What Produces It,
How It Acts And
What It Can Produce

What is Pressure?

Before pressure can be tested as a formative variable, it must be defined carefully.

Pressure is not a substance and it is not an independent source of energy. In its simplest mechanical definition, pressure is force distributed over area. But that definition does not tell us where the force came from, how the pressure developed, how long it persisted or what the pressured material was capable of doing.

Its origin may differ radically from one system to another, and its consequences depend upon the material, geometry, temperature, chemistry, confinement and duration involved.

This distinction is foundational to the Pressure Is of the Essence series.

What Can Produce Pressure?

Pressure is not restricted to gravitational loading or to the weight of overlying rock.

Gravity can produce pressure through the weight of overlying material. A confined fluid can exert pressure against its boundaries. Heating can increase pressure when thermal expansion is constrained. Chemical and mineral reactions can alter volume or release fluids into confined spaces. Tectonic loading can create large compressive and shear stresses. Impacts and shocks can generate intense transient pressure pulses. Electromagnetic radiation can exert radiation pressure upon matter.

These mechanisms must not be collapsed into one another.

Heat is not pressure. Chemistry is not pressure. Gravity is not pressure. But each can participate in a causal sequence that produces pressure.

We will distinguish pressure from the mechanism that produced the pressure.

Following the Causal Chain

The correct question is therefore not merely whether pressure existed. We must follow the sequence by which it arose.

Heating may cause expansion. If expansion is constrained, pressure or stress may rise. That pressure may then alter fracture behavior, fluid movement, phase stability or reaction conditions.

Gravitational loading may compress underlying material. Compression produces lithostatic or confining pressure, which may then alter mineral stability and rock behavior.

A chemical or mineral transformation may release fluid or change volume. If the system is sufficiently confined, pore pressure may increase, changing effective stress and potentially contributing to fracture, deformation or fluid migration.

An impact may generate a shock wave. The resulting transient pressure and temperature can reorganize crystal structures on extremely short timescales.

The sequence matters:

Cause → physical response → pressure or stress → structural consequence.

Major Forms of Pressure and Stress

Gravitational or lithostatic pressure is produced by the weight of overlying material.

Hydrostatic and pore-fluid pressure arise from liquids or gases within connected or confined spaces. Changes in pore pressure can strongly affect the stress carried by the solid framework.

Thermally generated pressure can arise when heating causes expansion that is restricted, especially in confined fluids. In solids, constrained thermal expansion is more precisely described as thermal stress.

Mechanical and tectonic stress results from directional loading, compression, tension or shear. Where the force is directional rather than approximately equal in all directions, this series will retain the technically correct term stress rather than calling everything pressure.

Shock pressure is intense transient loading produced by impacts or other extremely rapid mechanical events.

Reaction-generated pressure can develop when chemical or mineral transformations alter volume or generate or release fluids inside a confined material.

Radiation pressure is force per unit area produced when electromagnetic radiation interacts with matter. It is physically real, but its relevance to any geological process must be demonstrated rather than assumed.

What Can Pressure Produce?

Once produced, pressure can become an active physical condition governing what matter is capable of doing.

Pressure can compact materials and reduce pore space. It can influence fluid movement. Pressure differences can contribute to fracture. Pressure changes can alter mineral stability, phase boundaries, melting conditions and chemical equilibria. Together with temperature, chemistry and composition, pressure can permit structural arrangements that cannot persist under ordinary surface conditions.

Pressure can therefore be formative without being the sole formative variable.

That distinction must remain explicit throughout this series.

Pressure May Be Necessary
Without Being Sufficient

Diamond provides a useful example.

Carbon does not become diamond merely because it is old. Diamond formation requires an appropriate combination of pressure, temperature, composition and chemical environment. At the same time, extreme pressure alone does not guarantee that diamond will form.

Pressure may therefore be necessary without being sufficient.

The same discipline must be applied to every domain we investigate.

If temperature is necessary, we will identify temperature independently. If chemistry controls the reaction, we will identify chemistry. If fluid activity governs transport, we will identify fluid activity. If directional stress rather than isotropic pressure controls deformation, we will say stress.

We will not enlarge the meaning of pressure simply to make the hypothesis succeed.

The Four-Part Pressure Test

For every phenomenon examined in Pressure Is of the Essence, pressure will be classified according to the evidence.

Necessary; the transformation cannot occur under the relevant conditions without pressure reaching a required range.

Sufficient; pressure alone, within the stated boundary conditions, is capable of producing the observed transformation.

Contributory; pressure materially affects the transformation but additional variables are required.

Incidental; pressure is present but is not demonstrated to control the transformation.

This four-part test prevents the investigation from finding pressure everywhere merely because pressure is what we are looking for.

The Governing Rule

Pressure has no single origin and no single consequence.

It can arise through weight, confinement, thermal expansion, fluids, reactions, deformation, shock and other physical interactions. Once produced, it can become an active condition affecting structure, phase, transport, reaction and preservation.

Therefore, throughout this series, we will never be satisfied with the statement: “Pressure did it.”

We will ask:

• What produced the pressure?
• What kind of pressure or stress was present?
• How much pressure existed?
• For how long did it persist, insofar as the evidence can establish that duration?
• What temperature and chemical conditions accompanied it?
• What did the pressure physically change?
• Was pressure necessary, sufficient, contributory or incidental?

We will follow pressure wherever it is physically produced, but we will not rename its causes pressure.

That is the boundary and discipline.

And with that definition established, the investigation can begin.

Produced by The Lilborn Equation Team:

Michael Lilborn-Williams

Thomas Jackson Barnard

Audrey Williams