GL 1075 Earth Systems Science
K. Hannula
Discussion questions for 10/22/07
1. Define:
zone of saturation: the zone underground where all open spaces in rock (including cracks and spaces between grains) are filled with water; the area where groundwater is found.
water table: the top of the zone of saturation. Below the water table, all open spaces in rock are filled with water.
aquifer: a permeable rock unit that lies beneath the water table. A good thing to drill a well into.
2. What is porosity? Which types of rocks and sediment commonly have high porosity?
Porosity is the percentage of open space in a rock.
Some sediments and rocks and their porosities:
Relatively high porosity:
| Sediments: | Gravel | 25-40% |
| Sand | 30-50% | |
| Silt | 35-50% | |
| Clay | 35-80% | |
| Rocks: | Conglomerate | 10-30% |
| Sandstone | 0-30% | |
| Limestone (with caves) | up to 50% | |
| Volcanic rocks | 0-50% |
Low porosity:
| Sediments: | Glacial till (poorly sorted, from clay to boulders) | 10-20% |
| Rocks: | Shale | 0-30% |
| Limestone without caves | 0-20% | |
| Plutonic igneous rocks and metamorphic rocks | 0-5% |
3. What is permeability? Which types of rocks and sediment commonly have high permeability?
Permeability is the ability of rock to transmit water. (Water flows more quickly through rock with high permeability.)
Sediments with relatively high permeability include:
Gravel
Sand
Rocks with relatively high permeability include:
Conglomerate
Sandstone
Limestone (with caves)
Some volcanic rocks (if bubbles are connected)
4. Name two rocks or sediments that have high porosity but low permeability.
Clay (very low permeability because pore spaces are very small, so water drains very slowly from it)
Pumice (if bubbles are not connected, water can't travel from one bubble to another. Low permeability allows pumice to float in water.)
5. What causes groundwater to flow? (Be prepared to draw the direction of groundwater flow from a sketch of the groundwater table.)
Groundwater flows because of 1) gravity (flow from high elevation to low elevation) and 2) water pressure (flows from high pressure to low pressure -- this can allow groundwater to flow up, towards a well, for instance.)