McKnight's Physical Geography

Chapter 17

Karst and HydrothermalProcesses

Modified by AJ Allred

for SLCC Fall 2013

© 2014 Pearson Education, Inc.

Andrew MercerMississippi State University

Solution and Precipitation• Water in its pure form is a poor solvent.

• Chemical impurities make water a good solvent of a few underground minerals.

• Carbonic acid

Solution and Precipitation

• Dissolution – carbonic acidcarbonic acid– Most effective on carbonate sedimentary rocks (e.g.,

limestone)

– Calcium carbonate reaction• CaCO3 + HH22O + COO + CO22 = Ca(HCO3) 2

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

– These are most notable dissolution processes

– Occurs more rapidly in humid regions

– Possible role of sulfuricsulfuric acid and acids from nitrogen oxides

Solution and Precipitation• Role of bedrock structure– Profusion of joints allows for groundwater penetration

• Precipitation processes– Mineralized water trickles along cavern roof or wall.

– Reduced air pressure precipitates mineral material.

– High mineral content present in hot springs.

– Hot water has more minerals, except carbon dioxide.

Caverns and Related FeaturesOften in limestone deposits – calcium/carbonate rocks

Five stages1. Initial excavation – water finds a way in

2. Decoration stage – mild acid erosion caves

3. Dissolved solution precipitates back into “drip-dry” rockspeleothems form (stalactites and stalagmites)

4. Structural collapse - caves eventually “cave in”

5.Eventually, most

of the formation

washes away,

leaving isolated

“tower karsts”

Karst Topography• Mild acid easily dissolves

hard limestone

• Typical landforms– Sinkholes– Disrupted surface

drainage

• Ten percent of Earth’s land surface – soluble rock

[Insert Fig. 17-7 p. 412]

Isolated “tower karsts” after caves wash away

Karst Topography• Sinkholes– Rounded depressions – sinking areas

– Collapse dolines

– Disappearing streams

Karst TopographyExtent of karst topography

Hydrothermal Features

• Hydrothermal activity– Geysers and hot springs

• Hot springs– Water heated by magma

– Forced upward from pressure resulting from heating

– Resulting topography from hot springs

– Algae growth

Hydrothermal Features

• Geysers– Intermittent hot spring

– Accumulation of superheated water and steam builds pressure

– Tremendous heat required for geyser formation

– Variable eruption times

– Variable deposits; most are sheets of deposits scattered irregularly over ground

Hydrothermal FeaturesFumaroles– Surface crack connected to a deep-seated heat source

– Little water drainage

– Water that is drained – converted to steam

– Steam vent, either continuously or sporadically

Natural travertine layering – often used for building decoration

Soda Springs, Idaho. This deposition is provided by a 100 foot geyser that erupts every hour.

Hydrothermal Features• Hydrothermal features in Yellowstone – 225 geysers

– Volcanic bedrock materials

– Shallow magma chamber, mantle plume (heat source)

– Copious summer rain and winter snowmelt (water source)

– Numerous fractures and weak zones from earthquakes

Many geothermal features seem weaker than in past decades. What might global warming have to do with it?

<DROUGHT?>

Hydrothermal FeaturesHydrothermal features in

Yellowstone

– Geyser basins in same watershed on west side of park

– Geyser basins drained by three rivers

– Geyser basins have extensive geyserite

– Mammoth Hot Springs Terraces