Introduction
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Imagine the following scenario. During subduction, an andesitic melt is produced (see the Chapter on Igneous Rocks). It rises away from the subduction zone (driven by the density contrast between the melt and the overlying rock) and is emplaced in crust at a depth of 5 kilometers where it cools giving rise to a Diorite pluton. Erosion eventually exposes the top of this body to the reactive atmosphere of the Earth. The minerals that formed - primarily sodic plagioclase, pyroxene, amphibole and a small amount of alkali feldspar - were in equilibrium at an elevated temperature (approximately 800 degrees C) and may (or may not) be in chemical equilibrium at the Earth's surface. In addition to the possibility of chemical change, processes acting at the Earth's surface may (or may not) produce physical changes. Weathering is the chemical alteration and physical breakdown of rocks and minerals at or near the Earth's surface.
Erosion involves the transportation - movement of weathered particles. Keep in mind the fact that some materials are carried as solids (clastics) where as some ions in solution (chemical) are usually carried away from the weathering area.
Properties of Water
- Water is a remarkable compound. Near the surface of the Earth water can exist in three phases - ice (solid), water (liquid), and vapor (steam). Isaac Asimov once noted that the Earth was at just about the right distance from the sun so that water, on which life as we know it is based, exists as a liquid at the surface. Any closer to the sun and only steam would be present and further away would result in ice.
- solid - liquid boundary : melting - crystallization
- solid - gas boundary : sublimation
- liquid - gas boundary : boiling - condensation
- Most solid substances you are familiar with are more dense than the liquid that is produced when melting occurs. A phase diagram of such a substance would exhibit a positive slope of the boundary curve between solid and liquid. Sketch such a diagram and, using the pencil method defined above, convince yourself that the solid is indeed more dense than the liquid.
- Sketch a phase diagram with a vertical boundary curve between solid and liquid phases. Which phase has the highest density? [answer: they have the same density].
- Note that the boundary curve between liquid and vapor is constantly changing slope; that is, it is a curve and not a straight line. What is happening to the density contrast between liquid and vapor as T and P increase? Note that this boundary curve is approaching a vertical line. When this boundary curve becomes vertical there is no longer a density difference between liquid and vapor; there is no longer a distinction between the two phases. The T and P at which this occurs defines the critical point which for water is some 300 degrees C and 300 atm.
Note that there is one point on the diagram for water where solid, liquid and vapor are in equilibrium - triple point.
- Note that the Earth environment is close to the triple point and that water, steam and ice can all exist at the surface.
- Note that the Earth environment is close to the triple point and that water, steam and ice can all exist at the surface.
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It takes a lot of thermal energy to heat one gram of water one degree centigrade. Water has a high heat capacity. Imagine a shoreline. The land and the water surfaces receive equal thermal energy from the sun but the land heats up quicker than the water; in general, rocks have lower heat capacities than water. Thus, water may remain quite cool even in the summer.
- Water can be pictured as an ellipsoidal molecule (non spherical) in which the two hydrogen ions (positive charges) are isolated at one end of the molecule and the oxygen (negative charge) at the other. Thus, water is a polar molecule. Water is a good solvent; many materials are at least partially soluble in water and the polar nature of the molecule aids in this behavior. The positive end of the molecule attaches itself to negatively charged molecules, for example, which helps to promote breakdown of the solid structure.
When ice melts 80 calories of heat are absorbed. A calorie is the amount of energy needed to raise the temperature of 1 gram of water one degree centigrade. Imagine an experiment in which you put a thermometer on a block of ice and increase the temperature to 50 degrees centigrade. The temperature of the ice will rise until a temperature of 0 degrees is reached. The temperature then remains constant until the ice melts to water. Heat is absorbed until all the ice is melted. Then, and only then will the temperature rise. When water freezes heat is released (again 80 calories per gram of water). Notice how much heat is required to change liquid into vapor (gas).
Where does this heat come from? Think of water as the system and everything else are the surroundings. The heat comes from (or goes to) the surroundings.
A plot of temperature versus pressure provides a useful framework in which to examine some of the characteristics of these three phases. Note the direction of increase of the temperature and pressure axes. Think about the following questions. An understanding of the behavior of water is essential for an understanding and appreciation for this and the following sections.
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The fields (range of temperature and pressure) in which the three phases are stable are marked on the diagram. Each field is bounded by a boundary curve which represents the conditions (T and P) needed to achieve equilibrium between two of the phases.
Thus, if the temperature and pressure are such that they define a point on the boundary curve between the solid and the liquid forms of water, ice and liquid are in equilibrium. We tend to think of melting, for example, as a process that takes place at a known temperature. Note, however, that melting takes place along a straight line with a negative slope. The standard melting temperature is defined as the temperature at which ice and water are in equilibrium at a pressure of 1 atm. Similarly, the standard boiling point is defined as the temperature in which liquid and vapor are in equilibrium at 1 atm.
An increase in pressure favors the phase with the greatest density (smallest volume). Put your pencil on the melting curve. If the pressure increases (pencil moves upwards) the field in which water is stable is entered. Therefore, water is more dense than ice. This should confirm your observation that ice cubes float in water.
In addition to the properties noted above, there are some other properties of water which help to explain its behavior.