RockOn #6
A place such as central Pennsylvania, home of Penn State's University Park campus, is fairly typical of the world in terms of rainfall. What happens to the rain that falls on central Pennsylvania each year? Most of it flows directly across the surface, causing car crashes when people drive through too rapidly. Most of it flows through the spaces deep in the ground to streams, lakes and rivers. Most of it is sold to Pepsi and Coke bottlers, who make the secret fluids used in "fracking" for natural gas, which are really soft drinks. Most of it is evaporated. Most of it falls on the surface of streams, lakes and rivers.
Most of it is evaporated. Feedback: Water gives life, and life is very good at using water. When their leaves are out, trees use almost all the rain that falls, and tree roots reach down into the ground and pull up some of the water from cold-season rain and snowmelt. An important amount of water does soak into the ground and flow to streams (maybe 1/3 of the total), but plants still get the majority. The amount of water that flows across the surface is increasing as we pave the landscape, but most of the land is not paved, and flow across natural surfaces to streams is small. Streams, lakes and rivers cover only a tiny part of the landscape, so direct rainfall into them is small. "Frackers" are adding other chemicals to water and using it in natural gas recovery, but they are not using nearly as much water as trees. And, while the exact chemical composition of the frack waters is an industrial secret, we're pretty sure that it isn't made of soft drinks!
The map above shows the Birdfoot Delta of the Mississippi River, where it empties into the Gulf of Mexico. The river is shown in blue, as is the Gulf of Mexico. The river "wants" to leave this delta, and flow somewhere else, far to the west of the area covered by this map. Why? Humans have been damming the river at the end, so the river must go elsewhere. The meandering of the river has tied it in a knot, so it has to take a different path. The delta has built up as well as out, and that makes some other path to the Gulf steeper and shorter than the one now being taken, and during a flood the river tends to take that shorter path and cut a new channel. Hurricane Katrina, in 2005, plugged many of the river channels, so the river must go elsewhere. As the mud of the delta sinks, the river loses its river banks, so it flows elsewhere.
The delta has built up as well as out, and that makes some other path to the Gulf steeper and shorter than the one now being taken, and during a flood the river tends to take that shorter path and cut a new channel. Feedback: The river very nearly broke through the Old River control structure in a big flood, to take the shortcut down the Atchafalaya. The long path out to the end of the delta is not very favorable for the river, which has switched naturally in the past and would switch if humans allowed it to.
Both of the above pictures are along the Colorado River. The clear water of picture 1 and the muddy water of picture 2 appear quite different. What's going on? 1 is upstream of the Glen Canyon Dam, and 2 is also upstream of the dam. 2 is upstream of the Glen Canyon Dam, and 1 is downstream of the dam. 2 is downstream of the Glen Canyon Dam, and 1 is also downstream of the dam. 1 is 7-UP, and 2 is Yoo-Hoo. 1 is upstream of the Glen Canyon Dam, and 2 is downstream.
2 is upstream of the Glen Canyon Dam, and 1 is downstream of the dam. Feedback: The naturally muddy river is seen clearly in Canyonlands in 2. The river dumps its sedimentary load in the reservoir above the dam, so downstream of the dam the water is clear, as shown in 1.
If you went swimming in the single channel of this river, and grabbed a sample of the river bank, what would you likely come up with? Boulders, that pile together to hold up river banks. Sand, that collapses to plug channels . Sand, that can make really steep slopes such as are seen in sand castles. Clay, that sticks together and can hold up steep slopes. A mixture of clay, sand and boulders, called till.
Clay, that sticks together and can hold up steep slopes. Feedback: This is a meandering channel, and these normally are fairly deep and narrow, so the materials of the banks should be able to stick together and support a steep slope. Sand can be steep when damp, but slumps to nearly flat when wet, and boulders or too much sand plug streams and make a braided pattern, whereas clay can make very steep slopes.
A University of Michigan student visiting Penn State's University Park campus drinks too much Diet Pepsi, wanders out in a pouring rainstorm, and takes a leak in a sinkhole behind the nearby Nittany Mall. The trout in the stream to which the sinkhole drains will notice the dastardly deed: In a few centuries. Never, because sinkholes don't drain to trout streams. In a few hours to days. In a few thousand years. Never, because all sinkholes drain to Michigan.
In a few hours to days. Feedback: Sinkholes often connect directly and quickly to underground caves or big cracks, and thus to streams, allowing rapid drainage. There are rock units that would hold their water for centuries or millennia, but such units have small spaces, not caves and sinkholes. Local sinkholes do drain to trout streams, and Michigan has to make their own water pollution because pollution from Pennsylvania does not reach them. (Fun thing to do if you're bored: fit this question into the Michigan fight song.)
Given the materials presented in this class about the formation of caves, it is likely that most large caves are formed: In sandstone in moist climates. In sandstone in dry climates. In limestone in moist climates. In limestone in dry climates. In granites under Diet Pepsi.
In limestone in moist climates. Feedback: Caves require easily dissolved rock, and water to dissolve that rock. In really dry climates, limestone is a resistant rock that stands in huge cliffs. In wet climates, limestone dissolves to yield caves. Sandstone is not a good cave-former because sandstone does not dissolve easily. (Yes, there are very shallow rock-shelter caves in sandstone, which is why the question specifically notes "large caves".) And while Diet Pepsi actually would be marvelous at dissolving limestone, Diet Pepsi attacks granite rather slowly and won't make caves well.
When humans build or raise levees along big rivers such as the lower part of the Mississippi, we are likely to cause: Land to emerge from the sea at the mouth of the river, because the levees stop compaction of sediment downstream. Fields and roads on the flood plain to rise above the surface of the river, because compaction of flood-plain mud will no longer be balanced by sediment accumulation during floods. The surface of the water in the river to become slanted to the side, as mud compaction beneath the river increases meandering. Fields and roads on the flood plain to drop below the surface of the river, because compaction of flood-plain mud will no longer be balanced by sediment accumulation during floods. Madonna and Don McLean to drive their Chevys to the levees, but the levees will be dissolving rapidly in Pepsi.
Fields and roads on the flood plain to drop below the surface of the river, because compaction of flood-plain mud will no longer be balanced by sediment accumulation during floods. Feedback: "Long, long, time ago, I can still remember how the river used to mend with mud..." The continuing compaction of the sediment along the lower Mississippi should be balanced by new deposits on top; stopping the new deposits does not stop the compaction, so the surface of the flood plain sinks. Even racing around curves, rivers don't tilt much, and mud compaction doesn't do much to meandering. "With a pink carnation and a pickup truck..."
The Mississippi River: Has built a delta, which is several miles thick at its thickest point, from near St. Louis, MO to the Gulf of Mexico over millions of years. Has split the southern part of the continent by eroding a small trench from near Baton Rouge, LA to the Gulf of Mexico over millions of years. Has built a delta, which is almost a mile thick at its thickest point, from near Baton Rouge, LA to the Gulf of Mexico over millions of years. Has split the continent by eroding a great trench from near St. Louis, MO to the Gulf of Mexico, causing earthquakes to occur at the tip of this trench near St. Louis, MO. Is colored brown because of people spitting up massive quantities of Yoo Hoo.
Has built a delta, which is several miles thick at its thickest point, from near St. Louis, MO to the Gulf of Mexico over millions of years. Feedback: Amazing as it may seem, the Mississippi has been taking the debris from the vast area from the Rockies to the Appalachians, and dumping that debris into the Gulf of Mexico, building a pile of sediment that is miles thick in places and extends from St. Louis to the Gulf. The mud has filled an old crack in the continent from when the Atlantic and Gulf of Mexico opened, but the mud doesn't stop the earthquakes that occasionally occur near the tip of the crack. And as for the Yoo Hoo, Yuck!
What happened in the picture above? Divers have built the mud piles to slow down the river water and protect endangered clams that live along the coast. Turbulence where the rivers enter the sea has caused suspended sediment in the seawater to flocculate and settle, forming the deposits seen on the right. Rivers have delivered sediment to the sea, forming flat-topped deposits called deltas. Rivers have delivered sediment to the sea, forming deltas that built up as they built out so that they still slope slightly downhill toward the sea. A certain financially ailing airline that flies to Cincinnati and then Atlanta built this from thrown-out beverage cups as an advertisement.
Rivers have delivered sediment to the sea, forming deltas that built up as they built out so that they still slope slightly downhill toward the sea. Feedback: Two deltas have formed where streams carry sediment from the hillside into the fjord in South Greenland. The main source of sediment is the streams from the land, not suspended sediment already in the sea water. And the deposits cannot be purely flat-topped, or the rivers would not flow across to get to the sea water in the fjord. Mollusks do live along the coast but are not being protected by intrusive changes to the coastline, nor do enough people visit here to be worth advertising Delta Airlines.
Years may pass with no major damage to the US mainland from hurricanes, but other years bring huge damages. A terrible event happened in 2005, when levees around New Orleans failed in the rising waters of Hurricane Katrina. More than 1400 people died, and the damages were in the neighborhood of $300 for each person in the US, or about $100 billion. As discussed in the text, history shows that: No one could have foreseen a huge disaster in New Orleans; acts of nature just happen sometimes. Mardi Gras causes hurricanes. The disaster happened only because humans had been pumping oil into the ground, raising the Mississippi Delta above sea level so that the waves from the storm could use the Delta as a ramp and jump easily into the city. Scientists and serious planners had warned about such an event for decades, based on the known size of hurricanes and the sinking of the Mississippi Delta and much of New Orleans. Planners always respond fully to scientific warnings, so you never need to worry about dangers from weather events again.
Scientists and serious planners had warned about such an event for decades, based on the known size of hurricanes and the sinking of the Mississippi Delta and much of New Orleans. Feedback: Scientists and planners did not know exactly when a big hurricane would hit New Orleans and threaten the city, but serious assessments had consistently highlighted the possibility for decades. The failures at New Orleans happened despite the fact that Katrina was NOT the "Big One"—on a scale of 1-5, Katrina was a 3 when it made landfall.
Geologically speaking, the water table: Separates the water-filled region below the Earth's surface from the region closer to the surface in which some air exists in the spaces. Rises during droughts, and sinks during rainy times. Sits next to the coffee table in the Capitol Building. Separates the water-filled region near the Earth's surface from the deeper region with some air in the spaces. Never changes its elevation, because it is pinned by the creeks.
Separates the water-filled region below the Earth's surface from the region closer to the surface in which some air exists in the spaces. Feedback: The water table is the surface below which all the spaces are full of water, but above which there is generally some air in the spaces. During droughts, water drains away from the ground to the creeks, so air enters spaces previously occupied by water, and the water table drops in elevation. Creeks do change in elevation between rain and drought (floods happen...). And while there might be bottled water in the Capitol, geologically speaking, that is not the right answer.
Suppose that all the rainfall that fell during an average year on a typical surface in central Pennsylvania just stayed there as a layer of water (and all the snow melted, and the melt just stayed there). If at the end of the year you were standing on that surface (assuming you are a typical-sized human being), what would be true? (Pennsylvania gets about the same amount of precipitation as the average for the world.) You would need your SCUBA gear, or a really long snorkel, to keep from drowning because the water would be way over your head. The water would be up around your waist or chest, but you'd still be able to breathe. The water would be over your ankles, but no deeper. The bottom of your toes would be wet, but not much else. The water would just cover your toes, but no deeper.
The water would be up around your waist or chest, but you'd still be able to breathe. Feedback: A typical rainfall supplies about an inch of water, or just under 0.1 foot. 30 feet of rain would be a big storm every day, about equal to the wettest place on Earth, and while sometimes it may seem the rain in Pennsylvania will never end, there really are clear days. 0.3 feet is a mere 3 or 4 rainfalls per year, and is a dry desert. 0.03 feet would be the driest place on Earth, and 0.003 doesn't occur on Earth. 3 feet is a nice number, which would be up to your chest, but you'd be able to breathe.
In the picture above, Dr. Alley is on the South Rim of the Grand Canyon. What problem with the Canyon is he discussing? A dire shortage of Pepsi has developed at the Canyon, forcing people to actually drink water, and the people have found they like water. Arizona has raised the tax on well-drilling, and the Park Service is having trouble paying for the water used at the Canyon. Water is being pumped out of the ground on the plateau south of the Canyon, and used by humans and evaporated or dumped in streams, so the water does not flow to the springs in the Canyon. Human wastes are being dumped on and pumped into the ground, polluting the springs in the Canyon. A dire shortage of Pepsi has developed at the Canyon, forcing people to actually drink water.
Water is being pumped out of the ground on the plateau south of the Canyon, and used by humans and evaporated or dumped in streams, so the water does not flow to the springs in the Canyon. Feedback: Water soaks into the ground on the plateaus beside the canyon, seeps down to hit a rock layer that blocks the flow, and flows along that layer to feed beautiful and biologically important springs in the Canyon. Pumping water out of the ground on the plateaus to use for humans generally allows the water to evaporate (say, from the grass of a golf course) or run down a stream (say, below a sewage treatment plant), so the water doesn't flow through the ground to the springs.
Large rivers sometimes have natural levees because: The mud deposited by the river compacts and sinks. Point bars run together to make levees. There are no natural levees. Water slows and deposits sediment as the water leaves the main river channel during floods. Oxbow lakes run together to make levees.
Water slows and deposits sediment as the water leaves the main river channel during floods. Feedback: The initial slowdown as water spreads into the trees deposits sediment to form natural levees. Sinking mud doesn't make ridges. Point bars are the sand bars on the insides of meander curves, and oxbow lakes are abandoned meander curves, but neither makes the natural levees that are observed.
A dam is built on a river that has a river bed that is primarily sand. You have a house just downstream of the dam, and you like to go trout fishing in the river in front of your house. A few years after the dam is built, it is likely that: You will have moved, because sand deposits will have buried your house as the clean water from the dam washed sand out of the river bottom and onto the river banks. You will have quit trout fishing as the huge rocks released from the reservoir behind the dam during floods built up the bed of the river. You will have quit trout fishing as the Diet Pepsi released from the dam dissolved the sand so that your house fell down the cliff forming in front of your house. You will have built a ladder or steep path to get down to the river, because the clean water released by the dam will have washed a lot of the sand away and lowered the elevation of the river in front of your house. You will have quit trout fishing as the Diet Pepsi released from the dam dissolved your house during floods.
You will have built a ladder or steep path to get down to the river, because the clean water released by the dam will have washed a lot of the sand away and lowered the elevation of the river in front of your house. Feedback: The stream will slow where it enters the new lake, and so will deposit sediment to form a delta rather than cutting downward or having no change. As the delta builds out into the lake, the upstream end of the delta must build up so that the stream still slopes downward, and this will tend to bury fields upstream. Meanwhile, moving water can carry sediment. Sediment-free water is released from a dam but often later observed to have sediment, so erosion must be occurring. Loss of sand bars below the Glen Canyon Dam shows that sand is carried away downstream of dams. Dams stops floods that are needed to move the big pieces (boulders, cobbles), and dams cause sedimentation upstream, while erosion occurs downstream.