the units Qls&Qdf composed of in-tact bedrock or landslide deposits?What is the difference between the two units? Hint: refer to the map key.
Qls and Qdf and not intact bedrocks. Instead they are deposits of landslide!
Qls is Debris Flow process
Qdf represents translational slide
- Name one geologic unit that appears to be resistant to mass wasting. What type of rock is it?
Ks is likely to be the geological unit
Not only is quartz the most stable of the common rock forming minerals in chemical weathering, its high hardness and lack of cleavage make it quite resistant to mechanical weathering. This rock is quartz!
SECTION 2 Earthquakes
An earthquake is the sudden release of energy stored as lithospheric stress that radiates seismic waves. At the surface, earthquakes may manifest with a shaking or displacement of the ground. The majority of earthquakes occur at tectonic plate boundaries, of which there are three main types: divergent, convergent, and transform. Plate boundaries in different localities are subject to different inter-plate stresses, producing these three types of earthquakes. Each type has its own special hazards.
The point where the movement of rock occurs is called the focus and directly above it on the earth’s surface is called the epicenter.
The strength of shaking from an earthquake diminishes with increasing distance from the earthquake’s focus, so the strength of shaking at the surface from an earthquake that occurs at 500km deep is considerably less than if the same earthquake had occurred at 20 km depth.
Earthquake magnitudes are based on a logarithmic scale, so for each whole number you go up on the magnitude scale, the amplitude of the ground motion recorded by a seismograph goes up 10 times. Fortunately, most of the earthquakes that occur each year are magnitude 2.5 or less, too small to be felt by most people. The illustration to the right compares the energy released during some historic earthquakes with other events and phenomena.
Lastly, consider the map Seismicity of Hawaii, 1962-1985.
- What type of plate tectonic feature is responsible for the Hawaiian chain? Think back to the Igneous Rocks lab when we discussed how different types of mafic melts are created.
The combined processes of magma formation, eruption, and continuous movement of the Pacific Plate over the stationary hot spot have left the trail of volcanoes across the ocean floor that is now called the Hawaiian-Emperor Chain.
- On which island have the largest and most numerous earthquakes occurred during this time period?
The overwhelming majority of earthquakes in Hawai’i occur on and around the Island of Hawai’i, especially in the southern districts of the island where Kīlauea, Mauna Loa, and Lō’ihi volcanoes are the most active in the State.
To what geologic process do you think that many of them are related?
Volcanic eruption and solidifications
- From what you know about the process of formation of the Hawaiian Islands and their ages, explain why you would expect to see the largest and most frequent earthquakes on the island that you identified.
Thousands of earthquakes occur every year in the State of Hawaii. They are caused by eruptive processes within the active volcanoes or by deep structural adjustments due to the weight of the islands on Earth’s underlying crust. Most are so small that they can only be detected by sensitive instruments, known as seismometers. Some are strong enough to be felt on one or more of the islands. A few earthquakes are large enough to cause significant damage and impact residents across the State.
SECTION 3 Volcanoes
There are hundreds of active volcanoes on earth today, and any one of them can cause hazards that place humans at risk. Primary volcanic hazards include falling ejected material, lava flows, and volcanic gases. Secondary volcanic hazards can include earthquakes, tsunami, and landslides.
Explosive volcanic activity is usually associated with intermediate and felsic lavas. As shown in the table below, these lavas solidify into volcanic rocks with 52%-65% and >65% silica (SiO2), respectively. Consequently, geologists measure the amount of silica in volcanic rocks to better-understand the threat of explosive volcanic activity. In this activity, you will use the amount of silica in volcanic rocks to help you evaluate the threat of explosive volcanic activity at five sites (A through E) identified on the geologic map of Costa Rica provided.
Table 1. The composition of volcanic rocks.
- Use the information in Table 1 to assign a composition to each rock sample in Table 2. In the COMPOSITION column of Table 2, write “U” for ultramafic, “M” for mafic and so on. The compositions of the four samples from Site E have been entered for you as an example.
- At which site are all samples intermediate?
As for the case of Site B all the sites were intermediate
- At which site are all samples mafic?
In the case of Site A, all were mafic
- Based on your answers to #2 and #3 above, at which site is the potential for explosive volcanic activity least? (Assume the samples are representative of the geology at the site.)
Explosive volcanic activity is usually associated with intermediate and felsic lavas. This implies that the lest occurrence will be for the case of those regions dominated with ultrasonic(U) and Mafic(M). As can be seen in table 2, site A will experience the least volcanic activity.
- Which site(s) yielded rocks indicating at least some potential for explosive volcanic activity?
Site D and Site E indicated at least some potential of experiencing explosive volcanic activity.
- In general, is the volcanic threat greatest in northern, central, or southern Costa Rica? Consider the location of the most recent volcanic deposits in conjunction with the most populous areas.
As indicated previously, the higher chances of occurrence of the volcanic activities are associated with intermediate whose percentage range is between 52 and 65. As can be seen from the map, this class of rocks is within the central region. Hence the central region experiences the greatest processes of volcanic eruptions.
- Extremely explosive volcanic activity can form an ignimbrite – a kind of felsic flow. Examine the geologic map. Which two sites are relatively close to ignimbrites?
Northwestern Region/Site and part of the central
- Examine the Costa Rica nighttime image. Night lights indicate human settlements. List volcanic sites in or touching settled areas.
- Arenal Volcano
- Irazu Volcano
- Tenerio Volcano
- Barva Volcano
- Rincon de la Vieja Volcano
- Poas Volcano
- Turrialba Volcano
| SITE | SAMPLE | % SILICA | COMPOSITION |
| A | A-1 | 51.4 | M |
| A | A-2 | 51.2 | M |
| A | A-3 | 50.8 | M |
| A | A-4 | 50.6 | M |
| A | A-5 | 50.5 | M |
| A | A-7 | 50.2 | M |
| A | A-9 | 49.6 | M |
| A | A-10 | 49.4 | M |
| B | B-1 | 56.0 | I |
| B | B-2 | 55.2 | I |
| B | B-3 | 54.7 | I |
| B | B-4 | 54.6 | I |
| B | B-5 | 54.5 | I |
| C | C-1 | 52.9 | M |
| C | C-2 | 49.8 | M |
| C | C-3 | 47.1 | M |
| C | C-4 | 46.1 | M |
| D | D-1 | 55.7 | I |
| D | D-2 | 55.1 | I |
| D | D-3 | 55.0 | I |
| D | D-4 | 54.9 | I |
| D | D-5 | 54.4 | I |
| D | D-7 | 54.0 | I |
| D | D-8 | 53.5 | I |
| D | D-9 | 51.3 | M |
| E | E-1 | 52.5 | I |
| E | E-2 | 52.3 | I |
| E | E-3 | 51.7 | M |
| E | E-4 | 51.6 | M |
SECTION 4 Karst Topography
The term karst describes a distinctive topography that indicates dissolution of underlying soluble rocks by surface water or ground water.Although commonly associated with carbonate rocks (limestone & dolomite) other highly soluble rocks such as evaporates (gypsum & anhydrite) can be sculpted into karst terrain.
Rainwater is slightly acidic because it reacts chemically with carbon dioxide that occurs naturally in the atmosphere and soil. Rainwater seeps downward into the soil and through fractures in the rock in response to the force of gravity. The carbonic acid in the moving groundwater dissolves the bedrock along the surfaces of joints, fractures and bedding planes. Common karst features include:
- Disappearing streamsterminate abruptly by flowing or seeping into the ground. They are evidence of disrupted surface drainage and thus indicate the presence of an underground drainage system.
- Sinkholesare closed surface depressions draining underground in karst landscape. They are often “bowl-shaped” and can be a few to many hundreds of meters in diameter.These sinkholes or shallow basins may fill with water to form lakes or ponds.
- Springs are locations where groundwater emerges at the surface of the earth.The opposite of a sinking stream.
- Caves are natural openings in rock large enough to be entered by man and extending to points where daylight does not penetrate.Caves may reflect a complex underground drainage system.
Understanding caves and karst is important because 10% of the Earth’s surface is occupied by karst landscape and as much as a quarter of the world’s population depends upon water supplied from karst areas. Though most abundant in humid regions where carbonate rock is present, karst terrain occurs in temperate, tropical, alpine and polar environments.
About 18% of the area of the United States is underlain by soluble rocks having karst or the potential for development of karst features.
Study the Mammoth Cave (Kentucky) Quadrangle topographic map. This area is overlain by sandstone in the northern part of the mapped area (north of Park City).
- How can you identify the area on this map where limestone crops out at the earth’s surface?
In this particular map of topography, there is illustration of the region of limestone karst due to the existence of geomorphic features like Uvalas depression and sinkholes that can be observed clearly. Most critical, the disappearance of surface drainage or the stream river disappearance represents the presence of some blind valley. Blind valleys basically refers to those valleys which course the water to take it to the large subsurafe cave. This results into the termination of the stream at the blind valley.
- Imagine you are planning to build a home on top of one of the highest hills in the area where the sandstone crops out, and that you plan to use well water drawn from the limestone under your property. What is the deepest (approximately) you would have to drill your well (through the sandstone) to obtain water from the limestone aquifer?
As per the analysis of the geographical rock cores and logs, the top sandstone aquifer is basically located at 35 feet. Also the developed regional cross-section indicated that the top of the aquifer at the selected site pr the proposed site of building over sandstone will generally correlate with other parts or components. This implies that digging approximately 100 feet will be okay
- Find a disappearing stream at the end where it disappears within the Karst region. Label it below.
| Location of the Lake |
Find a lake that formed by flooding a sinkhole. Label it on the map below.
- Why does Florida experience more sinkholes than Louisiana?
Unlike the case of Louisiana, Florida is known to be a sunshine state although living in this particular state has been characterized by very dark side. The number of the sinkholes has been increasing as a result of the geology of the region. This particular state is underlain largely by the porous limestone with the potential of holding immense water quantity in the underground aquifers. As the ground water flow slowly through these limestone rocks, there is formation of landscape called karst. Karst formed is characterized with features such as springs, caves as well as sinkholes.
- Why do the more humid parts of the U.S. exhibit more karst-related topography?
Karst topography areas which are found in the United States are more dynamic and their occurrences are common in the humid places.. The places which are very humid allow for the greater quality of flowing water. More noticeable differences do exist as for the case between humid temperate and humid tropical environments. As for the case of the humid-temperate climates, there is likelihood of developing several sinkholes. Also, in the case of the humid tropical climates, there is dominance of the hills. The greater calcite concentration will determine most effectively the extent of the karst. This has been demonstrated clearly in the map.
- Most of the evaporite karst features we see in the U.S. are a direct result of human activity. How might drilling boreholes to acquire the evaporites where they are deep below the surface (e.g. Louisiana) lead to the development of karst features?
When the boreholes are dug deep underground, there is creation of the water ways or seepages down deep. This implies that the amount of water accessing the underground will me much. Also it is important t to note that the karst substances tend to be softer and their the rate of formation of the karst feature will be higher as the process of dissolution will have been enhanced properly.
Seismicity of Hawaii, 1962-1985
Highway Corridor Geologic Map and Highway Corridor Landslide Map from Big Sur, CA, along with the geologic unit key:
Costa Rica nighttime and geology image
Mammoth Cave (Kentucky) Quadrangle topographic map
