The Rock Cycle After this lesson you will be able to explain what happens during the rock cycle.
What is the rock cycle? Natural forces act on the Himalayas. In fact, rock in Earth’s crust is always changing. Forces deep inside Earth and at the surface produce a slow cycle that builds, destroys, and changes the rocks in the crust. The rock cycle is a series of processes that occur on Earth’s surface and in the crust and mantle that slowly change rocks from one kind to another. For example, weathering can break down granite into sediment that later forms sandstone.
One pathway through the rock cycle There are many pathways by which rocks move through the rock cycle. For example, Stone Mountain, near Atlanta, Georgia, is made of granite. After the granite had formed, the forces of mountain building slowly pushed the granite upward. Then, over millions of years, weathering and erosion began to wear away the granite. Today, particles of granite constantly break off the mountain and became sand. Streams carry the sand to the ocean. What might happen next?
One pathway through the rock cycle Over millions of years, layers of sand might pile up on the ocean floor. Slowly, the sand would be compacted by its own weight. Or perhaps calcite that is dissolved in the ocean water would cement the particles together. Over time, the quartz that once formed the granite of Stone Mountain could become sandstone, which is a sedimentary rock. Sediment could keep piling up on the sandstone. Eventually, pressure would compact the rock’s particles until no spaces were left between them. Silica, the main ingredient in quartz, would replace the calcite cement. The rock’s texture would change from gritty to smooth. After millions of years, the sandstone would have changed into the metamorphic rock quartzite.
The rock cycle and plate tectonics The changes of the rock cycle are closely related to plate tectonics. Recall that Earth’s lithosphere is made up of huge plates. These plates move slowly over Earth’s surface as the result of convection currents in Earth’s mantle. As the plates move, they carry the continents and ocean floors with them. Plate movements help drive the rock cycle by helping to form magma, the source of igneous rock Where oceanic plates move apart, magma formed form melted mantle rock moves upward and fills the gap with new igneous rock. Where an oceanic plate is subducted beneath a continental plate, magma forms and rises. The result is a volcano made of igneous rock. A collision of continental plates may push rocks so deep that they melt to form magma, leading to the formation of igneous rock.
The rock cycle and plate tectonics Sedimentary rock can also result from plate movement. For example, the collision of continental plates can be strong enough to push up a mountain range. Then, weathering and erosion begin. The mountains are worn away. This process leads to the formation of sedimentary rock. Finally, a collision between continental plates can push rocks down deep beneath the surface. Here, heat and pressure could change the rocks to metamorphic rock
Conservation of material in the rock cycle Constructive and destructive forces build up and destroy Earth’s landmasses. But as the rock in Earth’s crust moves through the rock cycle, material is not lost or gained. For example, a mountain can erode to form sediment, all of which can eventually form new rock.
Assignment Start on rock cycle poster activity