The word “clay” often suggests a uniform material. In reality, clays include a wide diversity of minerals and geological formations.

From a grain-size perspective, clays correspond to the finest particles in soils and sediments. But their specific behaviour mainly comes from their mineralogical structure.

Most clay minerals belong to the phyllosilicate family: minerals organised as stacked microscopic sheets. This structure gives them a very large exchange surface with water.

Some clays can absorb water between their crystalline sheets and change volume. Not all clays therefore react in the same way.

Some remain relatively stable. Others have a strong capacity to swell and shrink. This difference is fundamental.

Clays form in very diverse environments: ancient marine deposits, lakes, alluvial plains, wetlands or the progressive weathering of pre-existing rocks.

In France, many clay-rich formations come from large sedimentary basins such as the Paris Basin or the Aquitaine Basin.

Over millions of years, these basins accumulated clays, marls, limestones, sands, silts and organic deposits.

These formations are never perfectly homogeneous. Over just a few tens of metres, thickness, composition or continuity of layers may already vary strongly.

Two neighbouring sites can therefore respond differently to the same drought. Geology controls a large part of this variability.

The behaviour of clay soils strongly depends on their mineralogy.

Some mineral families, such as smectites, have a structure able to incorporate water between crystalline layers.

When humidity increases, spacing between sheets increases: the material swells. When the soil dries, water progressively leaves these interlayer spaces: the material shrinks.

Smectites, especially montmorillonite, have among the highest swelling capacities. Other clays, such as kaolinite or illite, are generally less expansive.

But natural soils are always more complex than theoretical models. Real geological formations often mix several mineral families, grain sizes, carbonates, organic matter and successive episodes of deposition or alteration.

The final behaviour of a site therefore depends on mineralogy, clay proportion, layer thickness, hydrological context and local geological history.

Shrink–swell is primarily a hydrological dynamic.

Soils continuously exchange water with their environment: rainfall, evaporation, plant evapotranspiration, groundwater, drainage and urbanisation.

These exchanges are never uniform. A tree can dry several metres of soil. A road or building can locally modify water flows. A buried leak can maintain abnormal humidity for years.

The ground is not static. It constantly reacts to water variations.

Clay shrink–swell corresponds to volume changes in clay soils in response to moisture variations.

During wet periods, some clays absorb water and swell. During dry periods, they lose this water and shrink.

These movements often remain small at human scale. But repeated over years, they can produce differential settlement, progressive deformation, cracks or damage to infrastructure.

Climate change is now amplifying these dynamics. Longer and more intense droughts increase the depth of soil desiccation and disturb older hydrological equilibria.

Hazard maps are essential prevention tools. But they should never be interpreted as absolute truths.

A geological map mainly describes a dominant formation, lithology, regional context or general susceptibility.

It never perfectly represents local variations, clay lenses, artificial fills, hydrological conditions, vegetation or the real dynamics of the ground.

Geological potential does not automatically imply observed movement. This distinction is fundamental.

Movements related to shrink–swell can now be observed through complementary approaches: geology, geotechnics, hydrological monitoring, instrumentation and satellite Earth observation.

Satellite radar interferometry, or InSAR, can measure millimetric ground displacement from radar images acquired regularly by satellites.

InSAR time series reveal seasonal cycles, long-term trends, spatial differences and some hydrological responses of the ground.

The main value is not only to produce a map. It is to follow a dynamic.

But these observations must always be interpreted in the light of geology,
hydrology and territorial context. To understand what satellite radar observation
actually measures, see also:

understanding InSAR and ground motion
.

Clay shrink–swell is a geological, hydrological and territorial process.

Understanding it requires connecting formation history, clay mineralogy, the role of water, climate cycles and ground motion observed through time.

Cracks are only the visible manifestation of a deeper process, inherited from geological history and reactivated by contemporary hydrological constraints.

These articles extend this reading: understanding what InSAR actually measures, comparing the 2026 RGA map update, then crossing hydrological and CatNat signals at territorial scale.

Understanding InSAR and ground motion

2026 clay shrink–swell map: geology has not changed, risk has

When physical drought and insurance signals tell different stories