A Trip Through the Rock Cycle
The rock cycle is a continuous process with no beginning and no end. To better understand how the cycle works, let’s look at one possible path that begins at Earth’s surface with an intrusive igneous
rock formation called a dike that lies exposed on the side of a mountain. The dike is composed of granite.
Weathering and Erosion
Granite is an igneous rock made up of minerals that are bonded tightly together. This makes granite highly resistant to the processes of weathering and erosion. Time, however, is on the side of weathering
and erosion. In the case of the aforementioned granitic dike, constant exposure to the elements—sun, wind, rain, and changes in temperature—slowly causes the rock to break down into dissolved minerals,
clay, sand grains, and rock fragments. As the larger rock fragments are transported by wind and water, they bump against each other and the sharp-cornered fragments become rounded pebbles and sand.
Deposition and Burial
The igneous rock has now weathered into sediments, which are being carried downhill by wind and water. Eventually, these sediments enter a stream at the base of the mountain. They break into smaller
pieces as they tumble along with the current. The stream enters a river; the river enters an ocean. The sediments—now mere particles of sand—settle to the ocean floor. Meanwhile, back on the mountain,
weathering and erosion are causing more sediments to enter streams and ultimately, the ocean. These sediments are deposited in layers upon existing layers of sediment. The pressure of overlying layers
squeezes the bottom sediments closely together. As burial depth increases, the layers of sand and mud become cemented into sandstone and shale—sedimentary rocks.
LINK UP: Explore the processes of the rock
cycle.
Heat and Pressure
At this point, the rock cycle proceeds beneath Earth’s surface. As the layers of sediment are buried deeper in the crust, they encounter steadily increasing temperatures and greater pressure. At depths
below 10 km, the temperature exceeds 300°C, and the chemical composition of the minerals begins to change. New minerals that are stable at higher temperatures and pressures begin to crystallize. Heat
and pressure—the forces behind metamorphism—alter the texture and mineralogy of the rock. The layers of shale may recrystallize into layers of schist that contain muscovite and biotite micas, while the
layers of quartz sandstone metamorphose into quartzite.
Melting and Cooling
The original igneous granite has been transformed into the sedimentary rocks shale and sandstone, and finally, into the metamorphic rocks schist and quartzite. However, the cycle is not over. If temperatures
continue to increase, the metamorphic rocks may melt and form magma with a rhyolitic composition. Magma is less dense than surrounding rock. Thus, pressure will cause the magma to be slowly forced upwards.
Eventually, it will cool and form igneous intrusions with the composition of granite. Later, uplift and erosion may expose one of these granitic dikes, and the journey around the rock cycle will be complete.
Activity
As a group, describe a possible path along the rock cycle in which a metamorphic rock becomes a sedimentary rock. Illustrate the process in your science journal. Must the rock go through a melting
phase before it becomes a sedimentary rock? Explain.
