rocks
The rock cycle is usually drawn as a circle. It is better understood as a network: many possible pathways between materials, and no requirement that any particular rock travel the whole loop.
About: Metamorphism · Lithification · Magmatic crystallisation · Granite · Marble
The Earth is a connected system in which changes in one part produce changes in another, continuously through geological time. That is the substance of the rock cycle.[1]
But a rock does not have to pass through every textbook stage in order. Metamorphic rocks can form from igneous rock, from sedimentary rock, or from earlier metamorphic rock; limestone can form biologically or by precipitation; and a rock hot enough to melt exits the metamorphic path entirely and re-enters as igneous.[1,2,3]
Each transformation in the knowledge graph asserts that a pathway exists, never that every rock must take it. Limestone can become marble; shale can become slate; granite weathers and erodes into the grains that become sandstone, carrying its quartz, feldspar and mica with it.[1,2,3,4]
Sedimentary rock buried deeply enough can enter metamorphic conditions; metamorphic rock heated further can melt and crystallise as igneous rock. The links are conditional, and the conditions are the science.[1,2,3]
The cycle joins the tectonic engine at both ends. Weathering and erosion break rock down and feed the sedimentary limb; subduction returns lithosphere to the mantle, and new crust is made at spreading centres.[1,2,3]
Often said
“The rock cycle is a fixed circular sequence every rock passes through.”
What the evidence supports
It is a network of possible transformations. A metamorphic rock can form from another metamorphic rock; limestone has two independent origins; melting removes a rock from the metamorphic path altogether. Each pathway is conditional on the conditions being met.
Supporting records: rock-cycle-network · metamorphic-protolith · biologic-sedimentary
Bracketed references link to the structured entity records whose claims support each paragraph. Last updated 2026-09-06.