Active Tectonics and Mountains

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What is the difference between weathering and erosion?

Weathering: the chemical or mechanical breakdown of soil, rock or dissolved material in place. Erosion: the removal and transport of that material from one location to another where it is deposited

What is the fundamental principle governing mountain building?

Mountain building is dictated by the balance between the tectonic flux of material into a mountain range and the erosional flux of material out of it

What is the surface uplift equation?

Surface uplift = rock uplift − denudation. If denudation is faster than rock uplift, mountain elevation decreases even if tectonic activity continues

What is isostatic rock uplift and what causes it?

The buoyancy-driven rebound of the crust when mass is removed from it. Erosion (denudation) removes mass, causing the crust to rise — approximately 80% of total denudation is compensated by isostatic rebound

How can erosion cause mountain peaks to rise higher than their original elevation?

If erosion is non-uniform (valleys erode faster than peaks), isostatic rebound lifts the entire crust. Peaks can rise above their original height even without any tectonic activity

How does the direction of precipitation affect mountain range morphology?

On the precipitation (pro) side, intense erosion pulls material to the surface and concentrates tectonic activity there. On the dry (retro) side, little erosion means little faulting. Topography and precipitation feedbacks are bidirectional

How does topography influence precipitation?

Mountains interact with and disrupt atmospheric circulation, creating persistent spatial differences in rainfall. Example: the Tibetan Plateau drives the strength of the South Asian Monsoon

What are the four main ways sediment is removed from mountain ranges?

Suspended load (fine silt/clay in river flow); solute load (dissolved ions, ~20% of suspended load); bed load (coarse sand to boulders rolling along riverbeds, ~10–15% of suspended load); aeolian transport (wind-blown sand, silt, clay)

What is the difference between sediment load and sediment yield?

Sediment load: total mass of sediment leaving a catchment per unit time. Sediment yield: load divided by catchment area — allows comparison between basins of different sizes

How does sediment load relate to drainage area?

Sediment load increases with drainage area (large rivers carry larger total loads, e.g. Amazon, Brahmaputra, Yangtze). Sediment yield decreases with drainage area — small mountain rivers have the highest yields per unit area

What controls sediment yield at a global scale (Pelletier, 2012)?

Topographic slope is the strongest control — steep slopes produce high yields. Rainfall and vegetation are also important but show less variation than slope

Why are mountain rivers particularly high in suspended sediment relative to solutes?

Steep slopes and rapid erosion (including landslides) deliver large volumes of unweathered rock fragments to rivers. Less time for chemical dissolution means proportionally more sediment and less dissolved material

What makes measuring long-term erosion rates difficult?

Modern measurements are snapshots over human timescales. Rare events like earthquakes can increase suspended sediment concentrations by 3–7× (e.g. Wenchuan earthquake, M7.9), making short-term data unrepresentative of long-term rates

What two approaches allow longer-term erosion rate measurements?

Cosmogenic radionuclide geochemistry (measures exposure age of surface rocks) and offshore sedimentary basin records (accumulation rates and provenance of sediment over geological timescales)

What can offshore sediment cores tell us about mountain building?

Sediment age, accumulation rates, grain size (transport distance), and geochemical provenance (source area) — together revealing timing, duration and drivers of mountain building episodes

What did Clift et al. (2005) discover using neodymium isotopes in the Indus Fan?

Sediment from the Himalayas (rather than Pakistan/Arabian Shield) became the dominant source after ~5 Ma, coinciding with a doubling of sedimentation rate — indicating major drainage reorganisation driven by tectonic uplift

What happened in the 'Three Rivers' area of southeastern Tibet around 40 Ma?

Motion along a fault in Tiger Leaping Gorge caused the Yangtze River to capture drainage that previously flowed through the Red River. The Three Gorges began to be cut ~40 Ma as the Yangtze reversed eastward — evidenced by neodymium isotope changes in river sediments

What is the key lesson from large-scale river capture events?

Even large river systems are transient — they can be diverted, blocked or captured over geological time. This changes sediment and nutrient delivery to the sea and affects distributions of plants and animals

What are hillslopes and channels in the context of mountain morphology?

Hillslopes: surfaces connecting ridgelines to streams, delivering water and sediment to the fluvial network; low upslope contributing areas, variable gradients

Channels: where flow concentrates; steepness decreases downstream as drainage area increases

How does slope change with drainage area for hillslopes versus channels?

Hillslope gradient increases or remains high with increasing drainage area (moving downslope from the ridgeline)

Channel gradient (steepness) decreases with increasing drainage area — a fundamental property of river systems

What is diffusive sediment transport on hillslopes?

Grain-by-grain movement driven by slope gradient, influenced by physical, chemical and biological processes (e.g. soil creep, rainsplash, burrowing animals). Produces smooth, rounded, soil-mantled hillslopes

What is the threshold slope concept in hillslope geomorphology?

When hillslopes become very steep, sediment transport transitions from slow grain-by-grain diffusion to rapid shallow landsliding. Transport rate increases dramatically as slope approaches a critical (threshold) value

What is the difference between shallow and deep-seated landslides?

Shallow landslides: occur in loose sediment on steep soil-mantled slopes; very efficient at removing material

Deep-seated/bedrock landslides: involve entire hillslopes (up to km across); produce planar rather than rounded hillslope forms

What are debris flows and why are they important in mountain systems?

Mixtures of sediment and water behaving as a slurry in steep channels (slopes >10%). They are the mechanism by which hillslopes 'hand off' sediment to the larger channel network, and are a major hazard capable of rapidly transporting large boulders (e.g. Sichuan 2019, Montecito, California)

What are the general downstream trends in mountain river channels?

Decreasing slope (gradient), decreasing bedrock exposure, increasing sediment cover, and decreasing grain size — reflecting the transition from erosional headwaters to depositional lower reaches

What is a sediment routing system?

The network of pathways sediment travels from mountain peaks to river outlets or ocean basins. Different processes dominate at different stages: diffusive hillslope transport → landslides → debris flows → fluvial channel transport

What is uniformitarianism and who developed it?

The principle that the present is the key to the past — modern processes and rates can be used to interpret Earth history. Originated with James Hutton and popularised by Charles Lyell in Principles of Geology (1830–1833)

What is neocatastrophism and when is it relevant?

The view that large, rare events with no modern analogue (e.g. bolide impacts, supervolcanoes, mass extinctions) have had profound effects on Earth history and cannot be understood through uniformitarian reasoning alone

What is the Gutenberg-Richter law?

LogN = a − bM, where N = number of earthquakes, M = magnitude, and a and b are constants. It describes the frequency-magnitude relationship: each unit increase in magnitude corresponds to a ~10× decrease in frequency

What does the b-value in the Gutenberg-Richter law tell us?

The slope of the frequency-magnitude line. A small b-value means proportionally more large earthquakes; a large b-value means proportionally more small earthquakes. It varies by region and fault

What is frequency of exceedance in earthquake hazard assessment?

How often an earthquake of at least a given magnitude is expected to occur. Used alongside peak ground acceleration (PGA) maps to assess seismic hazard for a region

Why is earthquake impact not simply a function of magnitude?

Impact depends on proximity to population, building quality, secondary hazards (e.g. tsunami, nuclear accidents) and social vulnerability. A M9 earthquake in Japan (2011, ~21,000 deaths via tsunami) vs a M8+ in Peru (2019, 2 deaths) illustrates this

What were the three models of earthquake recurrence proposed as alternatives to the simple periodic model?

Time-predictable model (constant strain threshold, variable earthquake size)

slip-predictable model (variable strain threshold, but earthquake always returns to same base state — longer gap = larger quake)

clustered model (earthquakes occur in clusters separated by quiet periods of 10–10,000 years)

What happened at L'Aquila in 2009 and what were the legal consequences?

A M6.3 earthquake killed 309 people. Prior seismic swarms were not acted upon; a government official told residents to stay home. Scientists and officials were convicted of manslaughter, then the scientists were cleared on appeal — highlighting the limits of probabilistic earthquake forecasting

What are the three phases of the earthquake cycle?

Pre-seismic: elastic strain accumulation, no fault movement.

Coseismic: rapid strain release during the earthquake (seconds to hours)

Post-seismic: decaying relaxation and strain accumulation (hours to years)

What is exhumation in the context of mountain building?

The movement of rock to the Earth's surface through the combined effects of tectonic uplift and erosion (denudation). Erosion removes overlying material, allowing deeper rocks to be exposed at the surface

What drove the 2015 Gorkha earthquake in Nepal and what were its impacts?

Movement along the Main Himalayan Thrust (continental-continental convergence). The earthquake caused several metres of fault slip, killed ~9,000 people, injured 22,000, and caused ~$10 billion in damage (~50% of Nepal's GDP)