6.2 The Rock Cycle

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 6.2). 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.
  2. The hydrological cycle, which is the movement of water, ice, and air at the surface. The hydrological cycle is powered by the sun.

 

Figure 6.1 Processes that change rocks from one form to another [SE]

Figure 6.2 The rock cycle consists of processes that change rocks from one form to another [SE]

 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 (e.g., Mercury), 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, but it’s convenient to start with magma. Magma is melted rock located within the Earth.  Rock can melt at between about 800 °C and 1300 °C, depending on the minerals in the rock, and the pressure the rock is under.  If it cools slowly within the Earth (over centuries to millions of years), magma forms intrusive igneous rocks.  If magma erupts onto the surface, we refer to it as lava.  Lava cools rapidly on Earth’s surface (within seconds to years) and forms extrusive igneous rocks (Figure 6.3).[1]

Figure 6.2 Lava forming pahoehoe basalt at Kilauea Volcano, Hawaii [SE]

Figure 6.3 Lava forming pahoehoe (ropy) basalt at Kilauea Volcano, Hawaii [SE]

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, Figure 6.4) and chemically (by reacting with minerals and changing them to something else).

Mechanical weathering near La Madaleta glacier in Spain. Cycles of freezing and thawing causes ice to wedge open rocks along pre-existing cracks. [Luis Paquito, CC-BY-SA]

Figure 6.4 Mechanical weathering near La Madaleta glacier in Spain. Cycles of freezing and thawing cause ice to wedge open rocks along pre-existing cracks. [Luis Paquito, CC-BY-SA]

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 (Figure 6.5), and sediments are deposited in rivers, lakes, deserts, and the ocean (Figure 6.6).

Wind transports sediment in a dust storm near Okotoks, Alberta, Canada in July of 1933. [Glenbow Archives NA-2199-1, Public Domain]

Figure 6.5 Wind transports sediment in a dust storm near Okotoks, Alberta, Canada in July of 1933. [Glenbow Archives NA-2199-1, Public Domain]

 

Sand ripples deposited by the South Saskatchewan River near Saskatoon, Saskatchewan, Canada [KP]

Figure 6.6 Sand ripples deposited by the South Saskatchewan River near Saskatoon, Saskatchewan, Canada [KP]

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 (Figure 6.7).

Ripples preserved in 1.2 Ga old sandstone. Notice the wavy lines above the coin. This is a side view of the ripples. [Anne Burgess, Geograph, CC-BY-SA]

Figure 6.7 Ripples preserved in 1.2 Ga old sandstone. Notice the wavy lines above the coin. This is a side view of the ripples. [Anne Burgess, Geograph, CC-BY-SA]

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 (Figure 6.8).

Metamorphosed Triassic-aged limestone, Quadra Island, B.C. [SE]

Figure 6.8 Metamorphosed Triassic-aged limestone, Quadra Island, B.C. [SE]


  1. Remember the difference between intrusive and extrusive igneous rocks by recalling that INtrusive rocks form withIN the Earth, and EXtrusive rocks form when lava EXits the Earth's crust.

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