How Is Clay Formed Naturally?

Clay is a fine-grained earthy material that is plastic when wet and hard when fired. It consists primarily of hydrated aluminosilicates, decomposed minerals containing silicon, aluminum, and oxygen along with other impurities. The formation of clay occurs over geologic time and involves a complex series of geologic processes.

The most common pathway for natural clay production begins with the chemical weathering and physical disintegration of rocks at the earth’s surface. The broken down particles are transported by water, wind or ice to sites of deposition where they accumulate. Further burial, compaction, and cementation consolidates these materials into sedimentary rock units. Later uplift and exposure opens up the clay deposits to erosion and human use.

Weathering of Rock

The formation of clay begins with the weathering of rocks over long periods of time. Weathering is the process by which rocks are broken down into smaller particles through mechanical and chemical means. Mechanical weathering includes physical processes like freezing, thawing, erosion, and abrasion by wind, water, and ice that gradually break rock into smaller pieces. Chemical weathering involves chemical reactions that alter the mineral composition of rocks. Water, air, acids, and salts react with rock minerals, causing them to decompose and disintegrate.

For example, feldspar is a common mineral found in igneous and metamorphic rock like granite. It chemically weathers into clay minerals like kaolinite. Carbon dioxide and organic acids from the air, water, and soil dissolve feldspar, freeing up atoms that then combine with water to form new clay minerals. Over time, chemical weathering transforms hard crystalline rock into soft, earthy clay material through these chemical changes.

Mechanical and chemical weathering work together to gradually break down large rock formations into smaller fragments and new clay minerals. This weathering process is the first key step in the natural formation of clay.

Transportation

Once rocks have been broken down into smaller particles through weathering, erosion begins to transport these particles. Erosion is the removal and transportation of weathered rock and soil. Water is the main agent of erosion and acts to carry away weathered rock particles.

Flowing water such as streams, rivers, and ocean currents are particularly effective at eroding and transporting weathered rock sediments. As water flows, it picks up and carries particles in the process of hydraulic action. The faster the water flows, the larger the particles it can transport. Particles bounce along the stream or river bed in saltation, gradually breaking into smaller pieces through abrasion. Over long distances, rivers carry significant loads of sediment to deposit in areas like river deltas or floodplains.

Strong waves along ocean coasts also erode rock particles and carry them away in coastal currents. Turbulence helps keep particles suspended in the water as they are transported. The swirling motion of waves striking the coast prevents particles from settling out of the water column.

Wind can also erode and transport fine clay, silt, and sand particles once rocks have been weathered down. Abrasion within sand dunes or dust storms breaks particles into ever smaller sizes. Wind transports these fine particles long distances in suspension.

In all cases, the transportation stage of the rock cycle spreads weathered rock sediments to new locations for potential deposition and lithification into new sedimentary rock.

Deposition

As water transports the clay particles, its velocity eventually decreases to the point that it can no longer hold the particles in suspension. At this point, gravity takes over and the particles begin to settle out. The larger particles like sand tend to settle first, while the smallest clay particles remain in suspension the longest. The clay particles are so small that they essentially never fully settle out until the water becomes completely still. This is why clay deposits tend to accumulate in slow moving water environments like lakes, backwater areas of rivers, and coastal lagoons.

The composition of the deposited clay depends on the source rocks it originated from and how far it was transported. Clay deposits close to the source rock will more closely resemble its composition. The farther the clay travels, the more mixing occurs with other source materials. This gives rise to regional differences in clay composition and properties.

Burial

Once the clay particles have been deposited in the ocean, lake, or other water body, they become buried under successive layers of sediment over time. As more and more sediments from ongoing erosion accumulate, the clay deposits get buried deeper and deeper. These accumulating sedimentary layers exert pressure on the underlying clay deposits and squeeze out water, causing compaction. Burial to depths of 1-2 miles is common before the clay becomes lithified into rock.

The process of burial is driven by continual deposition of eroded sediments on top of previously deposited material. Major sediment sources include erosion from rivers carrying weathered rock debris into the ocean, sand and silt carried by ocean currents, volcanic ash deposits, and the skeletal remains of marine organisms. Over millions of years, these sediments gradually build up in successive layers, burying earlier deposits underneath.

Being buried under thick sediment layers isolates the clay from any meteoric water at the surface. This prevents further interaction with rain, groundwater or surface water, allowing the clay chemistry and composition to be preserved. The weight of overlying sediments pressing down causes compaction and lithification of the buried clay into rock.

Compaction

As sediment accumulates on top of the deposited clay particles, the weight and pressure increase significantly. With more and more layers building up over time, the clay particles become compressed and packed together under the sheer load. This compression squeezes out any remaining pore water and brings the clay particles into closer contact with each other.

The immense pressure compacting the clay also realigns the clay particles so they become flattened and aligned parallel to the direction of increasing pressure. This reorientation and tighter packing of the clay particles reduces the porosity and permeability of the sediment dramatically.

Over long periods of time under great pressure, the clay becomes an increasingly dense and cohesive mass. The compaction process transforms the loose clay sediment into a dense, hard mudstone or shale. The compressed clay particles are now bound together in a tight formation that is nearly impermeable to water.

Cementation

During the cementation stage, dissolved minerals crystallize between particle contacts, cementing the clay particles together. This process turns the clay into a hard, solid rock. The cementing agents are typically silica, iron oxides, calcium carbonate, or other minerals that were dissolved in groundwater. As the mineral-rich water flows through the spaces between the clay particles, the minerals come out of solution and crystallize. The most common cementing agent is silica, which forms the mineral quartz. Quartz and other cemented minerals fill the pores between clay particles and bind them tightly together. Over long periods of time, this process lithifies the clay into shale or mudstone. The length of time for complete cementation to occur can vary greatly, from thousands to millions of years depending on mineral concentration in the waters passing through.

Uplift

After the original sedimentary rock has become buried and cemented into a solid mass, tectonic forces can push up sections of the earth’s crust, raising the rock and forming mountains, plateaus, and ridges. This uplift process brings deep buried sedimentary layers back up to the earth’s surface. Uplift occurs at plate boundaries and is related to large-scale movements and collisions of the earth’s tectonic plates. As plates shift and collide, compressed sections are pushed upwards. Uplift can raise rocks buried under many kilometers of overlying material back up to the surface, exposing them to agents of weathering and erosion once again.

Uplifted sedimentary rock that contains clay deposits can therefore be exposed at the surface again after millions of years underground. The uplift process is key to making buried clay accessible again where it can continue the cycle of weathering, transportation and redeposition.

Exposure

After geologic uplift, erosion brings the clay deposits back up to the surface. Streams, rivers, ocean waves, wind, and glaciers all erode overlying materials to expose buried clay deposits. Through erosion, river valleys are cut deeper, hillsides are worn back, and coastlines move inland. This exposes clay that had been buried and formed underground.

Rock units containing clay minerals are uplifted over millions of years through plate tectonics. As mountains form and continents collide, previously buried sediments get pushed up and folded. Erosion then strips away the overlying rock and soil, revealing the clay beneath. Cliff faces, hillsides, and river cuts expose the cross sections of uplifted clay layers.

Clay exposed at the surface undergoes weathering as water interaction breaks the material down. However, new clay continued to be exposed over geologic timescales through the uplift and erosion cycle. This interplay of burial and exhumation is key to bringing sedimentary clay deposits to the surface where they can be utilized.

Conclusion

In summary, clay formation is a long process that starts with the weathering and breakdown of feldspar-rich rocks like granite. As these rocks are exposed to water, oxygen, carbon dioxide and organic acids, the feldspar minerals slowly decompose into clay minerals like kaolinite. These clay particles are then transported downstream by water and wind, eventually settling out and accumulating on riverbeds, lake bottoms and ocean floors in a process called deposition. Over long periods of time, hundreds to thousands of feet of sediments can accumulate on top of the deposited clay particles. The weight and pressure from overlying sediments causes the clay deposits to compact and harden through the process of cementation, expelling water and fusing clay platelets together. Tectonic forces can then uplift and expose these compacted clay sediments, now turned to rock, at the earth’s surface once again. With renewed exposure to weathering and erosion, portions of the clay-rich rock breaks down into new clay soil, completing the full cycle of clay formation from start to finish.