The terminology used to describe fractures in rocks can be confusing, in part because older terms such as “fault” and “joint” – inherited from pre-scientific mining – have evolved in meaning over time, becoming more specific.
“Fracture” is the most general term, referring to any planar break in rock. Fractures are typically classified into two types: faults and opening-mode fractures, distinguished by the direction of displacement across the break (see accompanying figure, ‘F’, ‘Fz’). Faults involve displacement parallel to the fracture walls. Opening-mode fractures involve displacement perpendicular to the fracture walls.
This distinction, based on displacement direction, provides a clear way to categorize fractures. Even if a fault includes an opening component, it remains a fault. Similarly, fractures with even minimal parallel displacement are still considered faults.
The terms “fracture,” “fault,” and “opening-mode fracture” form the core vocabulary for describing rock deformation and generally meet most terminological needs. However, distinguishing between faults and fractures, characterizing their various attributes and understanding how they occur together – across different scales, tectonic settings, thermal regimes and detection methods – is central to modern structural geology. Indeed, the Journal of Structural Geology publishes work on faults and fractures in nearly every issue.
Both faults and opening-mode fractures occur across a wide range of sizes, from microscopic features to structures at the scale of tectonic plates. A key to interpreting them reliably is to maintain awareness of scale – and how different parts of the size spectrum are sampled by different tools such as seismic imaging, boreholes or outcrop observations. For instance, fault populations often span a wide displacement range, but seismic data typically reveal only the larger faults. Accounting for smaller, unmapped faults can significantly influence applications such as fault strain estimation, burial history analysis and seal integrity assessments.
Similarly, opening-mode fracture populations might follow consistent scaling relationships across many orders of magnitude. However, chemical processes like mineral precipitation can preferentially seal narrower fractures, making attributes such as open pore space size-dependent and potentially concealing the smallest fractures. In some contexts, scale is reflected in specialized terminology like “microfracture,” though definitions vary by discipline. Geologists often use the term for fractures visible only under a microscope (millimeter scale or smaller), whereas engineers and geophysicists may apply it to much larger fractures – centimeter to decimeter scale. Because of this variability, it is essential to clearly specify the scale you are referring to in any discussion.
Avoiding Confusion Across Disciplines
Terminology for opening-mode fractures can be particularly tricky. While the basic distinction between faults and opening-mode fractures, based on the direction of displacement relative to the fracture plane (see accompanying figure), is straightforward, the terminology used for opening-mode fractures has often been inconsistent. Terms like “joint,” “cleat” and “vein” have been used inconsistently, contributing to confusion. For example, veins are typically defined as fractures filled with mineral cement, whereas joints have been used loosely to refer to fractures that are barren, partially filled or fully sealed.
A clearer and more consistent approach is to use the descriptive term “opening-mode fracture” and explicitly state whether mineral fill is present and what type it is. Similarly, process-based terms like “tectonic fracture” should be avoided. One of the key challenges in studying opening-mode fractures is determining the processes that formed them, so labeling them with generic terms can lead to circular reasoning and misinterpretation.
Detection method limitations also affect how fractures are classified. For example, distinguishing between faults and opening-mode fractures is rarely possible using flow data from well tests alone. Similarly, conductive or resistive linear anomalies observed on well logs might not reveal the fracture mode. While seismic imaging can offer valuable insights into the size and spatial patterns of subsurface fractures, it often lacks the resolution needed to determine fracture type. In such cases, using the general term “fracture” is appropriate. If the nature of the feature remains unclear, terms like linear anomaly or lineament may be more suitable.
Fracture terminology varies across geoscience subdisciplines, often reflecting different perspectives and needs. For instance, field structural geologists typically use the terms “fracture” and “microfracture,” whereas fracture mechanics researchers – including experimentalists and theoreticians – tend to use “crack” and “microcrack.” In geomorphology and related fields, terms like “fissures,” “caves,” “cracks” and “crevices” often carry a topographic connotation. In certain earth-surface process applications, “fracture” can refer to any open discontinuity with a high length-to-aperture ratio, regardless of its scale, location (for instance, within clasts or bedrock) or origin. These distinctions are not inherently problematic – they reflect the specific priorities of different subfields and often capture important nuances. However, for clarity and consistency, it is essential to know your audience and explicitly define your terms when crossing disciplinary boundaries.
Clear and unbiased terminology is particularly important in applied contexts, such as oil and gas exploration, where miscommunication can lead to serious interpretive errors. Terminological care helps avoid circular reasoning and supports more reliable predictions about the behavior and impact of fractures. A notable example is the widespread use of the term “natural fracture” in subsurface analysis. This label is generally meant to distinguish fractures formed by natural geological processes (tectonic stress and pore pressure changes, for example) from those induced by engineering activities like drilling or core handling. However, many natural and induced opening-mode fractures are extremely difficult to differentiate, particularly in core data, without rigorous descriptive criteria.
Thus, the first step in understanding fractures – whether using cores, logs, seismic data, outcrops or other tools – is to adopt clear, descriptive and consistent terminology. Avoiding assumptions about fracture origin unless supported by evidence helps maintain objectivity and improves communication across disciplines.
Fracture studies are a natural arena for cross-disciplinary collaboration, and effective communication across those disciplines begins with shared, well-defined language.