1. The Rock Cycle
Interlude C

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Figure C.1
The Rock Cycle
Rock cycle: the progressive transformation of Earth materials from one rock type to another over time.
There can be numerous pathways within the rock cycle.
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Figure C.2
The Rock Cycle
Lava erupts from a volcano, forming igneous rock.
Weathering and erosion form sediment, which becomes sedimentary rock.
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Figure C.2
The Rock Cycle
During continent-continent collision, sedimentary rocks are metamorphosed into metamorphic rock.
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Figure C.2
The Rock Cycle
Metamorphic rock is exposed at Earth’s surface, where some of it undergoes weathering and erosion.
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Figure C.2
The Rock Cycle
During continental rifting, some of the metamorphic rock melts, creating igneous rock.
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7. Rock-Forming Environments and the Rock Cycle
Geology at a Glance
Rock-Forming Environments and the Rock Cycle
This is a good time to discuss the driving forces of the rock cycle, which include: Earth’s internal heat (helps drive plate movements and creates mantle plumes), gravity (helps drive plate movements and affects surface processes), the Sun (affects wind, rain, ice, and other aspects of weathering and erosion), and life (adds oxygen to the atmosphere, aids in weathering).
These driving forces can be illustrated in the different rock-forming environments shown in the figure.
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8. Cycles of the Earth System
Another View
Cycles of the Earth System
It is important at this time to define what a cycle is and the terminology associated with cycles.
Cycle: a series of interrelated events or steps that occur in succession and can be repeated
Mass transfer cycle: materials that are physically transferred among regions of the Earth System
Reservoir: a region that holds material
Residence time: the time a material remains in a reservoir
This is also a good time to let students know there are different types of cycles on Earth that will be discussed in class. Two examples are:
Geochemical cycles—involve physical components
Biogeochemical cycles—involve physical and living components
A reef off the coast of a Caribbean island consists of calcite extracted from sea water by living organisms during the process of growing shells. The calcite becomes biochemical rock. This process is part of a biogeochemical cycle.
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Think–Pair–Share
Does Mars have a rock cycle? Explain how and why a Martian rock cycle is similar to, and different from, the terrestrial one.
Would your answer be different if we were asking about Mars early in its geologic history?
Mars does have a rock cycle. The volcano Olympus Mons has produced igneous rock, and there is also sediment and sedimentary rock on the planet. Notable things that will make a Martian rock cycle different include: the lack of plate tectonics, lack of liquid water, and a thinner atmosphere with a different composition from Earth’s.
Early in Mars’s geologic history, plate tectonics did operate and there was liquid water present, which would make the rock cycle more similar to the one on Earth today.
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Think–Pair–Share
How is the rock cycle on Earth today different from one that operated early in Earth’s history?
Students should think about the fact that the atmosphere today is different than the one found early in Earth’s history, which would affect weathering and erosion of rocks. Heat flow was also significantly greater, which could affect how fast some parts of the rock cycle operated. Lastly, there is some debate as to when plate tectonics began on Earth. The presence or absence of plate tectonics would affect parts of the rock cycle.
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11. Rock Cycle Narrated Figure Video
Links are active only in Slideshow mode. Click the image to launch the animation (requires an active Internet connection).
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12. Interlude C Photo Credits
1 (all): Stephen Marshak
8 Stephen Marshak
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