20 DSST Introduction to Geology practice questions, with answers and video walkthroughs

Twenty practice questions for DSST Introduction to Geology, in the real exam's style. Score 16 of 20 here and you are in passing range. Miss more than four, and the pattern of your misses points straight at the topic to review next. Every question below is explained on video, including why the wrong answers trap most test-takers.

What students preparing for this exam tell us is that steady practice in the exam's format, with an explanation for every miss, is what moves a score. Use these the same way: answer, check, and read why the wrong choices are tempting. For the full plan around them, see the DSST Introduction to Geology pillar guide.

Watch the full video walkthrough above, then test yourself on the twenty questions below. Each one is explained on the video, including why the wrong answers trap most test-takers.

Questions 1 to 5 (questions 1 to 5)

1. The boundary that separates the Earth's crust from the mantle below it is called the: Mohorovicic Discontinuity. The Mohorovicic discontinuity, often shortened to the Moho, is the boundary between the crust and the mantle. Seismic waves speed up sharply as they cross it, which is how geologists first detected it.

2. The rigid outer shell of the Earth that is broken into the moving tectonic plates is the: Lithosphere. The lithosphere is the cool, rigid outer layer that includes the crust and the top of the mantle. It is broken into the tectonic plates that ride on the softer asthenosphere beneath them.

3. New oceanic crust is created at a: Divergent Plate Boundary. At a divergent boundary two plates pull apart and magma rises to fill the gap, creating new oceanic crust. The Mid-Atlantic Ridge is the classic example of seafloor spreading.

4. The San Andreas Fault in California, where two plates slide horizontally past each other, is an example of a: Transform Boundary. At a transform boundary, plates grind sideways past one another rather than colliding or separating. The San Andreas is the best-known transform fault, and that grinding is what produces its earthquakes.

5. The Hawaiian Islands formed over a stationary plume of rising magma far from any plate boundary, known as a: Hot Spot. A hot spot is a fixed plume of magma rising through the mantle. As the Pacific plate drifts over it, a chain of volcanic islands forms, which is why the Hawaiian Islands line up in age order.

Questions 6 to 10 (questions 6 to 10)

6. On the Mohs hardness scale, which mineral is the hardest, with a value of ten? Diamond. The Mohs scale ranks scratch resistance from one to ten. Diamond sits at ten as the hardest, while talc sits at one as the softest, with quartz at seven in between.

7. Granite, which cools slowly deep underground and has large, visible crystals, is what kind of rock? Igneous. Granite is an igneous rock, formed from cooled and hardened magma. Because it cools slowly deep below the surface, its mineral crystals grow large enough to see, giving it that speckled, coarse texture.

8. Limestone and sandstone, formed from compacted layers of sediment, are classified as: Sedimentary Rocks. Sedimentary rocks form when sediment is deposited in layers and then compacted and cemented together. Limestone and sandstone are common examples, and their layering is why they often hold fossils.

9. Marble forms when limestone is exposed to intense heat and pressure deep in the crust. Marble is therefore a: Metamorphic Rock. Metamorphic rocks form when an existing rock is changed by heat and pressure without fully melting. Limestone transforms into marble this way, and shale transforms into slate.

10. The process by which an igneous rock can be weathered into sediment, then later melted back into magma, is described by the: Rock Cycle. The rock cycle describes how the three rock types change from one to another over time. Igneous rock can weather into sediment, become sedimentary rock, and later melt back into magma to start again.

Questions 11 to 15 (questions 11 to 15)

11. The principle that in undisturbed layers of sedimentary rock, the oldest layer is at the bottom, is called the: Principle of Superposition. The principle of superposition states that in an undisturbed sequence of sedimentary layers, each layer is older than the one above it. So the deeper you dig, the further back in time you go.

12. Radiometric dating measures the decay of unstable atoms to find the age of a rock. The time it takes for half of those atoms to decay is called the: Half-life. A half-life is the time it takes for half of the radioactive atoms in a sample to decay. Because each isotope has a constant, known half-life, geologists can use it as a clock to date very old rocks.

13. A fossil of a species that lived during only a short, well-known span of geologic time, used to date the rock layer it is found in, is called a: Index Fossil. An index fossil comes from a species that was widespread but existed for only a brief span of time. Finding one lets geologists assign a narrow age to the rock layer that contains it.

14. The slow breakdown of rock at the Earth's surface by water, ice, and air is called: Weathering. Weathering is the breakdown of rock in place by physical and chemical processes. Erosion is the next step, the transport of that broken material away by wind, water, or ice.

15. Which agent is responsible for carving most large river valleys and canyons over long periods of time? Running Water. Running water is the most powerful agent of erosion on land. Over millions of years a river cuts downward through rock, which is how the Colorado River carved the Grand Canyon.

Questions 16 to 20 (questions 16 to 20)

16. The point on the Earth's surface directly above where an earthquake begins is called the: Epicenter. The focus, or hypocenter, is the underground point where an earthquake actually starts. The epicenter is the spot on the surface directly above the focus, which is where shaking is usually strongest.

17. Which type of seismic wave travels fastest and is the first to be recorded at a seismograph station? P-wave. P-waves, or primary waves, are the fastest seismic waves and arrive first. They are compressional waves that can pass through both solids and liquids, unlike the slower S-waves.

18. The Richter scale and the moment magnitude scale are used to measure an earthquake's: Energy or Strength. Magnitude scales like the Richter and moment magnitude scales measure the energy released by an earthquake. Each whole number step represents a large jump in the size of the seismic waves.

19. Thick, sticky, silica-rich magma tends to trap gas and produce which kind of volcanic eruption? Explosive. Thick, silica-rich magma traps gas instead of letting it escape, so pressure builds until the volcano erupts explosively. Runnier, low-silica magma like Hawaii's lets gas escape and flows out gently.

20. A broad volcano with gently sloping sides built from many layers of thin, runny basaltic lava, like those in Hawaii, is called a: Shield Volcano. A shield volcano is built from repeated flows of thin, runny basaltic lava that spread far before hardening. That produces a broad, gently sloping shape, as seen in the Hawaiian volcanoes.

What to do with your score

The point of twenty questions is not the twenty, it is the pattern. Mark which topic each miss came from, then spend your next session on that topic alone. None of it requires starting over, only tightening the spots that cost you points.

A single twenty-question set is a snapshot, not a study plan. To pass with margin you need volume: enough questions, in the exam's format, with explanations that turn a wrong answer into a correction. That is what Flying Prep's DSST Introduction to Geology practice is built for: every question explained, with a free trial before you decide.