The Rock Cycle: Types, Driving Forces, and Plate Tectonics

Introduction

Most processes on Earth are cyclic in nature, seemingly designed to keep ecological systems in balance. We see this with the water cycle, photosynthesis, and even weather patterns. The rock cycle is a basic geological concept that describes the manner in which rock types — sedimentary, metamorphic, and igneous — evolve and change in response to stimuli from the atmosphere, pressure, heat, plate tectonics, the water cycle, and more. The fundamental principle is that rocks do not remain in equilibrium; they must change in response to new environmental conditions.

The rock cycle also shows us how, over longer periods of geological time, the differing types of rocks are related to one another and how they can undergo a varied set of environmental changes. In this way, the rock cycle demonstrates how geological materials on Earth, much like flora and fauna, reflect the diversity of natural processes that shape our planet.

The Three Stages of the Rock Cycle

The original concept of the rock cycle is usually attributed to the "father" of geology, James Hutton. The cycle is made up of three parts, corresponding to the three basic types of rocks: igneous, metamorphic, and sedimentary. The development of cycle theory was part of Hutton's understanding of uniformitarianism, which dominated geological thought until the 1960s, when the theory of plate tectonics shifted the view from one of simple repetition to one of gradual, ongoing evolution.

Transition to igneous rock: As rocks are pushed deeper and deeper into the Earth, they often melt into magma. If heat lessens, this magma cools and solidifies into igneous rock. As the rocks begin to cool, they change: gases within magma deposits alter the rock's composition, and as material is pushed upward it can begin to break down due to weathering from rain, frost, oxygen, and similar forces. Rocks can also change based on other minerals that have been deposited into the magma.

Transition to metamorphic rock: Rocks exposed to both high temperatures and high pressures may be physically transformed into metamorphic rock. Metamorphic rocks typically display banding of mineral types and are often formed when rock comes into contact with igneous layers that heat the surrounding material. In fact, any pre-existing rock may be modified through the process of metamorphism.

Transition to sedimentary rock: Rocks that are continually exposed to weathering and erosion — breaking down into smaller fragments — usually accumulate and become buried under additional material such as sand. When these deposits accumulate sufficiently and are subjected to pressure, they become sedimentary rock, often incorporating other organisms and minerals in the process (Rocks and the Rock Cycle, 2011).

Driving Forces: Water and Plate Tectonics

There are two major driving forces now understood to be integral to the rock cycle: water and plate tectonics. Both involve forms of pressure that also generate heat and chemical change. Water, for instance, is so abundant on Earth that it drives much of the weathering and erosion observed across the surface. Precipitation, acidic soil water, and groundwater dissolve minerals and rocks; serpentinization from heated seawater causes the destruction of volcanic rock or changes in other seabed rocks; and the combined presence of water and carbon dioxide further transforms rock over time. This is the mechanism by which the carbon cycle and the water cycle continually interact to reshape geological material.

Plate tectonics, on the other hand, describes the large-scale motions within the Earth that move, converge, and drive material from deep inside the planet toward the surface and back again. Subduction zones within the eight or nine major tectonic plates form slabs of crust that become embedded in the mantle; when subjected to sufficient heat and pressure, these contribute to the evolution of rock. In addition, one of the closing phases of this process — sometimes referred to as the Wilson Cycle — occurs when two plates meet, generating tremendous force that distorts and modifies rock through regional metamorphism, and at times produces mountain-building events (All About Plate Tectonics, 2010; The Wilson Cycle, 2000).