Es 17: Karst And Hydrothermal Processes

Front 1

How do dissolution and precipitation processes affect bedrock such as limestone?

Back 1

??

Front 2

How do limestone caverns develop?

Back 2

??

Front 3

How do subsurface processes influence surface landforms in areas of karst topography?

Back 3

??

Front 4

What conditions are necessary for hydrothermal features to develop?

Back 4

??

Front 5

The Impact of Solution Processes on the Landscape?

Back 5

Mechanical effects of underground water have limited topographic influence

Some subsurface mechanical weathering does take place, but the surface landscape is rarely directly affected by it, although certain forms of mass wasting (such as earthflows and slumps) are facilitated when loose materials are lubricated by underground water.

Through its chemical action, however, underground water is an effective shaper of the topographic landscape. Water’s solvent properties allow it to dissolve certain chemicals from rock

Underground water also affects surface topography via the creation of such hydrothermal features as hot springs and geysers, formed when hot water from underground is discharged at the ground surface.

Front 6

What is Underground Water?

Back 6

underground water refers to any water beneath the surface

Front 7

What is Groundwater?

Back 7

groundwater specifically refers to water below the water table within the zone of saturation

Front 8

Waters Impact on the Topographic Landscape.

Back 8

Through its chemical action, however, underground water is an effective shaper of the topographic landscape. Water is a solvent for certain rock-forming minerals, dissolving them from rock and then carrying them away in solution and depositing them elsewhere. Under some circumstances, the aboveground results of this dissolution are widespread and distinctive.

Underground water also affects surface topography via the creation of such hydrothermal features as hot springs and geysers, formed when hot water from underground is discharged at the ground surface.

Front 9

Solution and Precipitation.

Back 9

Although pure water is a relatively poor solvent, almost all underground water is laced with enough chemical impurities to make it a good solvent for the compounds that make up a few common minerals

Front 10

??

Back 10

Basically, underground water is a weak solution of carbonic acid (H2CO3) because it contains dissolved carbon dioxide gas - the resulting carbonation can lead to the dissolution of bedrock.

Front 11

??

Back 11

Relative solubility in water of some common rock-forming elements. The “lumps” in each beaker represent the proportion of the element that remains undissolved in a given amount of water. Sodium and calcium can be almost completely dissolved, for instance, whereas iron and aluminum are essentially insoluble.

Front 12

Dissolution Processes.

Back 12

Removal of bedrock through chemical action of water; includes removal of subsurface rock through action of groundwater.
– Most effective on carbonate sedimentary rocks (i.e., limestone)
– Calcium carbonate reaction
• CaCO3 + H2O + CO2 = Ca(HCO3)2
– Dolomite reaction
• CaMg(CO3)2 + 2H2O + 2CO2 = Ca(HCO3)2 + Mg(HCO3)2
– These are the most notable dissolution processes
– Occur more rapidly in humid regions
– Role of sulfuric acid

Front 13

What is Dissolution?

Back 13

the removal of bedrock through the chemical action of water.

Dissolution is an important weathering and erosional process for all rocks, but it is most effective on carbonate sedimentary rocks, especially limestone.

Front 14

What is Calcium Carbonate and its Reaction?

Back 14

Calcium carbonate reaction
• CaCO3 + H2O + CO2 = Ca(HCO3)2

A common sedimentary rock, limestone is composed largely of calcium carbonate (CaCO3), which reacts strongly with carbonic acid solution to yield calcium bicarbonate, a compound that is very sol- uble in (and thus easily removed by) water.

Front 15

What is Dolomite and its Reaction?

Back 15

– Dolomite reaction
• CaMg(CO3)2 + 2H2O + 2CO2 = Ca(HCO3)2 + Mg(HCO3)2

Dolomite is a calcium magnesium carbonate rock that dissolves almost as quickly as limestone.

Front 16

What are the two (2) most notable dissolution processes?

Back 16

1. Calcium Carbonate Reaction
2. Dolomite Reaction

Front 17

??

Back 17

Water percolating down into carbonate bedrock dissolves and carries away a part of the rock mass. Because limestone and related rocks are largely composed of soluble minerals, great volumes of rock are sometimes dissolved and removed, leaving conspicuous voids in the bedrock. This action occurs more rapidly and on a larger scale in humid climates, where abundant precipitation provides plenty of water containing dissolved carbon dioxide necessary for dissolution.

Front 18

??

Back 18

In arid regions, evidence of dissolution action is unusual except for relict features dating from a more humid past.

Front 19

What is the Role of Sulfuric Acid in Dissolution Processes?

Back 19

Although reactions with carbonic acid are the most common processes involved in the dissolution of carbonate rocks, recent studies suggest that in some locations sulfuric acid (H2SO4) may also be important. For example, it appears that Lechuguilla Cave in New Mexico was at least in part enlarged through dissolution of limestone by sulfuric acid, formed when hydrogen sulfide (H2S) from deeper petroleum deposits combined with oxygen in the groundwater.

Front 20

LC 17-1 Why is Limestone so susceptible to dissolution?

Back 20

??

Front 21

What is the Role of Bedrock Structure in Dissolution Processes?

Back 21

Bedrock structure is also a factor in dissolution.

A profusion of joints and bedding planes permits groundwater to penetrate the rock readily. (Profusion of joints allows for groundwater penetration)

That the water is moving also helps because, as a given volume of water becomes saturated with dissolved calcium bicarbonate, it can drain away and be replaced by fresh unsaturated water that can dissolve more rock.
Such drainage is enhanced by some outlet at a lower level, such as a deep subsurface stream.
Most limestone is resistant to mechanical erosion and often produces rugged topography. Thus, its ready solubility contrasts notably with its mechanical durability—a vulnerable interior beneath a durable surface.

Front 22

Precipitation Processes.

Back 22

Complementing the removal of calcium carbonate is its precipitation from solution.

Mineralized water may trickle in along a cave roof or wall.

The reduced air pressure in the open cave induces precipitation of the minerals the water is carrying.

Front 23

Precipitation Processes.

Back 23

– Mineralized water trickles along cavern roof or wall
– High mineral content in hot springs

Front 24

Precipitation Processes.

Back 24

One other type of precipitation is worth mentioning despite its scarcity because of its dramatic distinctiveness. Hot springs and geysers nearly always provide an accumulation of precipitated minerals, frequently brilliant white but sometimes orange, green, or some other color due to associated algae.

Hot water is generally a much better solvent than cold, and so a hot spring or geyser usually contains a significant quantity of dissolved minerals. When exposed to the open air, the hot water precipitates much of its mineral content as its temperature and the pressure on it decrease, and as the dissolved gases that helped keep the minerals in solution dissipate or as algae and other organisms living in it secrete mineral matter.

Front 25

??

Back 25

It should be noted, however, that the solubility of carbon dioxide decreases as water temperature increases. Thus, cool water often is more potent than hot water as a solvent for calcium carbonate.

Front 26

LC 17-2: How do rocks such as travertine and tufa form?

Back 26

??

Front 27

What is a Cavern?

Back 27

Solution along joints and bedding planes in limestone beneath the surface often creates large open areas called caverns. (Large openings beneath the Earth’s surface that result from solution processes)

Caverns are found almost anywhere there is a massive limestone deposit at or near the surface.

The largest of these openings are usually more expansive horizontally than vertically, indicating a development along bedding planes. In many cases, however, the cavern pattern has a rectangularity that demonstrates a relationship to the joint system.

A stream may flow along the floor of a large cavern, adding another dimension to erosion and deposition.

Front 28

How are Caverns Formed?

Back 28

Caverns are formed by solution action of underground water as it trickles along bedding planes and joint systems.

Front 29

What are the two (2) principal stages in Cavern Formation/Development?

Back 29

1. Initial Excavation
2. Decoration Stage (speleothems)

Front 30

What is the Initial Excavation Stage in Cavern Formation/Development?

Back 30

First there is the initial excavation, wherein percolating water dissolves the carbonate bedrock and leaves voids.

Front 31

What is the Decoration Stage in Cavern Formation/Development?

Back 31

This dissolution is followed, often after a drop in the water table, by a “decoration stage” in which ceilings, walls, and floors are decorated with a wondrous variety of speleothems.

Front 32

What are Speleothems?

Back 32

A feature formed by precipitated deposits of minerals on the wall, floor, or roof of a cave

Front 33

Discuss Cavern Formation/Development in relation to the two principal stages: Initial Excavation and Decoration Stage.

Back 33

First there is the initial excavation, wherein percolating water dissolves the carbonate bedrock and leaves voids. This dissolution is followed, often after a drop in the water table, by a “decoration stage” in which ceilings, walls, and floors are decorated with a wondrous variety of speleothems. These forms are deposited when water leaves behind the compounds (principally carbon dioxide and calcite) it was carrying in solution. Once out of solution, the carbon dioxide gas diffuses into the cave atmosphere, and calcite is deposited. Much of the deposition occurs on the sides of the cavern, but the most striking features are formed on the roof and floor.

Front 34

Stalactites vs. Stalagmites?

Back 34

Where water drips from the roof, a pendant structure grows slowly downward like an icicle—a stalactite. Where the drip hits the floor, a companion feature, a stalagmite, grows upward. Stalactites and stalagmites may extend until they meet to form a column. In some caverns, long, slender soda straws hang down from the ceiling; little more than one water drop wide, these delicate hollow tubes may eventually grow into stalactites.

Front 35

Stalactites vs. Stalagmites: Stalactites?

Back 35

Where water drips from the roof, a pendant structure grows slowly downward like an icicle—a stalactite.

A pendant structure hanging downward from a cavern’s roof.

Front 36

Stalactites vs. Stalagmites: Stalagmites?

Back 36

Where the drip hits the floor, a companion feature, a stalagmite, grows upward.

A projecting structure growing upward from a cavern’s floor.

Front 37

LC 17-3: Explain the two (2) stages of cavern development.

Back 37

??

Front 38

Karst Topography.

Back 38

In many areas where the bedrock is limestone or similarly soluble rock, dissolution has been so widespread and effective that a distinctive landform assemblage has developed at the surface, in addition to whatever caves may exist underground.

Front 39

What is Karst Topography?

Back 39

Topography developed as a consequence of subsurface solution. (Topography that results from underground dissolution)

Front 40

What does the term "karst" connote?

Back 40

The term karst connotes both a set of processes and an assemblage of landforms. The word is used as the catchall name of a cornerstone concept that describes the special landforms that develop on exceptionally soluble rocks, although there is a broad international vocabulary to refer to specific features in specific regions.

Front 41

Karst Landforms

Back 41

Karst landscapes usually evolve where there is massive limestone bedrock.
However, karst features may also occur where other highly soluble rocks—dolomite, gypsum, or halite—predominate.

Front 42

What are some of the Typical Landforms in Karst Regions?

Back 42

Sinkholes
Disrupted Surface Drainage
Underground Drainage Networks that have openings formed from solution actions - the openings range in size from enlarged cracks to huge caverns.

Front 43

10% of the Earth’s land surface is soluble rock

Back 43

Karst landforms are worthy of study not only because of their dramatic appearance but also because of their abundance:
10% of the Earth’s land surface is soluble rock

Front 44

What are Sinkholes?

Back 44

Sinkholes are rounded depressions formed by the dissolution of surface carbonate rocks, typically (but by no means always) at joint intersections.

The sinkholes erode more rapidly than the surrounding area, forming closed depressions.

A sinkhole that results from the collapse of the roof of a subsurface cavern is called a collapse sinkhole (or collapse doline); these may have vertical walls or even overhanging cliffs.

Sinkholes range in size from shallow to deep.

Indeed, sinkholes are the karst equivalent of river valleys, in that they are the fundamental unit of both erosion and weathering.

Front 45

The most common surface features of karst landscapes are sinkholes (also called dolines),

Back 45

– Chains of sinkholes: uvala
– Tower karst
– Disappearing streams and swallow holes

Front 46

What are Uvalas?

Back 46

Where sinkholes occur in profusion, they often channel surface runoff into the groundwater circulation, leaving networks of dry valleys as relict surface forms. The Serbo-Croatian term uvala refers to such a chain of intersecting sinkholes. In many cases, sinkholes evolve into uvala over time.

Front 47

What are Disappearing Streams?

Back 47

In many ways, the most notable feature of karst regions is what is missing: surface drainage. Most rainfall and snowmelt seep downward along joints and bedding planes, enlarging them by dissolution. Surface runoff that does become channeled does not usually go far before it disappears into a sinkhole or joint crack—such streams are often termed disappearing streams.

Stream that abruptly disappears from the surface where it flows into an underground cavity; common in karst regions.

Front 48

What are Swallow Holes?

Back 48

The water that collects in sinkholes generally percolates downward, but some sinkholes have distinct openings at their bottom, called swallow holes, through which surface drainage can pour directly into an underground channel, often to reappear at the surface through another hole some distance away. Where dissolution has been effective for a long time, there may be a complex underground drainage system that has superseded any sort of surface drainage network. An appropriate generalization concerning surface drainage in karst regions is that valleys are relatively scarce and mostly dry.

The distinct opening at the bottom of some sinkholes through which surface drainage can pour directly into an underground channel.

Front 49

What is a Tower Karst?

Back 49

Tall, steep-sided hills in an area of karst topography.

Residual karst features, in the form of very steep-sided hills, dominate some parts of the world. These formations are sometimes referred to as tower karst because of their almost vertical sides and conical or hemispheric shapes. Such towers are sometimes riddled with caves.

Front 50

Explain the Development of Karst Topography.

Back 50

(a) Landscape dominated by sinkholes and disappearing streams. Dissolution of bedrock below the water table may leave openings that develop into caverns.
(b) Where underground caverns collapse, a collapse sinkhole forms. Streams may disappear into sinkholes through swallow holes. If the water table drops enough to leave open caverns, speleothems may gradually develop.
(c) Tower karst topography consists of residual towers of limestone (haystack hills or mogotes).

Front 51

LC 17-4: What is a sinkhole and how does one form?

Back 51

??

Front 52

Groundwater Extraction and Sinkhole Formation:

Back 52

??

Front 53

??

Back 53

??

Front 54

What is Hydrothermal Activity?

Back 54

In many parts of the world, there are small areas where hot water comes to the surface through natural openings. Such outpouring of hot water, often accompanied by steam, is known as hydrothermal activity and usually takes the form of either a hot spring or a geyser.

Front 55

What are Hot Springs?

Back 55

Hot water at Earth’s surface that has been forced upward through fissures or cracks by the pressures that develop when underground water has come in contact with heated rocks or magma beneath the surface.

Front 56

Hot Springs?

Back 56

The appearance of hot water at Earth’s surface usually indicates that the underground water has come in contact with heated rocks or magma and has been forced upward through a fissure by the pressures that develop when water is heated. The usual result at the surface is a hot spring, with water bubbling out either continuously or intermittently. The hot water invariably contains a large amount of dissolved mineral matter, and a considerable proportion of this load is precipitated out as soon as the water reaches the surface and its temperature and the pressure on it both decrease.

Front 57

What are some Topographies that Result from Hot Springs?

Back 57

The deposits around and downslope from hot springs can take many forms. If the opening is on sloping land, terraces are usually formed. Where the springs emerge onto flat land, there may be cones, domes, or irregular concentric deposits.

Sometimes the water bubbling out of a hot spring builds a continually enlarging mound or terrace. As the structure is built higher, the opening through which the hot water comes to the surface also rises, so that the water is always emerging above the highest point.

Front 58

Algae Growth as a Result of Hot Springs?

Back 58

As the water flows down the sides of the structure, more deposition takes place there, thus broadening the structure as well, often with brilliantly colored algae, which add to the striking appearance as well as contribute mineral secretions to the deposit

Front 59

What are Geysers?

Back 59

A specialized form of intermittent hot spring with water issuing only sporadically as a temporary ejection (eruption) , in which hot water and steam are spouted upward for some distance. Then the geyser subsides into apparent inactivity until the next eruption.

Front 60

What is the Basic Principle of Geyser Activity?

Back 60

The basic principle of geyser activity involves the building up of pressure within a restricted subterranean tube until that pressure is relieved by an eruption.

Front 61

Eruption Mechanisms of a Geyser?

Back 61

The process begins when underground water seeps into subterranean openings that are connected in a series of narrow caverns and shafts. Heated rocks and/ or magma are close enough to these storage reservoirs to provide a constant source of heat. As the water accumulates in the reservoirs, it is heated to 200°C (400°F) or higher, which is much above the boiling point at sea level and normal pressure. (Such superheating is possible because of the high underground water pressure.) At these high temperatures, much of the water becomes steam. The accumulation of steam deep in the tube along with boiling water higher up eventually causes a great upward surge that sends water and steam showering out of the geyser vent. This eruption releases the pressure, and when the eruption subsides, underground water again begins to collect in the reservoirs in preparation for a repetition of the process.

Front 62

??

Back 62

A tremendous supply of heat is essential for geyser activity.

Front 63

LC 17-5: Describe the conditions that typically lead up to a geyser eruption.

Back 63

??

Front 64

Eruption Patterns of Geysers?

Back 64

Some geysers erupt continuously, indicating that they are really hot springs that have a constant supply of water through which steam is escaping. Most geysers are only sporadically active, however, apparently depending on the accumulation of sufficient water to force an eruption.

Front 65

Time Frame Surrounding Geyser Activity?

Back 65

Some eruptions are very brief, whereas others continue for many minutes. The interval between eruptions for most geysers is variable. Most erupt at intervals of a few hours or a few days, but some wait years or even decades between eruptions.

Front 66

What is the Temperature of the Erupting Water for Geysers?

Back 66

The temperature of the erupting water generally is near the boiling point for pure water.

Front 67

Mineral Deposits from Geyser Activity?

Back 67

Some geysers erupt from open pools of hot water, throwing tremendous sheets of water and steam into the air but usually producing relatively minor depositional features. Other geysers are of the “nozzle” type and consequently build up a depositional cone and erupt through a small opening in it. Most deposits resulting from geyser activity are simply sheets of precipitated mineral matter spread irregularly over the ground.

Front 68

Mineral Deposits from Hot Spring Activity vs. Mineral Deposits from Geyser Activity.

Back 68

The deposits resulting from geyser activity are usually much less notable than those associated with hot springs.

Front 69

What is a Fumarole?

Back 69

A hydrothermal feature consisting of a surface crack that is directly connected with a deep-seated source of heat.

For some reason, very little water drains into the tube of a fumarole. (Little water drainage). The water that does drain in is instantly converted to steam by the heat, and a cloud of steam is then expelled from the opening —often with an accompanying roaring or hissing sound.

Thus, a fumarole is marked by steam issuing either continuously or sporadically from a surface vent.

A fumarole is simply a hot spring that lacks liquid water.
A fumarole is like a geyser except that it erupts no liquid water; it sends out only steam.

Front 70

Hydrothermal Features in Yellowstone?

Back 70

By far the largest concentration of Hydrothermal Features, occurs in Yellowstone National Park, located mostly in northwestern Wyoming, which contains about 225/425 of the world’s geysers as well as more than half of the world’s other hydrothermal phenomena.

Front 71

Geologic Setting of Yellowstone?

Back 71

The Yellowstone area consists of a broad, flattish plateau bordered by extensive mountains on the east and by more limited highlands on the west.

The bedrock surface of the plateau is almost entirely volcanic materials, although no volcanic cones are in evidence.

Front 72

Hydrothermal Conditions of Yellowstone?

Back 72

The uniqueness of Yellowstone’s geologic setting stems from the presence of a large, shallow magma chamber beneath the plateau (only ~8 km below the surface)—thought to be the result of a hot spot formed by a mantle plume (heat source) rising up from the mantle.

Front 73

What are the Three (3) Necessary Conditions for Hydrothermal Development?

Back 73

1. This shallow magma pool provides the heat source—the most important of the three conditions necessary for the development of hydrothermal features.

2. The second requisite is an abundance of water that can seep downward and become heated.

3. The third necessity for hydrothermal development is a weak or broken ground surface that allows water to move up and down easily.

Front 74

What is the First Requisite (Necessary Condition) for Hydrothermal Development?

Back 74

This shallow magma pool provides the heat source—the most important of the three conditions necessary for the development of hydrothermal features.

Front 75

What is the Second Requisite (Necessary Condition) for Hydrothermal Development?

Back 75

The second requisite is an abundance of water that can seep downward and become heated. Yellowstone receives copious summer rain and a deep winter snowpack.

Front 76

What is the Third Requisite (Necessary Condition) for Hydrothermal Development?

Back 76

The third necessity for hydrothermal development is a weak or broken ground surface that allows water to move up and down easily. Here, too, Yellowstone fits the bill, for the ground surface there is very unstable and subject to frequent earthquakes, faulting, and volcanic activity. Consequently, many fractures and weak zones provide easy avenues for vertical water movement.

Front 77

LC 17-6: Identify three conditions that make Yellowstone idea for the development of hydrothermal features.

Back 77

??

Front 78

Geyser Basins in Yellowstone?

Back 78

The principal geyser basins are all in the same watershed on the western side of the park.

The Gibbon River from the north and the Firehole River from the south unite to form the Madison River, which flows westward into Montana, eventually to join two other rivers in forming the Missouri.

Approximately two-thirds of the hydrothermal features of Yellowstone are in the drainage area of the Firehole River.