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DSST Introduction to Geology practice questions

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Question 1Core knowledge

Which layer of Earth's interior is composed mostly of solid iron and nickel and lies at the center?

  1. The asthenosphere
  2. The outer core
  3. The lower mantle
  4. The inner core Correct answer.

Explanation

The inner core is solid because pressure there is extreme even though temperature is very high, and its composition is dominated by iron with some nickel. The outer core is also iron-rich but liquid, which is why it generates the magnetic field through convection.

Question 2Core knowledge

A geologist finds a rock containing abundant olivine and pyroxene with very little silica. This composition is best described as:

  1. Evaporitic
  2. Felsic
  3. Clastic
  4. Mafic Correct answer.

Explanation

Mafic rocks are rich in magnesium and iron silicates such as olivine and pyroxene and are relatively low in silica. Felsic rocks are the opposite, dominated by quartz and feldspar with high silica content.

Question 3Core knowledge

Which type of plate boundary is characterized by plates moving apart, with new oceanic crust forming at a mid-ocean ridge?

  1. Convergent boundary
  2. Transform boundary
  3. Subduction zone
  4. Divergent boundary Correct answer.

Explanation

At divergent boundaries, tensional forces pull plates apart and magma rises to fill the gap, creating new crust along mid-ocean ridges. Convergent boundaries instead involve plates colliding, and transform boundaries involve lateral sliding.

Question 4Core knowledge

Which mineral is defined by a hardness of 10 on the Mohs scale and is the hardest naturally occurring mineral?

  1. Topaz
  2. Corundum
  3. Diamond Correct answer.
  4. Quartz

Explanation

The Mohs hardness scale ranks diamond at 10, the top of the scale, because of its strongly bonded carbon lattice. Corundum sits at 9 and quartz at 7, so both are softer than diamond.

Question 5Core knowledge

A sedimentary rock made of rounded gravel-sized particles cemented together is classified as:

  1. Quartzite
  2. Conglomerate Correct answer.
  3. Shale
  4. Limestone

Explanation

Conglomerate is a clastic sedimentary rock composed of rounded gravel-size clasts held together by a matrix or cement. Shale is made of fine clay particles, and quartzite is a metamorphic rock.

Question 6Core knowledge

The process by which water, wind, and ice loosen and remove rock and soil from one location is called:

  1. Metamorphism
  2. Deposition
  3. Erosion Correct answer.
  4. Lithification

Explanation

Erosion involves the transport of weathered material away from its source by agents such as water, wind, and ice. Deposition is the later dropping of that material, and lithification is the compaction and cementation of sediment into rock.

Question 7Core knowledge

Which seismic wave type is a compressional wave that travels fastest and can pass through solids, liquids, and gases?

  1. Rayleigh wave
  2. Love wave
  3. S-wave
  4. P-wave Correct answer.

Explanation

P-waves are longitudinal compressional waves and are the fastest seismic waves, capable of traveling through all three states of matter. S-waves are slower shear waves that cannot pass through liquids.

Question 8Core knowledge

A mineral that breaks along smooth, flat planes reflecting weak bonds in its crystal structure exhibits which property?

  1. Streak
  2. Cleavage Correct answer.
  3. Luster
  4. Fracture

Explanation

Cleavage is the tendency of a mineral to break along planes determined by weaker atomic bonds in its crystal lattice. Fracture describes irregular breakage with no preferred planes.

Question 9Core knowledge

A magma chamber beneath a volcano contains magma that cools slowly at depth, allowing large crystals to form. The resulting rock most likely has which texture?

  1. Aphanitic
  2. Phaneritic Correct answer.
  3. Vesicular
  4. Glassy

Explanation

Slow cooling at depth gives crystals time to grow large enough to be seen with the unaided eye, producing a coarse-grained phaneritic texture. Rapid cooling at the surface instead produces fine-grained aphanitic or glassy rocks.

Question 10Core knowledge

A metamorphic rock shows distinct alternating light and dark mineral bands. This texture is called:

  1. Porphyroblastic
  2. Foliation Correct answer.
  3. Granoblastic
  4. Slaty cleavage

Explanation

Foliation is the parallel alignment of platy or elongate minerals under directed pressure, producing banding such as the light and dark layers of gneiss. Slaty cleavage is a specific type of foliation in low-grade slate, but banding of light and dark minerals is broader foliation.

Question 11Core knowledge

In a subduction zone, oceanic lithosphere descends beneath a continental plate. Which feature is most likely to develop on the overriding plate above the descending slab?

  1. A transform fault
  2. An abyssal plain
  3. A mid-ocean ridge
  4. A volcanic arc Correct answer.

Explanation

Water released from the descending slab lowers the melting point of the overlying mantle, generating magma that rises to form a volcanic arc on the overriding plate. Mid-ocean ridges form at divergent boundaries instead.

Question 12Core knowledge

A river slows as it enters a wide, flat valley, dropping its sediment load. Which depositional feature is most likely to form where the river meets the flatter land?

  1. An alluvial fan Correct answer.
  2. A delta
  3. A moraine
  4. A sea arch

Explanation

When a stream leaves a steep mountain front and spreads onto flatter ground, velocity drops sharply and sediment spreads into a fan-shaped deposit called an alluvial fan. A delta forms specifically where a river enters a standing body of water such as a lake or sea.

Question 13Core knowledge

A geologist uses the principle of superposition to interpret a sequence of undeformed sedimentary strata. This principle states that:

  1. Fossils are the same age as the rock that surrounds them
  2. Any intrusion is younger than the rock it cuts
  3. Sedimentary layers were originally deposited horizontally
  4. Older layers lie below younger layers in an undisturbed sequence Correct answer.

Explanation

The principle of superposition holds that in an undisturbed sequence of layered rocks, the lowest layers are the oldest and the highest are the youngest. The other statements describe different principles of relative dating.

Question 14Core knowledge

A radioactive parent isotope has a half-life of 100 million years. If a mineral crystal contains 25 percent parent isotope and 75 percent daughter product, approximately how old is the crystal?

  1. 50 million years
  2. 200 million years Correct answer.
  3. 100 million years
  4. 400 million years

Explanation

After one half-life, 50 percent of the parent remains; after two half-lives, 25 percent remains. Since two half-lives have elapsed at 100 million years each, the crystal is about 200 million years old.

Question 15Core knowledge

Which of the following best describes the difference between a sill and a dike?

  1. A sill is parallel to existing rock layers and a dike cuts across them Correct answer.
  2. A sill is intrusive and a dike is extrusive
  3. A sill is always older than the surrounding rock and a dike is always younger
  4. A sill forms from lava and a dike forms from ash

Explanation

Both are tabular intrusive igneous bodies, but a sill is concordant, meaning it intrudes parallel to the existing layering, while a dike is discordant and cuts across the layers. Because both are intrusive, they are younger than the rocks they cut.

Question 16Core knowledge

A coastline experiences strong longshore drift moving sediment consistently in one direction. What is the most likely long-term effect on the shoreline?

  1. Rapid uplift of the coastline
  2. Uniform deposition along the entire coast
  3. Formation of a glacial moraine
  4. Beach erosion on the updrift side and deposition on the downdrift side Correct answer.

Explanation

Longshore drift transports sediment along the shore in the direction of the prevailing wave approach, removing material from the updrift area and depositing it downdrift. This produces erosion updrift and accretion downdrift over time.

Question 17Core knowledge

Which of the following is the best example of a chemical sedimentary rock?

  1. Conglomerate
  2. Rock salt formed by evaporation Correct answer.
  3. Sandstone
  4. Slate

Explanation

Chemical sedimentary rocks form when dissolved ions precipitate from solution, as when evaporating seawater deposits halite. Sandstone and conglomerate are clastic rocks made of transported fragments.

Question 18Core knowledge

A subsurface layer of permeable rock that transmits groundwater readily and can supply wells is called a(n):

  1. Permeability barrier
  2. Aquiclude
  3. Watershed
  4. Aquifer Correct answer.

Explanation

An aquifer is a body of rock or sediment that stores and transmits groundwater in usable quantities. An aquiclude is the opposite: a low-permeability layer that restricts flow.

Question 19Core knowledge

A mass of ice moves down a valley and deposits an unsorted mixture of clay, sand, and boulders at its terminus. This deposit is best described as:

  1. A chemical precipitate
  2. A well-sorted fluvial deposit
  3. Glacial till Correct answer.
  4. An evaporite

Explanation

Glacial till is unsorted and unstratified material deposited directly by ice, containing a wide range of particle sizes from clay to boulders. Fluvial deposits, by contrast, are typically sorted by water transport.

Question 20Core knowledge

Which factor most directly determines whether a volcano erupts explosively or effusively?

  1. The silica content and gas content of the magma Correct answer.
  2. The latitude of the volcano
  3. The age of the surrounding crust
  4. The depth of the nearest ocean

Explanation

High-silica, gas-rich magma is viscous and traps gas until pressure builds, producing explosive eruptions, while low-silica, gas-poor magma flows easily. Latitude, crustal age, and ocean depth do not control eruption style directly.

Question 21Core knowledge

An earthquake's epicenter is located by:

  1. Analyzing the color of the soil at the surface
  2. Comparing arrival times of P and S waves at three or more stations Correct answer.
  3. Determining the depth of the focus from one station
  4. Measuring the amplitude of a single seismogram

Explanation

The difference in arrival times of P and S waves gives the distance to the epicenter from each station. Triangulating distances from three or more stations locates the epicenter.

Question 22Core knowledge

Which of the following best explains why the Hawaiian Islands form a chain with progressively older islands to the northwest?

  1. The islands were all formed simultaneously by a single eruption
  2. The Pacific plate moves over a stationary mantle hotspot Correct answer.
  3. The islands are remnants of a continental collision
  4. The islands are the result of a divergent plate boundary

Explanation

A mantle hotspot stays fixed while the Pacific plate moves over it, so volcanoes form over the hotspot and are carried away as new ones form. This produces a chain of progressively older islands in the direction of plate motion.

Question 23Core knowledge

A rock sample contains minerals A and B. Mineral A shows a reaction rim of mineral C where it contacts mineral B, and mineral C is not found elsewhere in the rock. Which interpretation is most reasonable?

  1. C crystallized before A and B
  2. C was introduced by later weathering only
  3. A and B crystallized simultaneously without reaction
  4. A and B reacted to form C along their contact Correct answer.

Explanation

A reaction rim between two minerals indicates that they were not in equilibrium together and reacted at their boundary to produce a new mineral. The rim therefore records a reaction between A and B to form C.

Question 24Core knowledge

A sequence of sedimentary rocks shows a gradual change from coarse sandstone at the base to shale at the top. Assuming no unconformity, this pattern most likely indicates:

  1. A single volcanic eruption
  2. A period of glacial erosion
  3. A shallowing environment with increasing energy over time
  4. A deepening environment with decreasing energy over time Correct answer.

Explanation

Coarse sediment such as sand is deposited in higher-energy settings, while fine clay requires quieter water. An upward transition from sandstone to shale therefore suggests progressively deeper, lower-energy conditions.

Question 25Core knowledge

A confined aquifer is bounded above and below by impermeable layers. Which statement about a well drilled into this aquifer is most accurate?

  1. The aquifer cannot store groundwater
  2. The water table must coincide with the land surface
  3. The well will always be dry
  4. The water level in the well may rise above the top of the aquifer Correct answer.

Explanation

In a confined aquifer, water is under pressure, so the potentiometric surface can stand above the top of the aquifer, and in an artesian well water may rise above that level. The water table applies to unconfined aquifers, not confined ones.

Question 26Core knowledge

Which layer of Earth is composed mostly of solid iron and nickel and is under immense pressure?

  1. Crust
  2. Mantle
  3. Outer core
  4. Inner core Correct answer.

Explanation

The inner core is a solid sphere composed primarily of iron and nickel, kept solid by the immense pressure despite high temperatures. The outer core is liquid, the mantle is mostly solid silicate rock, and the crust is the thin outermost rocky layer. These layers differ in composition, physical state, and seismic wave behavior.

Question 27Core knowledge

A mineralogist observes that a mineral can be scratched by a copper coin but not by a fingernail. Which physical property is being tested?

  1. Streak
  2. Hardness Correct answer.
  3. Luster
  4. Cleavage

Explanation

Hardness is a mineral's resistance to scratching, commonly assessed using reference materials such as a fingernail (about 2.5 on the Mohs scale) and a copper coin (about 3.5). A mineral scratched by copper but not by a fingernail has a hardness between these values. Cleavage, luster, and streak describe other properties and are not determined by scratch resistance.

Question 28Core knowledge

Which of the following igneous rocks is extrusive and typically has a very fine-grained texture?

  1. Basalt Correct answer.
  2. Diorite
  3. Granite
  4. Gabbro

Explanation

Extrusive igneous rocks cool rapidly at or near Earth's surface, so their crystals are too small to see easily. Basalt is a common extrusive rock that forms from rapidly cooled mafic lava. Granite, gabbro, and diorite are intrusive rocks that cool slowly at depth and therefore have coarser crystals.

Question 29Core knowledge

Which of the following sedimentary structures is most useful for determining the original direction of sediment transport?

  1. Fossils
  2. Cross-bedding Correct answer.
  3. Mud cracks
  4. Ripple marks

Explanation

Cross-bedding consists of inclined layers within a sedimentary bed that dip in the direction the sediment was moving when deposited. Mud cracks indicate drying, ripple marks can be symmetric or asymmetric, and fossils indicate past life but not necessarily transport direction. Therefore, cross-bedding is the best indicator of paleocurrent direction among these choices.

Question 30Core knowledge

Which type of unconformity is characterized by sedimentary layers lying on top of eroded igneous or metamorphic rock?

  1. Angular unconformity
  2. Paraconformity
  3. Disconformity
  4. Nonconformity Correct answer.

Explanation

A nonconformity is an unconformity where sedimentary strata overlie eroded igneous or metamorphic rocks. An angular unconformity involves tilted layers below flat layers, a disconformity is an erosion surface between parallel sedimentary layers, and a paraconformity is a disconformity with no obvious erosion surface. The key distinction is the type of rock beneath the unconformity.

Question 31Core knowledge

Which of the following is a common metamorphic rock that forms from the metamorphism of limestone?

  1. Marble Correct answer.
  2. Schist
  3. Quartzite
  4. Slate

Explanation

Marble is a metamorphic rock composed mainly of calcite or dolomite and forms when limestone is subjected to heat and pressure. Quartzite forms from quartz sandstone, slate from shale, and schist from a variety of parent rocks under higher-grade metamorphism. The parent rock composition determines the metamorphic product.

Question 32Core knowledge

Which of the following best describes the focus of an earthquake?

  1. The point on Earth's surface directly above the epicenter
  2. The point within Earth where seismic energy is released Correct answer.
  3. The location of the seismometer that first records the earthquake
  4. The area where the most damage occurs

Explanation

The focus, or hypocenter, is the actual location within Earth where fault rupture begins and seismic energy is released. The epicenter is the point on the surface directly above the focus. Damage patterns and seismometer locations are consequences of wave propagation, not the definition of the focus.

Question 33Core knowledge

Which of the following is the primary driving force behind plate tectonics?

  1. Tidal forces
  2. Solar radiation
  3. Earth's magnetic field
  4. Mantle convection Correct answer.

Explanation

Mantle convection, along with slab pull and ridge push, is a major mechanism driving the motion of lithospheric plates. Heat from Earth's interior causes slow convection in the mantle, which helps move plates. Solar radiation, tides, and the magnetic field do not provide the primary energy for plate motion.

Question 34Core knowledge

A geologist finds a fault where the hanging wall has moved down relative to the footwall. Which type of fault is this?

  1. Thrust fault
  2. Normal fault Correct answer.
  3. Strike-slip fault
  4. Reverse fault

Explanation

In a normal fault, the hanging wall moves down relative to the footwall, typically due to tensional forces. A reverse fault has the hanging wall moving up, and a thrust fault is a low-angle reverse fault. Strike-slip faults involve horizontal movement parallel to the fault plane.

Question 35Core knowledge

Which of the following best describes the process of lithification?

  1. The compaction and cementation of sediment into solid rock Correct answer.
  2. The weathering of rock by chemical reactions
  3. The metamorphism of limestone into marble
  4. The melting of rock into magma

Explanation

Lithification is the process by which loose sediment becomes solid sedimentary rock through compaction and cementation. Melting produces magma, metamorphism alters existing rock, and weathering breaks down rock. Lithification specifically converts sediment to rock.

Question 36Core knowledge

A volcanic eruption produces a fast-moving, dense cloud of hot gas and ash that travels down the slope. What is this hazard called?

  1. Lahar
  2. Pyroclastic flow Correct answer.
  3. Tephra fall
  4. Lava flow

Explanation

A pyroclastic flow is a fast-moving current of hot gas and volcanic matter that moves down a volcano's slope. Lahars are volcanic mudflows, lava flows are molten rock streams, and tephra fall is the settling of airborne fragments. Pyroclastic flows are among the most dangerous volcanic hazards due to their speed and temperature.

Question 37Core knowledge

Which of the following best explains why the asthenosphere allows tectonic plates to move?

  1. It is a rigid layer that fractures easily.
  2. It is completely liquid and has no strength.
  3. It is a weak, partially molten layer that flows plastically. Correct answer.
  4. It is denser than the mesosphere and sinks.

Explanation

The asthenosphere is a partially molten, weak layer in the upper mantle that deforms plastically over long time scales, allowing the rigid lithospheric plates above to move. It is not completely liquid, nor is it a rigid fracturing layer. Its plasticity is key to plate motion.

Question 38Core knowledge

A geologist observes a sequence of sedimentary rocks where a layer of conglomerate is overlain by sandstone and then shale. Based on the principle of original horizontality, what can be inferred about the depositional environment?

  1. The sequence has been overturned by tectonic forces.
  2. The rocks were deposited in a single high-energy event.
  3. The environment changed from high energy to low energy over time. Correct answer.
  4. The conglomerate is younger than the shale.

Explanation

The principle of original horizontality states that sedimentary layers are deposited horizontally. A vertical sequence from conglomerate (high energy) to shale (low energy) indicates decreasing energy over time. The sequence is not necessarily overturned, and the conglomerate is older than the shale if undisturbed.

Question 39Core knowledge

A radioactive isotope has a half-life of 50 million years. If a rock contains 12.5% of the parent isotope, how old is the rock?

  1. 50 million years
  2. 100 million years
  3. 200 million years
  4. 150 million years Correct answer.

Explanation

After one half-life, 50% parent remains; after two, 25%; after three, 12.5%. Since three half-lives have elapsed, the age is 3 × 50 million = 150 million years. This assumes no loss or gain of parent or daughter isotopes.

Question 40Core knowledge

Which of the following best describes the difference between a joint and a fault?

  1. A joint is a fracture with no offset, while a fault has offset. Correct answer.
  2. A joint is a fold, while a fault is a fracture.
  3. A joint forms from compression, while a fault forms from tension.
  4. A joint is always vertical, while a fault is always horizontal.

Explanation

A joint is a fracture in rock along which there has been no significant displacement, whereas a fault is a fracture along which movement has occurred. Both are brittle structures, but only faults accommodate offset. Folds are bends in rock, not fractures.

Question 41Core knowledge

A soil profile shows a dark, organic-rich layer at the surface overlying a lighter, leached layer. Which soil horizon does the dark surface layer represent?

  1. B horizon
  2. A horizon Correct answer.
  3. C horizon
  4. O horizon

Explanation

The A horizon is the topsoil layer, typically dark and rich in organic matter and minerals. The O horizon is the organic layer above it, the B horizon is a subsoil accumulation zone, and the C horizon is weathered parent material. The dark surface layer in this profile is the A horizon.

Question 42Core knowledge

Which of the following best explains why deserts are often located at about 30° latitude?

  1. Warm ocean currents increase evaporation and rainfall.
  2. Descending dry air creates high pressure and inhibits precipitation. Correct answer.
  3. Mountains block moisture from reaching these regions.
  4. These regions receive the most direct sunlight year-round.

Explanation

At about 30° latitude, air that has risen at the equator and lost moisture descends, creating high-pressure zones with dry conditions. This descending air inhibits cloud formation and precipitation, leading to many of the world's deserts. Mountains can create deserts locally, but the global pattern is driven by atmospheric circulation.

Question 43Core knowledge

A stream flows from a mountainous area onto a flat plain. What feature is most likely to form at the base of the mountain?

  1. Delta
  2. Oxbow lake
  3. Alluvial fan Correct answer.
  4. Meander

Explanation

When a stream leaves a steep mountain front and enters a flat plain, its gradient decreases and it deposits sediment in a fan-shaped accumulation called an alluvial fan. Deltas form where streams enter standing water, and oxbow lakes and meanders form on low-gradient floodplains. The abrupt change in slope favors alluvial fan deposition.

Question 44Core knowledge

A retreating glacier leaves behind a ridge of unsorted sediment at its former terminus. What is this landform called?

  1. Esker
  2. Terminal moraine Correct answer.
  3. Drumlin
  4. Kettle

Explanation

A terminal moraine is a ridge of unsorted glacial till deposited at the maximum extent of a glacier. Drumlins are streamlined hills, eskers are sinuous ridges of meltwater deposits, and kettles are depressions from melted ice blocks. The ridge at the terminus is specifically a terminal moraine.

Question 45Core knowledge

Which of the following best describes the process of seafloor spreading?

  1. Continents move through the ocean floor without deforming it.
  2. New oceanic crust forms at mid-ocean ridges and moves away from the ridge. Correct answer.
  3. Oceanic crust is destroyed at mid-ocean ridges.
  4. Sea level rises due to melting glaciers.

Explanation

Seafloor spreading occurs at mid-ocean ridges where magma rises, solidifies to form new oceanic crust, and then moves laterally away from the ridge. This process creates new seafloor and is a key component of plate tectonics. It does not involve destruction at ridges or continents plowing through oceanic crust.

Question 46Core knowledge

A geologist finds a rock containing the minerals quartz, feldspar, and mica, with a coarse-grained texture. Which rock type is this most likely to be?

  1. Granite Correct answer.
  2. Obsidian
  3. Limestone
  4. Basalt

Explanation

Granite is a coarse-grained intrusive igneous rock composed mainly of quartz, feldspar, and mica. Basalt is fine-grained and mafic, limestone is sedimentary and composed of calcite, and obsidian is volcanic glass with no visible crystals. The mineral assemblage and texture point to granite.

Question 47Core knowledge

Which of the following best explains why the oceanic crust is subducted beneath continental crust at a convergent boundary?

  1. Oceanic crust is thicker than continental crust.
  2. Oceanic crust is denser than continental crust. Correct answer.
  3. Oceanic crust is hotter than continental crust.
  4. Continental crust is denser than oceanic crust.

Explanation

Oceanic crust is composed of denser mafic rocks (like basalt) compared to the less dense felsic rocks of continental crust. At a convergent boundary, the denser oceanic plate subducts beneath the less dense continental plate. Thickness and temperature differences are not the primary reason for subduction.

Question 48Core knowledge

A geologist is studying a landscape with limestone bedrock and observes caves, sinkholes, and underground drainage. Which process is primarily responsible for these features?

  1. Deposition by wind
  2. Erosion by glacial ice
  3. Mechanical weathering by frost wedging
  4. Chemical weathering by slightly acidic groundwater Correct answer.

Explanation

Caves, sinkholes, and underground drainage in limestone terrain are characteristic of karst topography, formed by chemical weathering as slightly acidic groundwater dissolves calcite. Frost wedging, glacial erosion, and wind deposition produce different landforms. The features described are classic karst.

Question 49Core knowledge

Which of the following best explains why some volcanoes have highly explosive eruptions while others have gentle lava flows?

  1. Explosive eruptions occur when magma is highly viscous and gas-rich. Correct answer.
  2. Gentle eruptions occur when magma is highly viscous and gas-rich.
  3. Explosive eruptions occur only at mid-ocean ridges.
  4. Explosive eruptions occur when magma is low-viscosity and gas-poor.

Explanation

Explosive eruptions are driven by gas that cannot easily escape from highly viscous magma, leading to pressure buildup and violent fragmentation. Low-viscosity, gas-poor magma allows gas to escape more easily, producing gentle lava flows. Viscosity and gas content are the key controls on eruption style.

Question 50Core knowledge

A geologist uses the principle of cross-cutting relationships. A fault cuts through a sedimentary layer but does not cut an overlying lava flow. What is the correct interpretation?

  1. The fault is older than the sedimentary layer but younger than the lava flow.
  2. The fault is younger than the sedimentary layer but older than the lava flow. Correct answer.
  3. The fault is older than both the sedimentary layer and the lava flow.
  4. The fault is younger than both the sedimentary layer and the lava flow.

Explanation

According to cross-cutting relationships, a fault is younger than the rocks it cuts and older than rocks that are not cut by it. The fault cuts the sedimentary layer, so it is younger than that layer. The overlying lava flow is not cut by the fault, so the fault is older than the lava flow.