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Unlock This CourseRivers don't just flow through landscapes - they actively shape them! But rivers aren't working alone. Two powerful forces called weathering and mass movement are constantly breaking down rocks and moving material around. These processes work together with rivers to create the valleys, cliffs and landforms we see today.
Think of weathering as nature's demolition crew - it breaks down rocks into smaller pieces. Mass movement is like gravity's delivery service - it moves all that broken material downhill towards rivers. Together, they're constantly reshaping our landscape!
Key Definitions:
This breaks rocks apart without changing their chemical composition. It's like smashing a biscuit - the pieces are still biscuit, just smaller! Common types include freeze-thaw weathering, where water freezes in cracks and expands, splitting rocks apart.
This changes the actual chemical makeup of rocks, like how iron rusts when it meets oxygen and water. Acid rain can dissolve limestone, creating caves and unusual rock formations.
Weathering happens in three main ways, each working differently but often together to break down rocks and prepare material for rivers to transport.
Physical weathering is all about breaking rocks into smaller pieces without changing what they're made of. It's particularly important in areas with extreme temperature changes or where water can get into rock cracks.
Water gets into cracks, freezes overnight, expands by 9% and splits the rock. Very common in mountain areas and during winter months.
Rocks heat up during the day and cool at night. This constant expansion and contraction creates stress that eventually cracks the rock.
Salt water evaporates in rock cracks, leaving salt crystals that grow and push the rock apart. Common in coastal areas.
Chemical weathering actually changes the minerals in rocks, creating new substances. It works faster in warm, wet climates and is responsible for creating clay soils and dissolving limestone landscapes.
Carbon dioxide mixes with rainwater to form weak carbonic acid. This dissolves limestone and chalk, creating caves and gorges.
Oxygen reacts with iron in rocks, causing them to rust and crumble. You can see this as the red-brown colour on weathered rocks.
Water reacts with minerals in rocks like feldspar, breaking them down into clay minerals. This is why granite eventually becomes clay soil.
The Yorkshire Dales show excellent examples of chemical weathering. Carbonation has dissolved the limestone bedrock, creating underground cave systems like Ingleborough Cave. Surface features include limestone pavements with deep cracks (grikes) separated by blocks (clints). The slightly acidic rainwater has slowly dissolved the limestone over thousands of years, creating a unique landscape that attracts thousands of visitors annually.
Once weathering has broken down rocks, gravity takes over! Mass movement is the downhill transportation of weathered material. It can happen slowly over years or suddenly in minutes, depending on conditions.
Several factors determine whether mass movement will occur and how fast it happens. Understanding these helps explain why some slopes are stable while others are prone to landslides.
Steeper slopes have stronger gravitational pull. Most mass movement occurs on slopes over 25°, though it can happen on gentler slopes if other factors are present.
Water adds weight and acts as a lubricant. Heavy rainfall or snowmelt often triggers mass movement events by saturating the soil and rock.
Mass movement can be classified by speed and the type of material involved. Each type creates different landforms and hazards.
Individual rocks break away from cliff faces and fall freely. Common where freeze-thaw weathering has loosened rocks. Creates scree slopes at cliff bases.
Large masses of rock and soil slide down slopes along a failure plane. Often triggered by heavy rain, earthquakes, or human activity like construction.
Saturated soil and rock debris flow downhill like thick liquid. Can travel several kilometres and bury everything in their path.
Very slow movement of soil particles downhill, just 1-2cm per year. You can spot it by bent tree trunks and tilted fence posts.
Saturated soil slides down curved failure surfaces, creating step-like features. Common on clay slopes and river banks.
Slow flow of waterlogged soil over frozen ground. Happens in areas with permafrost where the surface layer thaws but can't drain.
This famous mass movement event saw the four-star Holbeck Hall Hotel collapse into the sea after a major landslide. Heavy rainfall saturated the clay cliffs, causing them to slip along a curved failure surface. The hotel's gardens moved first, followed by the building itself over several days. The event highlighted how human development on unstable slopes can be dangerous and costly. The site remains empty today as a reminder of the power of mass movement processes.
Weathering and mass movement work as a team with rivers to create the valleys we see today. They provide rivers with the tools (loose rock and soil) needed for erosion and transportation.
River valleys don't just appear overnight - they're carved out over thousands of years through the combined action of weathering, mass movement and river processes.
Weathering attacks the valley sides, breaking down rock. Mass movement then transports this material downhill to the river, which carries it away. This process gradually widens valleys over time.
Material from mass movement accumulates on valley floors, providing rivers with sediment for erosion. Rivers then transport this material downstream, deepening and shaping the valley.
The interaction between weathering, mass movement and rivers creates distinctive landforms that geographers can identify and study.
Loose rock fragments accumulate at the base of cliffs after rockfall. These cone-shaped deposits are common in mountain areas and show ongoing physical weathering.
Steep valley sides created by river erosion at the base, with weathering and mass movement keeping them steep by removing loose material.
Fan-shaped deposits where rivers carrying mass movement debris suddenly slow down, often where mountain streams meet valley floors.
The Lake District's U-shaped valleys show how weathering and mass movement work with glacial and river processes. Helvellyn's eastern face demonstrates freeze-thaw weathering creating loose rock that falls to form scree slopes. Red Tarn below shows how this material is transported by streams. The combination of weathering on the fell sides and stream action in the valley bottom continues to shape this landscape today, making it a perfect outdoor laboratory for studying these processes.