How Do Igneous Rocks Turn Into Sedimentary

From Molten Rock to Sediment: The Journey of Igneous Rocks to Sedimentary Rocks

Igneous rocks, born from the fiery depths of volcanoes and the slow cooling of magma beneath the Earth's surface, might seem worlds apart from the layered sandstones and shales that form sedimentary rocks. Yet, the seemingly immutable nature of igneous rocks is ultimately transient. Through a long and complex process involving weathering, erosion, transportation, deposition, and lithification, igneous rocks can, over vast stretches of geological time, transform into sedimentary rocks. This journey is a cornerstone of the rock cycle, a fundamental process shaping our planet’s landscape and preserving the history of Earth.

The First Step: Weathering – Breaking Down the Giants

The transformation begins with weathering, the process of breaking down rocks at or near the Earth's surface. This isn't a single process, but a combination of several mechanisms acting in concert:

1. Mechanical Weathering: Physical Fragmentation

Mechanical weathering, also known as physical weathering, involves the physical breakdown of igneous rocks into smaller pieces without altering their chemical composition. Several factors contribute to this:

• Frost wedging: Water seeps into cracks in the rock, freezes, and expands, widening the cracks and eventually breaking the rock apart. This is particularly effective in colder climates.
• Exfoliation: As pressure on buried igneous rocks is reduced during uplift and erosion, the rocks expand and crack along parallel sheets, creating onion-like layers.
• Abrasion: Rocks are worn down by friction against other rocks, ice, or water. This is particularly common in fast-flowing rivers or glaciers.
• Biological activity: Plant roots growing into cracks can pry rocks apart, and burrowing animals can further break down the rock fragments.

2. Chemical Weathering: Altering the Composition

Chemical weathering, on the other hand, involves the alteration of the rock's chemical composition. This is driven primarily by reactions with water, oxygen, and acids:

• Hydrolysis: Water reacts with minerals in the igneous rock, breaking them down and forming new minerals, such as clays. Feldspars, a common mineral in many igneous rocks, are particularly susceptible to hydrolysis.
• Oxidation: Oxygen reacts with minerals containing iron, such as those found in many mafic igneous rocks, causing them to rust and weaken. This process is responsible for the reddish-brown coloration often seen in weathered rocks.
• Carbonation: Carbon dioxide in the atmosphere dissolves in rainwater, forming a weak carbonic acid. This acid reacts with minerals like calcite and feldspar, dissolving them and releasing ions into solution.
• Acid rain: Pollution can increase the acidity of rainwater, accelerating the chemical weathering process and causing more damage to igneous rocks.

The combined effect of mechanical and chemical weathering transforms the initially solid, coherent igneous rock into a collection of smaller fragments, ranging from large boulders to microscopic clay particles. These fragments, along with dissolved ions, represent the raw materials for the next stage in the journey to sedimentary rock formation.

Stage Two: Erosion and Transportation – Moving the Fragments

Erosion is the process of removing weathered rock material from its original location. This is achieved primarily through the actions of:

• Water: Rain, rivers, and streams carry away weathered fragments, transporting them downstream. The size and type of sediment transported depend on the velocity of the water. Faster-flowing water can carry larger particles, while slower water carries finer sediments.
• Wind: Wind can carry fine particles like sand and dust over long distances, forming deposits like sand dunes.
• Ice: Glaciers act as powerful agents of erosion, picking up and transporting large volumes of rock fragments, both large and small. Glacial deposits often contain a wide range of sediment sizes and types.
• Gravity: Mass wasting events, such as landslides and rockfalls, move weathered material downslope, contributing to sediment transport.

The transportation of these weathered fragments can cover vast distances, with sediment traveling from high mountain ranges to low-lying plains and even into oceans. During transport, further weathering and erosion may occur, modifying the size and shape of the particles and further refining the sediment. The distance of transport, as well as the transportation medium, profoundly influence the characteristics of the resulting sediment.

Stage Three: Deposition – Settling Down

Deposition is the process by which sediment is laid down or dropped off at a new location. This occurs when the energy of the transporting agent (water, wind, or ice) decreases, making it unable to carry the sediment any further. Factors influencing deposition include:

• Velocity of the transporting agent: As water slows down, for instance, it deposits larger particles first, followed by progressively smaller ones.
• Water depth: Deeper water allows finer sediments to settle, while shallower water favors coarser sediments.
• Changes in topography: Changes in the landscape, such as a bend in a river or a change in slope, can cause sediment to accumulate.

Deposition leads to the accumulation of layers of sediment in various environments, including rivers, lakes, oceans, and deserts. These layers build up over time, creating thick sequences of sediment. The arrangement of sediment layers reflects the conditions prevailing at the time of deposition. For instance, the presence of cross-bedding (inclined layers within a larger layer) indicates deposition by wind or water currents.

Stage Four: Lithification – Turning Sediment into Rock

The final stage in the transformation of igneous rock to sedimentary rock is lithification, the process by which loose sediment is converted into a solid rock. This involves two main processes:

1. Compaction: Squeezing Out the Water

As layers of sediment accumulate, the weight of overlying layers compresses the underlying sediment, squeezing out water and air from the pore spaces between the particles. This compaction reduces the volume of the sediment and makes it more dense.

2. Cementation: Gluing the Particles Together

Cementation is the process by which dissolved minerals precipitate from groundwater, filling the pore spaces between sediment particles and binding them together. Common cementing minerals include calcite, quartz, and iron oxides. The type of cement influences the properties of the resulting sedimentary rock. For example, calcite cement produces a rock that is relatively soluble in acid, while quartz cement produces a much more resistant rock.

The combined effects of compaction and cementation transform loose sediment into a solid, coherent sedimentary rock. The type of sedimentary rock formed depends on the type and size of the original sediment. For instance, sand-sized particles form sandstone, clay-sized particles form shale, and gravel-sized particles form conglomerate.

The Igneous Rock's Legacy: Clues in the Sedimentary Rocks

While the igneous rock itself is transformed, its chemical signature and mineral composition leave their mark on the resulting sedimentary rocks. The type of igneous rock undergoing weathering – whether it is granite, basalt, or another type – influences the mineral composition of the sediment produced. For instance, the weathering of feldspar-rich igneous rocks leads to the formation of clay minerals, while the weathering of mafic igneous rocks releases iron oxides, which can contribute to the coloration of the resulting sedimentary rock.

By studying the sedimentary rocks, geologists can trace the history of the igneous source rocks and gain insights into the processes that shaped the landscape over millions of years. The composition of the sedimentary rock can provide clues about the parent igneous rock, the weathering environment, and the transport mechanisms involved in its formation. The layering and sedimentary structures within the rock preserve a record of the depositional environment and can indicate past climate conditions. The story of an igneous rock’s transformation into a sedimentary rock is a powerful testament to the dynamic processes that continually shape and reshape the Earth.