DSST Introduction to Geology common mistakes
DSST Introduction to Geology · Understand the traps and how to avoid them.
Confusing the physical properties of minerals with the properties of rocks, such as calling a rock 'cleavable' because its constituent mineral has cleavage.
Candidates learn mineral identification (cleavage, hardness, luster) and rock classification (texture, composition) separately but fail to integrate them, so they apply mineral-scale properties to rock-scale descriptions.
What to do instead
Explicitly distinguish mineral from rock: minerals are naturally occurring, inorganic solids with a definite chemical composition and ordered atomic structure; rocks are aggregates of one or more minerals. Practice by describing a granite: it contains quartz (no cleavage, vitreous luster) and feldspar (two cleavages at ~90°), but the rock itself does not have cleavage. Use a two-column table for each specimen: mineral properties vs. rock properties.
Reversing the relationships in the rock cycle, such as claiming sedimentary rocks form directly from magma or that metamorphic rocks always melt to form igneous rocks.
The rock cycle is often memorized as a simple linear sequence rather than a network of processes, and the role of intermediate steps (weathering, erosion, deposition, lithification, metamorphism, melting) is overlooked.
What to do instead
Draw the rock cycle as a diagram with arrows labeled by process (e.g., weathering → sediment → lithification → sedimentary rock; heat/pressure → metamorphic rock; melting → magma → cooling → igneous rock). Note that any rock type can transition to any other through appropriate processes. Practice tracing multiple pathways, such as sedimentary → metamorphic → igneous, and identify the specific process at each arrow.
Misidentifying the plate boundary type associated with specific geologic features, for example placing mid-ocean ridges at convergent boundaries or deep-sea trenches at divergent boundaries.
Candidates memorize feature names without linking them to the underlying plate motion (divergent, convergent, transform) and the associated stress regime (tension, compression, shear).
What to do instead
Create a three-column chart: boundary type, relative motion, and characteristic features. Divergent: tension, mid-ocean ridges, rift valleys, shallow earthquakes. Convergent: compression, trenches, volcanic arcs, fold mountains, deep earthquakes. Transform: shear, offset features, shallow earthquakes. For each feature, ask: does this form where plates move apart, together, or past each other? Use examples like the Mid-Atlantic Ridge (divergent), Andes (convergent), San Andreas Fault (transform).
Confusing the terms 'weathering' and 'erosion', such as stating that rocks are weathered by transport or eroded by chemical alteration.
Everyday language uses these terms interchangeably, and textbook definitions are often glossed over, leading to imprecise usage on exam questions that test the distinction.
What to do instead
Memorize precise definitions: weathering is the in-situ breakdown of rock (physical or chemical) without movement; erosion is the removal and transport of weathered material by agents (water, wind, ice, gravity). Practice with examples: frost wedging is weathering; a river carrying sand is erosion. When reading a question, identify whether the process involves movement; if not, it is weathering.
Misinterpreting the principle of superposition or cross-cutting relationships when determining relative ages, such as assuming a fault is older than the layers it cuts.
The principles are learned as isolated statements, and candidates fail to apply the logic that a feature must exist before it can be cut or deformed.
What to do instead
State each principle in your own words and apply it to diagrams: superposition-in undisturbed sequences, oldest layers are at the bottom; cross-cutting-a fault or intrusion is younger than the rocks it cuts; inclusions-the included fragments are older than the host rock. Practice with block diagrams: identify the oldest and youngest events by listing them in order. For example, if a fault cuts layers A, B, and C, the fault is younger than all three.
Reversing the order of seismic wave arrival, claiming S-waves arrive before P-waves or that surface waves are the fastest.
The names 'primary' and 'secondary' are not connected to their physical properties, and the speeds are memorized without understanding why P-waves are faster (compressional, can travel through solids, liquids, gases) and S-waves slower (shear, only solids).
What to do instead
Use the mnemonic P before S: P-waves are faster and arrive first; S-waves are slower and arrive second; surface waves are slowest but most destructive. Relate to particle motion: P-waves compress and expand (like a slinky), S-waves move particles perpendicular to wave direction. Note that S-waves cannot travel through liquids, which is why they disappear beyond the outer core. Practice with a seismogram: identify the first arrival as P, second as S, and later surface waves.
Misclassifying folds and faults, such as calling an anticline a syncline or labeling a normal fault as reverse.
The terms are visually similar and candidates rely on shape alone without considering the direction of stress or the relative movement of blocks.
What to do instead
For folds: anticlines arch upward (oldest rocks in core), synclines dip downward (youngest in core). For faults: normal faults result from tension, with the hanging wall moving down relative to the footwall; reverse faults result from compression, with the hanging wall moving up. Thrust faults are low-angle reverse faults. Draw and label each, and practice with block diagrams: identify the stress type first (tension vs. compression), then determine the fault type.
Confusing the processes of erosion, transportation, and deposition in river systems, such as thinking that a river deposits sediment when its velocity increases.
The relationship between velocity and sediment transport is counterintuitive: faster water can carry more and larger particles, while slower water causes deposition. Candidates often reverse this.
What to do instead
Remember: higher velocity → higher competence (can carry larger particles) and capacity (can carry more total sediment); lower velocity → deposition. Use the Hjulström curve concept: fine particles require higher velocity to erode than to deposit. Practice with scenarios: a river entering a lake slows down and deposits its load, forming a delta; a steep mountain stream erodes and transports. Identify the velocity change and then the resulting process.
Misidentifying the agents and features of glacial erosion, such as attributing U-shaped valleys to river erosion or confusing till with outwash.
Glacial features are often learned from images without understanding the processes, and the distinction between unsorted till (deposited directly by ice) and sorted outwash (deposited by meltwater) is blurred.
What to do instead
Link process to feature: glaciers erode by abrasion and plucking, creating U-shaped valleys, striations, and cirques. Depositional features: till is unsorted and angular; outwash is sorted and rounded, deposited by meltwater streams. Moraines are ridges of till. Practice by sorting a list of features into erosional vs. depositional and glacial vs. fluvial. For example, a U-shaped valley is glacial, a V-shaped valley is fluvial.
Misapplying the concepts of porosity and permeability, such as assuming a highly porous material always has high permeability or that groundwater flows through impermeable rock.
Porosity (pore space) and permeability (connectivity of pores) are related but distinct, and candidates often treat them as synonymous. Additionally, the role of the water table and aquifers is misunderstood.
What to do instead
Define porosity as the percentage of void space and permeability as the ability of a material to transmit water. Note that clay has high porosity but low permeability because pores are not connected; sand and gravel have high porosity and high permeability. An aquifer is a permeable unit that yields water; an aquitard is impermeable. Practice with examples: a clay layer can hold water but not transmit it quickly, forming a confining layer. Use Darcy's law conceptually: flow depends on permeability and hydraulic gradient.