Lesson 5 of 6 · Transport, sediment & missing records
Erosion and deposition: follow the material
9 minute lesson · 3 checks along the way
Your goal for this lessonDistinguish material breakdown, transport, and accumulation, then use a bounded sediment budget to test a landscape interpretation.
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Three processes can leave different clues
breaks down or chemically changes material where it is. removes material, often with transport by wind, flowing liquid, ice, or gravity. Deposition is accumulation after transport. These processes may occur together across a landscape: a cliff loses fragments while a nearby basin gains sediment. Their names describe what happens to material, rather than three exclusive kinds of world.
The available agent matters. Earth’s liquid water is not a universal explanation for every channel-like feature. Wind moves particles on Mars, and gravity can move loose debris even without a thick atmosphere. An empty-looking valley may record a past environment that no longer exists. Identifying the agent requires patterns of shape, grain size, composition, slope, and context, not merely a familiar resemblance in one overhead image.
Check 1 of 3
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A resistant cap can preserve what surrounds it
Mars Express imaged eastern Arabia Terra, where cratered terrain, layered deposits, and erosion interact. Some pedestal craters stand on elevated platforms because protective helped preserve material while surrounding terrain was removed. The landscape records selective preservation: a raised patch need not have risen bodily from below. Different resistance to removal can create relief even when the starting surface was less uneven.
The mission’s account also discusses layered deposits whose origins could include volcanic processes or by wind or water. Layering is therefore evidence of a structured material history, not an automatic proof of a lake. A photograph can show exposed contacts and ; spectra, stereo topography, and regional mapping help test the proposed materials and processes. Similar-looking terraces can preserve quite different histories.
Mars Express HRSC observations of eastern Arabia Terra acquired October 26, 2009. Cratered platforms and contrasting textures motivate questions about preservation and removal. The scale bar is 20 kilometers and north points right; brightness alone does not identify a deposit’s material or transport agent.
Look closely
Locate a crater with a surrounding platform and compare it with adjacent lower-looking terrain. Does apparent relief alone prove that the platform was pushed upward?
See what to notice
No. Resistant ejecta can protect material while surrounding ground is eroded. Stereo topography can test heights, and material contrasts and regional contacts help distinguish selective erosion from uplift.
Work through a decision
Write a budget with a boundary and a time interval
Imagine a clearly bounded basin during one hypothetical interval. It begins with 1,000 tonnes of stored sediment. Another 600 tonnes enters from outside; 450 tonnes leaves the boundary. If no other gains or losses occur, final storage is 1,000 + 600 − 450 = 1,150 tonnes. Net accumulation is 150 tonnes. It is not 600 tonnes, because some transported material also exited.
Now suppose 100 tonnes of already-counted loose sediment is eroded from a slope inside the basin and deposited on its floor. That transfer changes where sediment lies within the boundary but does not change the basin’s total stored mass. A floor-only inventory would increase, so the answer depends on the chosen system. State whether measurements describe one landform, a whole catchment, or material crossing its edge before comparing their numbers.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Preservation can hide a large amount of change
A basin with zero net accumulation can still experience substantial incoming and outgoing transport. Likewise, a thin surviving layer might be the remnant of a much thicker deposit. changes sediment volume without necessarily changing its mass, so a thickness measurement cannot be converted into a mass budget without area and appropriate bulk density. Lost material and intervals of nondeposition create gaps in the geological record.
Use the Arabia Terra image to locate a resistant-looking platform and surrounding lower terrain, then propose a measurement rather than an instant explanation. Stereo heights could test relief; spectra could test contrasting materials; repeat imaging could reveal current movement at detectable scales. These observations cannot directly replay ancient erosion. They can, however, eliminate histories that fail to account for where material survives, where it is missing, and where it might have gone.
Check 3 of 3
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Put it together
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