ASBOG - Association of State Boards of Geology Geomorphology and Quaternary Geology Questions and Answers 2 — Questions and Answers
Question 1: What is the primary mechanism by which meandering rivers migrate laterally across their floodplains over time?
- Vertical incision into bedrock
- Lateral erosion of cut banks and deposition on point bars (Correct answer)
- Mass wasting of valley walls into the channel
- Avulsion — sudden channel abandonment and creation of a new course
Correct answer: Lateral erosion of cut banks and deposition on point bars
Meandering rivers migrate laterally by eroding the outer bank (cut bank) of bends through hydraulic action and depositing sediment on the inner bank (point bar) as a scroll bar sequence.
In a meandering river, centrifugal force and secondary helicoidal flow patterns direct erosion to the outer (concave) cut bank while slower near-bed flow deposits sediment on the inner (convex) point bar. Over time, this process migrates meander bends downstream and laterally outward. Meander migration rates vary from centimeters to meters per year depending on bank erodibility, discharge, and sediment load. Abandoned meander loops (oxbow lakes) form when a river cuts through a meander neck during flood.
Question 2: Which geomorphic surface is characterized by a broad, gently sloping bedrock erosion surface that truncates older structures and is mantled by thin residual sediments?
- Alluvial fan
- Pediment (Correct answer)
- Peneplain
- Fluvial terrace
Correct answer: Pediment
A pediment is a gently inclined bedrock erosion surface at the foot of a mountain range, cut by lateral planation of streams, commonly veneered by a thin sheet of alluvium in arid and semi-arid environments.
Pediments are characteristic landforms of arid and semi-arid regions, forming at the transition between mountain fronts and basin floors. They develop by sheet flooding and stream lateral planation that bevels bedrock to near-horizontal surface, often cut across multiple rock types. Pediments are typically veneered by a thin (< 1 m) layer of alluvium. They should be distinguished from alluvial fans (depositional, triangular, sediment-filled) and peneplains (regional low-relief surface produced by long-term subaerial erosion in humid climates).
Question 3: What Quaternary dating method is based on the accumulation of cosmogenic nuclides (such as 10Be) in surface rocks exposed to cosmic ray bombardment?
- Radiocarbon (14C) dating
- Optically stimulated luminescence (OSL) dating
- Cosmogenic nuclide exposure age dating (Correct answer)
- Uranium-series dating
Correct answer: Cosmogenic nuclide exposure age dating
Cosmogenic nuclide exposure dating uses the in-situ production of nuclides like 10Be, 26Al, and 36Cl in mineral surfaces by cosmic ray spallation to determine how long rock surfaces have been exposed at Earth's surface.
Cosmogenic nuclide surface exposure dating is a powerful Quaternary geochronology tool for dating landforms such as glacial moraines, lava flows, fault scarps, and river terraces. Cosmic rays penetrating the upper meter of Earth's surface produce nuclides like 10Be and 26Al in quartz by spallation. The concentration of these nuclides increases with exposure time at known production rates (which vary with latitude, altitude, and shielding). Ages from a few thousand to millions of years can be obtained, making it ideal for Quaternary geomorphic studies.
Question 4: What is the primary distinction between an alluvial fan and a debris fan in semi-arid mountain terrain?
- Alluvial fans are depositional while debris fans are erosional surfaces
- Alluvial fans are dominated by water-laid sediments; debris fans by mass movement deposits (Correct answer)
- Alluvial fans form only in humid climates; debris fans only in alpine environments
- Alluvial fans are steeper than debris fans
Correct answer: Alluvial fans are dominated by water-laid sediments; debris fans by mass movement deposits
Alluvial fans are constructed primarily by fluvial processes (stream flow and sheetflow depositing sorted sediments), while debris fans are built mainly by mass movements such as debris flows and rock avalanches depositing poorly sorted, matrix-supported material.
Both alluvial fans and debris fans form at the mouths of mountain canyons where channels widen and gradient decreases, causing deposition. Alluvial fans are dominated by fluvially deposited, moderately sorted stratified sediments deposited by sheet floods and braided stream systems. Debris fans are constructed primarily by debris flows, rock falls, and avalanches, producing poorly sorted, lobate deposits with surface boulders and matrix-supported fabric. Many fans in arid mountains are composite, showing evidence of both processes.
Question 5: What does a river's longitudinal profile describe, and what is a knickpoint?
- The cross-sectional shape of the channel; a point where channel width suddenly increases
- The elevation change along the river from headwaters to mouth; a steep reach or waterfall where the graded profile is interrupted (Correct answer)
- The gradient of the floodplain; a location where the floodplain narrows
- The sinuosity of the river; a sharp meander bend
Correct answer: The elevation change along the river from headwaters to mouth; a steep reach or waterfall where the graded profile is interrupted
A river's longitudinal profile plots elevation against distance downstream; a knickpoint is an abrupt steepening or waterfall that disrupts the smooth concave-up graded profile, typically due to base level change or resistant rock.
The longitudinal profile of a graded river is a smooth, concave-up curve representing equilibrium between erosion and deposition. Knickpoints (or nickpoints) are disruptions of this profile — steep reaches or waterfalls — caused by base level lowering (sea level drop, lake draining), tectonic uplift, or outcrops of resistant rock. Knickpoints migrate upstream over time by headward erosion. Their rate of migration can be used in tectonic geomorphology to date episodes of base level change.
Question 6: Which glacial depositional landform is an elongated, streamlined hill of till that formed parallel to ice flow direction?
- Esker
- Drumlin (Correct answer)
- Kame
- Kettle
Correct answer: Drumlin
Drumlins are streamlined, elliptical hills of till molded beneath an active glacier, with their long axis oriented parallel to ice flow, with a steeper upstream (stoss) face and gentler downstream (lee) face.
Drumlins are subglacial bedforms composed of till (and sometimes bedrock) that are sculpted by the flow of overriding glacial ice. Their characteristic asymmetric shape — steep blunt stoss end facing the direction of ice approach and tapered lee end — allows paleo-ice flow directions to be reconstructed. They commonly occur in swarms called drumlin fields. The exact mechanism of drumlin formation remains debated, with theories involving subglacial erosion, deposition, and meltwater erosion under deforming till.
ASBOG - Association of State Boards of Geology Geomorphology and Quaternary Geology Questions and Answers 2
What is the primary mechanism by which meandering rivers migrate laterally across their floodplains over time?