Engineering geology

The hydrologic cycle

Earth is characterized by energy flow and processes of change at every spatial and temporal scale of observation. Earth’s surface is energized by geothermal energy (from inside the planet) and solar energy (from outside the planet). The energy flows from sources to sinks (materials that store or convert energy) and drives processes of change. Most of these processes involve organic (biological; parts of living or once-living organisms) and inorganic (non-biological) materials in solid, liquid, and gaseous states, or phases (Figure 1).

Figure 1.Ternary diagram showing the three states (phases) of water, plus six common processes that change states of matter by heating (+) and cooling (–). Note the distribution and packing of atoms and molecules in fluid (liquid and gas) versus solid states

Note that many of the processes have opposites depending on the flow of energy to or from a material: melting and freezing, evaporation and condensation, sublimation and deposition, dissolution and chemical precipitation, photosynthesis (food energy storage) and respiration (food energy release or “burning” without flames). And while some chemical reactions are irreversible, most are reversible (as in the process of dissociation). Thus, opposing processes of change cause chemical materials to be endlessly cycled and recycled between two or more phases. One of these cycles is the hydrologic cycle, or “water cycle” (Figure 2).

Figure 2. The hydrologic cycle (water cycle). Note the relationship of processes of change in the states of water (evaporation, condensation, etc.) to Earth’s spheres (geosphere, cryosphere, hydrosphere, atmosphere, biosphere). Also, note that the hydrologic cycle is driven (forced to operate) by energy from the Sun (solar energy), energy from Earth’s interior (volcanoes and geothermal energy), and gravity.

The hydrologic cycle involves several processes and changes in relation to all three phases of water and all of Earth’s spheres (global subsystems). It is one of the most important cycles that geologists routinely consider in their work. The hydrologic cycle is generally thought to operate like this: water (hydrosphere) evaporating from Earth’s surface produces water vapor (atmospheric gas). The water vapor eventually condenses in the atmosphere to form aerosol water droplets (clouds). The droplets combine to form raindrops or snowflakes (atmospheric precipitation).

Snowflakes can accumulate to form ice (cryosphere) that sublimates back into the atmosphere or melts back into the water. Both rainwater and meltwater soak into the ground (to form groundwater), evaporate
back into the atmosphere, drain back into the ocean, or are consumed by plants and animals (which release the water back to the atmosphere via the process of transpiration).

In addition to water that is moving about the Earth system, there is also water that is stored and not circulating at any given time. For example, a very small portion of Earth’s water (about 2% of the water volume in oceans) is currently stored in snow and glacial ice at the poles and on high mountaintops. Additional water (perhaps as much as 80% of the water now in oceans) is also stored in “hydrous” (water-bearing) minerals inside Earth.
When glaciers melt, or rocks melt, the water can return to active circulation.

The endless exchange of energy and recycling of water undoubtedly has occurred since the first water bodies formed on Earth billions of years ago. Your next drink may include water molecules that once were part of a hydrous (water-bearing) mineral inside Earth or that once was consumed by a thirsty dinosaur!

Relating Scales of Understanding

The hydrologic cycle is a reminder that each thing on Earth is somehow related to everything else in space, time, or process. Geologists seek to understand these complex relationships relative to human lifetimes and the geologic time scale. For you to think like a geologist, you must consider many materials and processes over a broad range of temporal and spatial scales of observation. Some of these scales of observation may be unfamiliar to you, so you will need to convert unfamiliar sizes and rates to familiar ones.

For example, a rate of “1000 meters per million years” is much easier to conceptualize if it is converted to “1 millimeter per year.” You can also make scale models of things that are too large or small to visualize. A scale model is a physical representation of something that is actually much larger or smaller and has the same proportions as the actual object. For example, a toy car is a small model of an actual car.

The scale of the model is the ratio by which the actual object was enlarged or reduced to make the scale model. If a toy car is 20 centimeters long and the actual car was 800 centimeters long, then the ratio scale of the model to actual car is 20:800, which reduces to 1:40. The model has a fractional scale of 1/40, meaning that the actual car is 40 times (40*) larger than the model.

Figure 3. Some common processes of change on Earth

Authors: R.M.Busch; AGI; NAGT; Illustrated by D.Tassa

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