The rock components of the crust are slowly but constantly being changed from one form to another and the processes involved are summarized in the rock cycle (Figure 3.2). The rock cycle is driven by two forces: (1) Earth’s internal heat engine, which moves material around in the core and the mantle and leads to slow but significant changes within the crust, and (2) the hydrological cycle, which is the movement of water, ice, and air at the surface, and is powered by the sun. The rock cycle is still active on Earth because our core is hot enough to keep the mantle moving, our atmosphere is relatively thick, and we have liquid water. On some other planets or their satellites, such as the Moon, the rock cycle is virtually dead because the core is no longer hot enough to drive mantle convection and there is no atmosphere or liquid water.  In describing the rock cycle, we can start anywhere we like, although it’s convenient to start with magma. As we’ll see in more detail below, magma is rock that is hot to the point of being entirely molten. This happens at between about 800° and 1300°C, depending on the composition and the pressure, onto the surface and cool quickly (within seconds to years) — forming extrusive igneous rock (Figure 3.3). Magma can either cool slowly within the crust (over centuries to millions of years) — forming intrusive igneous rock, or erupt onto the surface and cool quickly (within seconds to years) — forming extrusive igneous rock. Intrusive igneous rock typically crystallizes at depths of hundreds of metres to tens of kilometres below the surface. To change its position in the rock cycle, intrusive igneous rock has to be uplifted and exposed by the erosion of the overlying rocks. Through the various plate-tectonics-related processes of mountain building, all types of rocks are uplifted and exposed at the surface. Once exposed, they are weathered, both physically (by mechanical breaking of the rock) and chemically (by weathering of the minerals), and the weathering products — mostly small rock and mineral fragments — are eroded, transported, and then deposited as sediments. Transportation and deposition occur through the action of glaciers, streams, waves, wind, and other agents, and sediments are deposited in rivers, lakes, deserts, and the ocean.
Exercise 3.1 Rock around the Rock-Cycle clock Referring to the rock cycle (Figure 3.2), list the steps that are necessary to cycle some geological material starting with a sedimentary rock, which then gets converted into a metamorphic rock, and eventually a new sedimentary rock. A conservative estimate is that each of these steps would take approximately 20 million years (some may be less, others would be more, and some could be much more). How long might it take for this entire process to be completed? Unless they are re-eroded and moved along, sediments will eventually be buried by more sediments. At depths of hundreds of metres or more, they become compressed and cemented into sedimentary rock. Again through various means, largely resulting from plate-tectonic forces, different kinds of rocks are either uplifted, to be re-eroded, or buried deeper within the crust where they are heated up, squeezed, and changed into metamorphic rock.
In many ways the Moon is a geologic Rosetta stone: an airless, waterless body untouched by erosion, containing clues to events that occurred in the early years of the solar system, which have revealed some of the details regarding its origin and providing new insight about the evolution of Earth. Although they also posed new questions, the thousands of satellite photographs brought back from the Moon have permitted us to map its surface with greater accuracy than Earth could be mapped a few decades ago. We now have over 380 kg of rocks from nine places on the Moon, rocks that have been analyzed by hundreds of scientists from many different countries. Data from a variety of experiments have revealed much about the Moon's deep interior. As it turns out, the Moon is truly a whole new world, with rocks and surface features that provide a record of events that occurred during the first billion years of the solar system. This record is not preserved on Earth because all rocks formed during the first 800 million years of Earth's history were recycled back into the interior. The importance of the Moon in studying the principles of geology is that it provides an insight into the basic mechanics of planetary evolution and events that occurred early in the solar system. Much of the knowledge we have of how planets are born and of the events that transpired during the early part of their histories has been gained from studies of the Moon. At the outset, it is important to note that we assume that the physical and chemical laws that govern nature are constant. For example, we use observations about how chemical reactions occur today, such as the combination of oxygen and hydrogen at specific temperatures and pressures to produce water, and infer that similar conditions produced the same results in the past. This is the basic assumption of all sciences. Moreover, much of what we "know" about the planets, as in all science, is a mixture of observation and theory---a mixture that is always subject to change. Scientific knowledge is pieced together slowly by observation, experiment, and inference. The account of the origin and differentiation of planets we present is such a theory or model; it explains our current understanding of facts and observations. It will certainly be revised as we continue to explore the solar system and beyond, but the basic elements of the theory are firmly established. 4.1 Major Concepts
In July 1969, a human stood for the first time on the surface of another planet, seeing landscape features that were truly alien and returning with a priceless burden of Moon rocks and other information obtainable in no other way. Nonetheless, many of the facts listed in Table 1 were known long before we began to explore space; they represent years of diligent study. For example, it was discovered centuries ago that the Moon revolves about Earth and not the Sun and is thus a natural satellite (the largest in the inner solar system). Long ago the distance from Earth to the Moon was measured and the diameter of the Moon determined. Early astronomers realized that the Moon's rotation period and its period of revolution are the same; thus it keeps one hemisphere facing Earth at all times. Moreover, many of the Moon's surface features have become well known, especially since the days of Galileo, the first to study the Moon through a telescope. Even the density and gravitational field of the Moon had been determined long before our generation. But not until the 1960s---and the inception of space travel with its sophisticated satellites and probes and the eventual Moon landing---did man begin to appreciate the significance of the Moon as a planet. In spite of its small size and forbidding surface, the Moon has revealed secrets that pertain to the ultimate creation of our planet, Earth, and our neighbors beyond.
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Which of the following represents the sequence in which the listed planets become geologically dead?
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