Earth’s Geodynamic Cycle: Rock Formation and Volcanoes
The Geodynamic Cycle: Earth Material Transformation
Also called the Geodynamic Cycle or Rock Cycle, this set of processes explains how Earth materials change and transform geological substances. This cycle relates to erosion and plate tectonics operating over vast temporal scales.
Stages of Rock Formation
The cycle involves various stages, resulting in different rock formations:
Stage 1: Formation of Igneous Rocks
Igneous rocks form when magma cools and solidifies into interlocking crystals. These rocks can be:
- Intrusive Materials: Formed deep beneath the Earth’s surface, penetrating older rocks through deep cracks before cooling. These include crystalline rocks.
- Extrusive Materials: Formed after volcanic eruptions, deposited on the surface after being ejected from volcanoes and fissures, grouping lavas.
Stage 2: Formation of Sedimentary Rocks
Sedimentary rocks (such as limestone and sandstone) form when igneous rocks are exposed to surface processes like weathering, erosion, transport, and sedimentation. These phenomena separate the rock material into small particles that are transported and accumulate as sediment in oceans and lakes.
These sedimentary deposits are compacted by the weight of successive layers and can also be cemented by water filling the pores. This process, known as lithification, transforms the deposits into solid rock.
Stage 3: Formation of Metamorphic Rocks
Metamorphic rocks form when sedimentary rocks are deeply buried or affected by mountain formation associated with crustal plate movements. They are exposed to varying degrees of heat and pressure, transforming them into metamorphic rocks.
Stage 4: Cycle Completion
The geodynamic cycle ends when metamorphic rocks are subjected to extremely high levels of heat and pressure, causing them to melt and become magma again, restarting the formation of igneous rocks.
Volcanoes and Associated Landforms
Volcanoes are cone-shaped or dome-shaped structures formed by the emission of lava and gases. Their shape and dimensions vary depending on the type of lava and the existence of tephra (fragments of rock and ash).
Basic Types of Volcanoes
We can distinguish two basic types: composite volcanoes and cinder cones or lapilli cones.
- Composite Volcanoes: Tall, steep structures produced by the stacking of acidic lava and ash. These are usually located in areas of subduction, where magma upgrades occur, but can also result from fractional crystallization phenomena.
Volcanic Hazards and Manifestations
One of the most important manifestations in composite volcanoes is the glowing cloud, also known as a pyroclastic flow (or nuée ardente). It is a compound formed by incandescent gases, ash, and rock fragments torn from the chimney and crater.
This superheated compound flows rapidly down the slopes of the volcanic cone, burning and destroying the landscape. Once settled, it forms an impermeable rock called tuff, which is a light rock composed of ash.
These structures are considered potential hazards for humanity due to the risk of pyroclastic flows and their ability to produce lahars (volcanic mudflows), as they tend to favor significant runoff processes. Despite the dangers, many of these buildings are admired for their beauty (e.g., Mount Kilimanjaro and Mount Fuji).
Characteristics of Volcanic Relief
Volcanic landforms are characterized by the presence of igneous rocks formed at the surface from cooling magma that has reached this point through conduits in the crust. The existence of these forms is determined by the type of magma.
Magma Types and Eruptions
The nature of volcanic eruptions (explosive or non-explosive) depends heavily on the magma type:
- Acidic Lavas: Cooler, more viscous, and contain large amounts of gas under pressure. These often lead to explosive eruptions.
- Basic Lavas: More fluid and contain less gas. These often lead to effusive (non-explosive) eruptions.
Upon reaching the surface, the igneous material can extend as a tongue of lava or be projected as tephra or ash under gas pressure.
Key Volcanic Landforms
Characteristic forms associated with volcanic relief include:
- Volcanic Cones (Composite Volcanoes and Cinder Cones)
- Calderas
- Lava Flows (Castings)