Lesson 2 of 6 · Read the order written into the ground
Read the order written into the ground
10 minute lesson · 3 checks along the way
Your goal for this lessonUse landform relationships to reconstruct a sequence without inventing an absolute age.
Words with a dotted underline open a short definition right here in the lesson.
Building a volcanic landscape
Olympus Mons is a : repeated flows of relatively fluid lava built a very broad edifice with generally gentle slopes. Its scale records accumulated geological activity, not one enormous explosion. Other volcanoes cluster around the Tharsis region. A photograph can reveal their shapes and relationships, but estimating height requires a reference surface and measurements such as stereo topography or laser altimetry.
Even a familiar statement like “the mountain is this high” hides a choice. Height above nearby plains differs from elevation above a planetary . Mars has no sea surface defining everyday sea level. Compare measurements using the same convention before ranking mountains. Large size alone also does not tell you whether an eruption is happening now; that requires separate evidence of recent or ongoing activity.
A canyon need not begin as a river
Valles Marineris stretches across a substantial part of Mars. Its development involved stretching, faulting, and collapse of the crust, associated with the broader Tharsis region. Later processes, including landslides and water-related modification, changed parts of the system. Calling it a larger version of Earth’s Grand Canyon can therefore be misleading if that comparison silently assigns the same river-carving origin to both.
Use the global Viking mosaic to locate the long canyon system and nearby volcanic terrain. It supplies geographic context, not a complete process history. Several episodes can contribute to one present-day shape. An initial fracture may guide later erosion; a landslide may bury older layers. A useful geological explanation identifies which observation supports each step instead of making every visible feature the product of one event.
Viking Orbiter observations assembled into a global mosaic. NASA combined roughly 1,000 frames, normalized the mosaics, interpolated gaps, and synthesized the green channel. The view places Valles Marineris alongside the broad volcanic terrain.
Look closely
Trace the long canyon and compare it with the nearby volcanic forms. Does their appearance establish that a river carved the entire canyon system?
See what to notice
No. The image locates and displays landforms. Explaining their origin requires geological relationships and other measurements; Valles Marineris includes tectonic stretching and collapse with later modification. The assembled mosaic is not a single instantaneous exposure.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Work through a decision
Work the evidence: before and after
Imagine a mapped exposure with three features. Impact crater A already exists when lava flow B crosses and buries part of its rim. Fracture C then cuts both the exposed crater material and flow B. Given those observed relationships, the sequence is A, then B, then C. The covering and cutting relationships provide the ordering; the letters themselves carry no age information.
Now ask how many million years separated the events. Nothing in the description supplies that answer. The flow could have followed the impact quickly or much later. A rock’s radiometric age, if available and correctly interpreted, could add a numerical constraint. A second crater on top of B would be younger than B, but its order relative to C would remain unknown without another relationship.
Read a relative geological sequence
Original instructional diagram, not a real Mars map. Burial places B after A, and cutting places C after B. The sequence gives relative order without numerical ages; shape and dimensions are schematic.
Open full-size diagram in a new tab: Read a relative geological sequenceCheck 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Counting craters with conditions attached
On comparable surfaces, a larger accumulation of impact craters can indicate a longer exposed history. However, lava, erosion, and burial can erase craters, while impacts can produce clusters of secondary craters. Image resolution sets which sizes are detectable. Comparing raw counts from unequal areas or different image quality can create a false age contrast before any geological interpretation begins.
A careful comparison uses similar mapped areas, a declared size range, and a model for how impacts accumulate and survive. The resulting age estimate has uncertainty and assumptions. Geological mapping and crater statistics complement one another: one can establish local ordering, while the other can help constrain exposure history. Neither turns a global image into a precise timestamp for every mountain, canyon, or resurfaced patch it contains.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.
- Read: ESA: at the foot of Olympus MonsShield volcanism, landslides, lava flows, and their overlapping histories.
- Read: ESA: the Solar System’s grandest canyonExtension, faulting, collapse, and later changes to Valles Marineris.
- Read: NASA: global color views of MarsHow the Viking mosaic was assembled and its colors processed.