Test 2 Review: Geosci 180, Geology of the National Parks  The Inner Earth: Volcanoes and Such in the National Parks

Volcanic Parks

We have covered the Following Parks-highlighted parks were discussed in class:

The Structure of the Earth: Chemical layers (distinguished by chemical composition): (Listed from the inside out) Iron core, Fe-Mg Silicate mantle, and Lighter Silicate crust.  At any one point on the surface of the Earth, The Crust is either oceanic (basaltic composition) or continental (granitic composition). Physical layers (distinguished by physical properties); --do not coincide with chemical layers: (also listed from the Inside out) solid inner core, liquid outer core, Solid mesosphere (= most of the mantle, lower part), plastic asthenosphere (within the upper mantle), and rigid lithosphere (= uppermost mantle and all of the crust)

Plate Techtonics:  Lithospheric Plates: The lithosphere is divided into plates. Each plate is a piece of lithosphere which can include both continental and oceanic crust. Lithospheric plates move relative to each other, and seem to be driven in their motion by convection in the asthenosphere. Plate Boundaries: The nature of the boundary between two plates is affected profoundly by the relative motion between those two plates (or is it the other way around?). Divergent boundaries are those where two plates are moving away from each other and are characterised by basaltic magmatism. The basaltic magma forms as the mantle material moves upward in the upwelling part of the asthenosphere convection cell. The upward motion reduces pressure on the material and the already "slushy" material begins to melt, but only the more silica rich parts, rendering a basaltic magma which is more silica rich than the parent material. New oceanic crust is created when this basaltic magma cools and solidifies. Convergent boundaries are those where the plates are moving together. Old Oceanic crust is destroyed or recycled at convergent plate boundaries through the process of subduction, where one plate is subducted under another one and is returned to the asthenosphere. As the plate descends, he increasing pressure (because it contains water) and heat cause it to melt--again, only partially and the resulting magma is andesitic. If the over riding plate is continental at the boundary, then subduction will form a mountain range dominated by volcanoes (Cascades, and the Andes). If the over riding plate is oceanic, then an island arc (Aleutian Ranges) will form. A deep oceanic trench will form where the underlying plated is subducted, and this trench will be on the same side of the island arc as the plate that is being subducted. Continental material does not subduct under other materials because it is so light that it cannot sink under the much heavier oceanic crust or the mantle.
 
 

Mantle Plumes or Hot Spots: Mantle plumes are small areas of upwelling mantle material. They orginate from the core-mantle boundary where heat builds up, they rise up to the lithosphere, and stop.  The plumes cause isolated volcanic activity--one or a few volcanoes--in a sense punching hole in the overlying lithosphere. As a lithospheric plate moves over the (apparently) stationary mantle plume, the volcanoes die, and the plumes forms a new center of volcanic activity.  Thus mantle plumes leave trails of volcanoes in the overlying plates, and by calculating the the age of these volcanoes we can tell both speed and direction of plate movement with respect to the mantle plumes.
Mantle plume volcanism in oceanic crust is exemplified by Hawaii. The plume results in basaltic magma by partial melting of plume material.  Thus, plume activity under oceanic crust results in basaltic volcanism.  This basalt differs slightly from the composition of the oceanic crust, which is derived from partial melting of the asthenosphere. The plume causes some uplift of the overlying crust, thus, as the lithospheric plate moves over the plume, volcanic islands form over it. As the island is conveyed away from the plume, the surface subsides causing the island to sink.  Many of these islands may be flat-topped because of wave erosion.
Plume activity in continental crust is exemplified by Yellowstone and the associated snake river plain. The plume produces basalt at the base of the lithosphere, as in oceanic crust. This basalt pools under the continental material and begins to melt it. The resulting magma is granitic in composition, and it rises to form magma chambers called stocks when they solidify. These magma chambers release rhyolite lava to the surface, frequently with extremely large, violent eruptions. These eruptions may empty enough of the chambers to result in collapse of  the land over the magma chamber, resulting in the formation of large calderas. Later, the basalt that had pooled may rise through a series of fissures (leaving a series of dikes) to the surface where flows may cover large areas, including the calderas, as has happened at craters of the moon. The snake river plain is the result of

 

Igneous Rocks: Pyroclastics are volcanic rocks formed as magma is fragmented during an eruption.  The fragments of magma solidify in the air and fall as ash (dust sized particles), Larger fragments can also form including lappilli, the size of a pea, and volcanic bombs, which range upward from fist-sized. Other Features: Columnar Jointing forms as lava flows (and some intrusions--Devil's Tower) cool and the material contracts (Devil's Postpile).  Exfoliation occurs when large intrusive rock bodies are exhumed by erosion and the pressure release causes the outer surface to expand and break from the parent rock, which causes the rock to weather in layers (Yosemite).

Intrusive Igneous Rock Bodies:  Dikes are vertical walls of rock formed when magma was injected into vertical fractures, Sills are horizontal layers of igneous rock that formed as magma was injected between sedimentary rock beds, Laccoliths are sills that have "blistered", thus they are flat on the bottom, and roundef on the upper surface.  Volcanic necks ore vertical cylindrical igneous rock bodies which formed as magma plugged the conduit to a magma chamger.  Massive intrusive rock bodies are blob-shaped and include stocks (about the size of a mountain e.g. little cottonwood canyon--the very rock that has been the subject of recent debate) and batholiths (Yosemite is in the Sierra Nevada Range, which is a batholith). Stocks are probably solidified magma chambers, while batholiths appear to be composed of several overlapping stocks.

Volcanoes:  Sheild Volcanoes (Hawaii), Cindercones (also Hawaii), and Composite Volcanoes (the Aleutian Ranges and the Cascades).

Properties of Volcanic Eruptions:

Lava Flows: Pahoehoe flows are smooth and ropy in texture and form from fluid lava. A'a (aa in mainland English) flows are rough and jagged and form as very viscous lava moves.  Fluidity or viscosity of the lava is affected by its composition and temperature. More silica rich lavas are more viscous, as are cooler lavas.

Explosive Eruptions: The explosive power of an eruption is determined by the amount and pressure of gas trapped in the magma as it rises to the surface; it is powered by the same force that blows up soda pop after the container is given a good shaking and then opened. Silica-rich (particularly andesitic) magmas can hold more gas under pressure than basaltic magmas and tend to cause explosive eruptions. This is why the andesitic eruptions of the Cascades and the Aleutians are so much more violent than the eruptions of Hawaii., and why the rhyolitic eruptions of Yellowstone were even larger and more violent. higher-silica volcanic eruptions are characterized by ash clouds which cause ash falls . The ash is formed as the magma explodes into the air forming tiny pieces.  Larger fragments can also form including lappilli, the size of a pea, and volcanic bombs, which range upward from fist-sized.  Hot ash flowing down the side of the volcano is called an ash flow. Heating of the snows at the tops of volcanic mountains can result in massive debris flows called lahars.

Caves

We have covered the Following Parks-highlighted parks were discussed extensively in class:

Aquifers, aquicludes, aquitards: These are classifications of rock or sediment bodies based on their functional role with respect to ground water. An aquifer bears water. Having relatively high porosities and permeabilities, aquifers store and transmit large amounts of water in a manner similar to a soaked sponge. Aquicludes are impermeable units that stop water, and serve to confine it. Aquitards are very low permeability units that impede the flow of groundwater but do not stop it completely. Note that groundwater does not travel perticularly fast in a typical aquifer, and may take many years to travel a mile.
 

Unconfined or water table aquifers: An unconfined or water table aquifer is an aquifer on the land surface.  Such an aquifer consists of a zone of saturation overlain by a zone of aeration. The boundary between these zones is the water table, and just above the water table is the capillary fringe. Water completely filled the pore space in the zone of saturation, while it only clings to the rock surface or sediment particles in the zone of aeration. Pore space in the capillary fringe is also completely full of water, but the water is held there by surface tension, having been wicked up from the zone of saturation.  If a hole is dug into the zone of saturation, water will fill the hole up to the water table. Recharge occurs when rain water trickles through the zone of aeration to the zone of saturation.  The water table is not a flat surface, but is a subdued mimic of the land surface. The surfaces of perenial streams and lakes are coincident with the water table surface, and such streams are generally fed by groundwater (discharge) through a mechanism called base flow.  The water table builds up higher away from the stream because it is replenished (recharged) by rain. Like surface water, ground water flows from high water table to low water table areas.  A perched water table forms when an aquiclude or aquitard occurs within the zone of aeration and percolating water accumulates above it. where such a unit intersects with the land surface, a spring or seep may form
Confined or artesian aquifers: A confined or artesian aquifer is one that is separated from the surface by an aquiclude or aquitard called a confining layer. The confining layer allows the water to build up pressure below the recharge area.  Because of this pressure, water will rise to a level called the pressure surface when the aquifer is punctured by a well. If the top of the well is above the pressure surface, the water will not flow out, but will rise in the well up to the pressure surface. If the top of the well is below the pressure surface, then the water will flow out, and well is said to be free flowing. Both would be termed artesian wells.
Solution or Cavern systems: Cavern systems differ from aquifer systems in that groundwater flows like a surface stream through open conduits. These conduits are termed caverns if they are large enough for a person to enter and long enough to be totally dark inside. Relative to aquifers, water flows through caverns very rapidly. Water enters a cavern through surface streams which are "swallowed" by sink holes, which are openings into the cave systems formed from the collapse of the cave roof or from the direct action of dissolution. The terrain above a cave system may be dominated by these sinkholes, and surface streams and  stream valleys may be rare or absent; this type of terrain is termed a karst terrain. Cave discharge is in the form of streams and rivers which emerge from the cave system. A generalized Mammoth Cave system is illustrated

 
Cave genesis: Karst and caves form from the dissolution of certain rock types, mostly limestone, gypsum, and salt. Limestone caverns are the most common. In humid climates the most important dissolving agent is carbonic acid, which forms as rainwater picks up carbon dioxide (CO2) from the air and from rotting vegetation.  These caves form as water trickles down from the surface down and tend to be shallow (Mammoth Cave). In drier climates sulfuric acid may be more important. The sulfuric acid forms as ground water picks up hydrogen sulfide (H2S also "rotten egg gas") which was ultimately derived from organic matter in the sediments. Many of these caves may form as water rises from below, and they can be very deep (Carlsbad Caverns). In general, the dissolution acts to enlarge joints  Mammoth Cave demonstrates several generations of cave formation as the green river downcut and the upper chambers were abandoned.

Speleothems: Speleothems, also known as "cave formations" are mineral deposits, generally calcite, (also called travertine or onyx), but also gypsum, salt and other other minerals. There are several varieties of speleothems, the most common being stalactites (thin sharp cones hanging from the ceiling), stalagmites (blunt cones on the floor) columns (cylindrical form joining ceiling and floor), and soda straws (essentially small hollow stalactites). Calcite speleothems form when calcium carbonate is precipitate after carbon dioxide is released from water in the cave. This can be induced by heating the water or simply exposing it to air.

Groundwater as Resource, Pollution: Most of the nation's water supply comes from the ground, and a vast majority of the liquid fresh water in the world is groundwater. In the past the cleanliness of groundwater was not considered and materials were disposed of in such a way as to contaminate this resource. Sinkholes have historically been used as dumping sites, and wastes have been buried underground. both practices have lead to groundwater contamination that has proven very difficult to remediate.
 
 

Review questions:

1.     Explain how basalt forms at divergent margins.

2.     Where does andesite come from and how does it form?

3.     What is the difference between aa and pahoehoe lava flows?

4.     Why is the upper part of the mantle included in the lithosphere?

5.     Why do continents not subduct?

6.     Explain what a lithospheric plate is.

7.     What is the relationship between the different islands in the hawaiian island chain?

8.     How is Yellowstone related to the snake river plain?

9.     Describe the plate tectonic setting of the Aleutian Range.

10.    Describe the plate tectonic setting of the Cascades.

11.    What is the origin of rhyolite at yellowstone?

12.    Compare the types of volcanic eruptions that occur(ed) at 1) ancient Yellowstone, 2) the cascades, 3) the Aleutians, 4) Hawaii.  Which are (were) the largest? most violent? Why?

13.    Why is Hawaiian basalt chemically different from the basalt on the ocean floor?

14.    Discuss the relationship between stocks and batholiths.

15.    Would you expect that there is a batholith under the Cascades? Would it have been formed in the same way that the batholith under the snake river plain was formed? What about the Sierra Nevada Batholith?

16.    Explain the mechanism of geyser eruption.

17.    Where does the travertine come from that forms the travertine terrace at mammoth hot springs?

18.    What is porosity?

19.    What is permeability?

20.    Why does an artesian aquifer have water under pressure?

21.    What is the difference between an aquifer and an aquiclude?

22.    What is an aquitard?

23.    How does a perched water table form?

24.    What is the origin of carbonic acid in the ground?

25.    Discuss how joints are related to the formation of caves.

26.    Consider the difference between cave and aquifer systems. Which would take longer to contaminate? Which would be harder to clean up?

27.    Why are carlsbad caverns so deep?

28.    What is the origin of the sulfuric acid which formed Carlsbad Caverns?

29.    Why is it not good to put dead cows in sinkholes?

30.    What is karst?

31.    Why are there so few surface streams above Mammoth Cave?

32.    Which levels of Mammoth Cave are the oldest? Why?

33.    What is a sinkhole anyway? How do they form?

34.    What is the relationship between speleothem formation and the carbon dioxide content of water?

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Last Updated April 28, 1998. This page maintained by the instructor. Comments or corrections may be addressed to: Ben Dattilo