Question

Develop a geological "life history" of a sedimentary rock. Begin with a mass of igneous bedrock in a mountain area and end with your sedimentary rock being collected by a future geology student. Be as complete as possible.

Short answer

The life history includes weathering, erosion, deposition, compaction, cementation, and finally uplift and exposure for collection.

Step-by-step solution

Weathering of Igneous Bedrock

Mountainous regions often contain igneous rock formed from cooled magma. The first step in the journey to becoming a sedimentary rock is weathering, where physical and chemical processes break down the igneous rock into smaller particles or sediments.

Erosion and Transportation

The sediments generated from the weathering process are then eroded, meaning they are carried away from their original location by agents like water, wind, or ice. This process is crucial as it moves sediments to new locations, often into rivers and streams.

Sedimentation and Deposition

As the sediments are transported, they eventually settle out of the transporting medium and accumulate in layers in a new environment, such as a lakebed, riverbed, or ocean floor. This process is known as sedimentation.

Burial and Compaction

Over time, more layers of sediments build up on top of each other. The weight of the overlying sediments causes compaction, reducing the pore space and expelling any water trapped between the grains.

Cementation to Form Sedimentary Rock

With continuing pressure and chemical processes, dissolved minerals precipitate in the spaces between sediment grains, effectively 'gluing' them together. This process, known as cementation, ultimately transforms the compacted sediments into solid sedimentary rock.

Uplift and Exposure

Tectonic forces can uplift sedimentary rocks, exposing them to the Earth's surface. This exposure allows future geological students to access and study the sedimentary rock layers.

Collection by a Geology Student

Finally, a geology student comes across these exposed sedimentary rocks during fieldwork, collects a sample, and begins to analyze its composition and history.

Key concepts

Weathering Processes

Weathering is the initial step in the formation of sedimentary rocks. It's like the slow but steady phase of rock transformation. There are two main types of weathering: physical and chemical.

Physical weathering involves the breakdown of rocks through physical forces. These can include temperature changes causing expansion and contraction, freeze-thaw cycles where water enters cracks, freezes, expands and breaks the rock apart, or even the actions of plant roots growing into crevices.

Chemical weathering, on the other hand, occurs when rock minerals react with water, air, or other chemicals. This can lead to the rock material dissolving or forming new minerals. Both types of weathering persistently chip away at the mountainous igneous rocks, forming smaller fragments or sediments.

Erosion and Transportation

After weathering breaks rock into smaller pieces, these sediments must be moved from their origin. This movement is known as erosion and transportation, a dynamic phase in the rock cycle that often feels like nature's conveyor belt.

Different agents such as water, wind, ice, or gravity play the roles of movers. Water is a particularly powerful agent. It can pick up and carry sediments in streams and rivers, moving them over long distances. Wind can lift lighter sediments and transport them through the air, often forming dunes or deposits along the way. Meanwhile, glaciers slide down mountains, grinding and carrying rock debris along with them.

This stage is essential because it relocates the sediments to new areas where they will undergo the next stages of rock formation.

Sedimentation and Deposition

Sedimentation and deposition describe the settling process where transported sediments come to rest. This is where the journey of sediments begins to slow down and prepare for transformation into sedimentary rocks.

When moving agents like water slow down enough, they lose the energy required to carry sediments, which then settle out of the water column. Heavier and larger particles settle first, forming layers over time in different environments such as lakes, ocean beds, riverbanks, or deltas.

• Rivers can create point bars, depositing sediment along curves in the river
• Lakes often form fine-grained layers from suspended particles

Gradually, repeated deposition builds up thick sequences of sediments, setting the stage for compaction.

Compaction and Cementation

Layers of deposited sediments soon undergo significant changes as more layers accumulate above, leading to compaction and cementation.

Compaction occurs as the weight of overlying sediments squeezes out water and reduces pore spaces between grains, making the sediment layer denser. Think of it as packing down fluffier sediments into a much tighter compact mass.

Cementation then follows, as minerals dissolved in water precipitate out and act like a glue, binding the compacted sediment grains together. This chemical process solidifies the sediment into a cohesive sedimentary rock, completing its transformation. Slowly but surely, these processes turn loose sediments into rock solid formations.

Geological Fieldwork

Once sedimentary rocks are formed and possibly uplifted to the Earth's surface, they become a treasure trove of information for geologists. Geological fieldwork offers hands-on experience and helps scientists unravel the history of the Earth.

Through fieldwork, students and geologists observe rock formations, collect samples, and analyze their composition and texture. This practice can lead to understanding historical climate conditions, past events like volcanic eruptions or floods, and even gives clues about ancient life through preserved fossils in the sedimentary record.

By preparing for the field, scientists arm themselves with the necessary tools and knowledge to read the vibrant stories etched in stone formations.