Chapter 2: Earth's Activity and History: Changes on the Earth's Surface
Hello! In this chapter, we will explore "changes on the Earth's surface"—the everyday landscapes around us, such as mountains, rivers, and coastlines. Even solid bedrock is continually weathered, transported, and transformed into brand-new landforms over immense spans of time by solar energy, the atmosphere, water, and gravity.
You might think, "There are so many terms to memorize, this sounds tough..." but don't worry! These processes are directly connected to the natural phenomena you see around you every day. Once you grasp the basic cycle, everything will click right into place!
1. The Four Fundamental Processes Shaping the Earth's Surface
Rocks on the Earth's surface are broken down, moved, and reshaped primarily through the following four steps:
① Weathering
The process by which rocks on the surface gradually break down and disintegrate due to air, water, temperature changes, or the action of living organisms.
・Physical (Mechanical) Weathering: The breakdown of rocks into smaller fragments by physical forces without changing their chemical composition. Examples include water seeping into rock fractures and freezing—expanding in volume and wedging the cracks open (frost wedging)—as well as expansion and contraction caused by extreme day-night temperature swings (active in arid, cold, and high-mountain regions).
・Chemical Weathering: The alteration of rock minerals or their dissolution in water through chemical reactions with water, atmospheric oxygen, carbon dioxide, and other substances (active in warm, humid regions).
② Erosion
The process by which rocks and sediment weakened by weathering are worn away and stripped by forces such as running water (rivers), waves, wind, and glaciers.
③ Transportation
The process by which eroded sediment and gravel are carried downstream or to other locations by river currents, waves, wind, and other agents.
④ Deposition
The process by which transported gravel, sand, and mud settle and accumulate in areas where the flow slows down (such as lower river reaches, lakes, coastlines, and ocean floors).
[Key Takeaway]
"Weathering" means breaking down in place, "erosion" means wearing away and removal, "transportation" means moving, and "deposition" means settling and accumulating. These are easy to mix up, so make sure you keep them distinct!
[Common Mistake]
Students often assume that "weathering" only means being worn away by wind! Weathering actually refers to the entire suite of processes—driven by water, temperature fluctuations, and atmospheric gases as well as wind—that naturally cause rocks to become brittle and crumble.
2. Fluvial Landforms and Sediments
As river water flows from upstream to downstream, its continuous actions of erosion, transportation, and deposition create a diverse array of landforms.
◆ Downstream Progression of Fluvial Landforms
・Upper Course (Dominated by Erosion): The river gradient is steep, and the flow velocity is very high, promoting intense downward erosion (vertical incision) into the riverbed. As a result, steep-sided V-shaped valleys are formed.
・Middle Course to Alluvial Fans: At the valley mouth where a river emerges from the mountains onto a plain, the slope abruptly flattens and the flow velocity drops. Coarser sediments (gravel and sand) carried from upstream are deposited all at once, creating a fan-shaped, gently sloping alluvial fan.
・Lower Course to Floodplains: As the gradient becomes even gentler, lateral erosion (wearing away riverbanks) overtakes downward erosion, and the river begins to meander (flow in broad, looping curves). The flat plain where floodwaters spill over and deposit sediment is called a floodplain. Right along the riverbanks, sand and silt accumulate to form slightly elevated natural levees. The low-lying, poorly drained terrain behind these levees is known as a backswamp.
・River Mouth to Deltas: Where a river flows into the sea or a lake, its velocity drops virtually to zero. Fine sand and mud settle to build a delta, named for its resemblance to the Greek letter delta (\(\Delta\)).
◆ Terrestrial vs. Marine Sediments
The characteristics of deposited materials vary depending on the depositional environment.
・Terrestrial Sediments: Sediments that accumulate on alluvial fans, riverbeds, and floodplains. They tend to have relatively large grain sizes (gravel and coarse sand) and are generally poorly sorted (meaning the grains are not uniform in size).
・Coastal and Marine Sediments: Clastic particles transported from rivers into the ocean are sorted by waves and coastal currents. Larger-grained "sand" settles in shallow waters near the coast, while extremely fine-grained "mud" settles quietly in deep waters offshore.
[Did You Know?]
The grain size of sediment transported by running water is closely tied to flow velocity. When water moves swiftly, it can roll heavy gravel along; but as soon as the current slows down, the heaviest gravel drops out first (deposits). That is why you find large boulders and gravel upstream, while fine mud gathers at river mouths and across the sea floor.
3. Terrace Formation (Tectonic Uplift and Climate Change)
Step-like landforms known as terraces provide vital clues for understanding surface dynamics and Earth's history.
◆ River Terraces (Fluvial Terraces)
These are stepped combinations of flat tread surfaces and steep scarps found along river valleys.
1. During periods when the river gradient is gentle, the river meanders, creates a flat valley floor (floodplain), and deposits sediment.
2. When tectonic activity causes land to uplift, or when sea levels drop, the river gradient steepens. This revives its erosive power (downward incision), carving deeply into the old riverbed.
3. A new, lower flat surface develops, leaving the former valley floor stranded above as an elevated, flat terrace tread atop a cliff.
As this cycle repeats, multi-tiered step-like river terraces are constructed.
◆ Marine Terraces (Coastal Terraces)
These are step-like landforms found along coastlines.
A wave-cut platform carved flat by wave erosion is uplifted above sea level by tectonic activity or left exposed by a sea-level drop. It becomes an elevated, flat terrace tread fronted by a preserved sea cliff.
[Key Takeaway]
Terraces follow a fundamental rule: "The higher the terrace tread, the older the era in which it formed." By investigating the elevation and number of terrace levels, geologists can decipher how a region has uplifted in the past.
4. Landscape Evolution and Sediment Disasters
While the four processes shaping the surface (weathering, erosion, transportation, and deposition) rework the landscape across tens of thousands to hundreds of millions of years, they can also unleash sudden, violent events triggered by heavy rain or earthquakes, culminating in sediment-related disasters (mass wasting) that severely impact human life.
◆ Major Types of Sediment Disasters
・Cliff Collapses (Debris Falls / Rockfalls): Sudden failures where steep-slope earth and rock, saturated with groundwater from torrential rain, collapse all at once. They strike with very little warning and develop with extreme speed.
・Debris Flows: Disasters in which sediment, boulders, and water accumulated in mountain gullies or stream beds churn together into a slurry during intense downpours, rushing downhill at tremendous speeds. High vigilance is required at the heads of alluvial fans.
・Landslides: Mass-movement phenomena where a broad section of a relatively gentle slope slides gradually downhill along a slip plane (such as an underlying clay layer), heavily influenced by groundwater.
[Common Mistake]
It is easy to view sediment disasters merely as "sudden strokes of bad luck." From an Earth Science perspective, however, they are "natural processes of weathering and erosion accelerating catastrophically under the triggers of heavy rainfall and gravity." Understanding how landforms originate—such as alluvial fans or steep slopes, and the depositional or erosional processes that created them—is the cornerstone of effective disaster prevention and mitigation.
*Note: The detailed structures and field-observation techniques for strata that have piled up over extended geological periods will be covered in depth in another chapter: "Observing Sedimentary Strata."
Summary (Review Checklist)
・The 4-Step Cycle of Surface Dynamics: Weathering (physical & chemical) \(\rightarrow\) Erosion \(\rightarrow\) Transportation \(\rightarrow\) Deposition
・Fluvial Landforms: "V-shaped valleys" upstream, "alluvial fans" at valley mouths, "floodplains (natural levees & backswamps)" downstream, and "deltas" at river mouths
・Origin of Terraces: Formed through repeated cycles of tectonic uplift or sea-level change combined with erosion and deposition (higher terrace treads are older)
・Natural Processes and Disasters: The rapid, catastrophic progression of weathering, erosion, and deposition triggers sediment disasters like cliff collapses, debris flows, and landslides