Transport and supply of fine particles
Heavy rainfall provides the kinetic movement necessary for the transport of clay, mud and silty particles. Southeast Asia, including Bangladesh and India, receives large amounts of monsoon rain, which then washes sediment from the Himalayas and surrounding areas into the Indian Ocean.
Warm, humid climates are better for eroding rocks, and there is more mud on ocean shelves off tropical coasts than on temperate or polar shelves. The Amazon system, for example, has the third largest sediment load on Earth, with intense rains that wash particles of clay, mud and silt from the Andes in Peru, Ecuador and Bolivia to the Atlantic Ocean.[6].
Rivers, waves and coastal currents separate silt, mud and clay from sand and gravel due to their different sedimentation rates. Longer rivers, with low slopes and large hydrographic basins, have a great capacity to carry mud. The Mississippi River is a good example of a long, low-gradient river with a large amount of water, capable of carrying particles from its headwaters in the north and depositing them in its delta.
Sediment formation
Below is a list of different areas that act as sources, modes of transport to the oceans and deposition environments necessary for the generation of clay rocks.
The Ganges in India, the Yellow River in China, and the Lower Mississippi in the United States are good examples of alluvial valleys. These systems have continuous sources of water and can provide materials through the sedimentation on their banks of the mud and silt that is deposited during floods, as well as when the choking of a meander occurs.[3].
For an alluvial valley to exist, there must be a very elevated area, usually uplifted by active tectonic movement, and a lower area, which acts as a conduit for water and sediments heading towards the ocean.
Glaciations generate large quantities of soil, which end up being deposited in glacial lakes.[3] The glaciers themselves can in turn easily erode clay rock formations, and this process increases their capacity to drag clays and silt.
The Northern Hemisphere contains 90 percent of the world's lakes over 500 km (310.7 mi) in length, mostly created by glacial dynamics. Lake deposits formed by glaciations, including areas of deep erosion, are abundant in numerous areas of the Earth's crust.[3].
Although glaciers have generated 90 percent of the lakes in the northern hemisphere, they are not responsible for the formation of ancient lakes, which are the largest and deepest in the world and hold up to twenty percent of today's oil reserves. They are also the second most abundant source of clay rocks, only behind those of marine origin.[3].
The ancient lakes owe their abundance of clay rocks to their long life and thick deposits, with strata conditioned in their generation by changes in oxygen concentration and rainfall, and which offer a consistent sequential chronology of the evolution of the paleoclimate.
A delta is a terrestrial or underwater deposit formed where rivers or streams deposit sediment into a body of water. Deltas, such as the Mississippi and Congo, have enormous potential for sediment deposition and can transport granular particles to the deep ocean waters. They are located at the mouths of rivers, where their waters become slower as they flow into a sea, depositing the silt and clay they carry.
Low-energy deltas, in which a large amount of mud is deposited, are found in lakes, gulfs, seas and small oceans, where coastal currents are not very strong. Deltas rich in sand and gravel are high-energy deltas, where waves dominate and mud and silt move away from the river mouth.[3].
Coastal currents, the contribution of sludge and waves are a key factor in the deposition of particles on the coast. The Amazon River carries 500 million tons of sediment, mostly clay, to the coastal region of northeastern South America. 250 million of these tons of sediment move along the coast, where they are deposited. The accumulated mud is more than 20 meters (65 feet) thick and extends 30 kilometers (18.6 mi) into the ocean.[3].
Much of the sediment transported by the Amazon may come from the Andes mountain range, so that the final distance traveled by these particles carried by the river is about 6,000 km (3,700 mi).[3].
70 percent of the Earth's surface is covered by oceans, and marine environments are where the largest proportion of clay rocks in the world are found. These deposits are characterized by their continuity on the ocean floor, unlike masses of continental clay rocks, which appear confined to certain regions.
In comparative terms, continents can be considered temporary managers of mud and silt, while the final long-term destination of clay rock-forming sediments is the oceans. The cycle of clay rocks shown below allows us to understand the processes of burial and re-emergence of the various particles.
There are various environments in the oceans, including deep-sea trenches, abyssal plains, volcanic seamounts, convergent, divergent, and transforming plate margins.[7] Continental masses are not the only important source of ocean sediments, but organisms living within the ocean itself also contribute to their formation.
The world's rivers transport the largest volume of suspended and dissolved loads of clay and silt to the sea, where they are deposited on ocean shelves. At the poles, glaciers and floating ice deposits fall directly to the sea floor. Winds can provide fine-grained material from arid regions, and explosive volcanic eruptions also contribute. All of these sources vary in the rate of their contribution to sediment generation.[7].
Particles move to the deeper parts of the oceans by gravity, and the processes in the ocean are comparable to those that occur in continental masses.
Location has a large impact on the types of clay rocks found in ocean environments. For example, the Apalachicola River, which drains into the subtropics of the United States, transports between sixty and eighty percent kaolinite mud, while the Mississippi transports only ten to twenty percent kaolinite.[8].