The word "geology" must sound old-fashioned, because for the last few decades colleges have been changing their geology departments to Earth science departments, or Earth and planetary science departments, or geoscience departments. But "geology" literally means the exact same thing as "Earth science." To figure out what is happening, I think we have to look around the edges of these two nameswhat are their connotations?
Geology is the older word and has a long history. In that sense, geology is the root of Earth science.
The word arose before today's scientific discipline. The first geologists weren't even geologists; they were "natural philosophers," academic types whose novelty lay in extending the methods of philosophy to the book of nature. The first meaning of the word geology, in the 1700s, was a treatise, a "theory of the Earth" like the cosmology or "theory of the heavens" that was Isaac Newton's triumph a century before. The still earlier "geologists" of medieval times were inquisitive, cosmological theologians who treated the Earth by analogy to the body of Christ and paid scant attention to rocks. They produced some erudite discourse and fascinating diagrams, but nothing that we would recognize as science. (Today's Gaia hypothesis might be thought of as a New Age version of this long-forgotten world view.)
Eventually, geologists shook off that musty medieval mantle, but their subsequent activities gave them a new reputation that was to haunt them later.
Geologists are the ones who explored the rocks, mapped the mountains, explained the landscape, discovered the Ice Ages, and laid bare the workings of the continents and the deep Earth. Geologists are the ones who found aquifers, planned mines, advised the extractive industries, and laid straight the road to wealth based on gold, oil, iron, coal and more. Geologists put the rock record in order, classified the fossils, named the eons and eras of prehistory, and laid out the deep foundation of biological evolution. It's a great legacy, both visionary and practical, that I embrace here at About.com.
On "Earth Science"
Earth science gained currency with newer, more interdisciplinary tasks that build upon the work of the geologists. In that sense, Earth science is the fruit of geology.
I tend to think of geology as one of the true original sciences, along with astronomy, geometry and mathematics. Chemistry began as a purified, laboratory child of geology. Physics originated as an abstraction of engineering. This is not to downplay their wonderful progress and great stature, but only to establish priority.
The twentieth century brought revolutionary progress to every field of science. It was the cross-fertilization of chemistry, physics and computation, newly applied to the old problems of geology, that opened up geology into the wider realm called Earth science or geoscience. It seemed like a whole new field in which the rock hammer and field map and thin section seemed irrelevant. And those traditional methods are less intensively taught today.
Today geoscientists do things that geologists of the past never contemplated. Earth scientists help oversee remediation of polluted sites. They study the causes and effects of climate change. They advise the managers of lands, wastes and resources. They compare the structures of planets around our Sun and around other stars.
Green and Brown Science
It appears that educators have had an extra effect as curriculum standards for primary and secondary-school students have grown more complex and involved. Among these educators, the typical definition of "Earth science" is that it consists of geology, oceanography, meteorology and astronomy. As I see it, geology is a burgeoning set of subspecialties that is expanding into these neighboring sciences (not oceanography but marine geology; not meteorology but climatology; not astronomy but planetary geology), but that's clearly a minority opinion. A basic Internet search turns up three times as many "Earth science lesson plans" as "geology lesson plans." From what I've seen of them, Earth science lesson plans involve more conceptual exercises than actual rocks.
So where are we today? I see the field dividing into two pedagogical tracks:
Geology is minerals, maps and mountains; rocks, resources and eruptions; erosion, sediment and caves. It involves walking around in boots and doing hands-on exercises with ordinary substances. Geology is brown.
Earth science is pollution, food webs, dinosaurs, volcanoes, habitats, plates, the environment. It involves watching videos and conducting discussions. Earth science is green.
But maybe it's all just a matter of language. "Earth science" is as straightforward in English as "geology" is in scientific Greek. And who speaks English? Everyone in the world now.