Earthquakes and Volcanoes Practice Questions

Correct answer: Hypocenter

The focus, also known as the hypocenter, is the point within the Earth where rocks snap or shift along a fault line, initiating an earthquake. The epicenter is the point on the Earth's surface directly above the focus. Seismic waves radiate outward from the focus in all directions.

Correct answer: P-wave (Primary wave)

Primary (P) waves are compressional waves that push and pull the ground in the direction of wave travel. Because they compress material, they can travel through solids, liquids, and gases, making them the first to register on seismographs. Secondary (S) waves are slower shear waves that can only travel through solid rock.

Correct answer: The outer core is liquid, so it cannot support the shearing motion S-waves require.

S-waves are shear waves that move particles perpendicular to the direction of the wave's travel, a motion that requires a rigid material to propagate. Because liquids have no shear strength, S-waves cannot pass through the liquid iron and nickel of the outer core. This phenomenon creates a vast 'S-wave shadow zone' on the opposite side of the globe from an earthquake.

Correct answer: Moment Magnitude Scale

The Moment Magnitude Scale (Mw) is the standard tool used by modern seismologists because it accurately estimates the energy of large earthquakes by assessing the physical dimensions of the fault slip. The Richter scale measures wave amplitude on a seismogram and can saturate during massive quakes. The Modified Mercalli scale evaluates subjective damage and human perception instead of raw energy.

Correct answer: Caldera

A caldera is a massive, basin-shaped depression formed when a large volume of magma is rapidly expelled, leaving the overhead volcano unsupported so that it collapses inward. Craters are much smaller openings formed by the explosive ejection of material around a vent. Famous examples of calderas include Yellowstone and Crater Lake.

Correct answer: Shield volcano

Shield volcanoes, like Mauna Loa in Hawaii, are shaped like a warrior's shield because their runny, basaltic lava travels long distances before solidifying, preventing the formation of steep cones. Stratovolcanoes are steeper because they erupt thick, viscous lavas alternating with ash layers. Cinder cones are small piles of loose volcanic debris built around a single vent.

Correct answer: Pyroclastic flow

Pyroclastic flows are high-density mixtures of scorching gas and rock fragments that can reach speeds exceeding 400 miles per hour and temperatures over 1,000 degrees Fahrenheit. They represent one of the most destructive and immediate hazards of explosive volcanism. A lahar is a volcanic mudflow composed of water and ash debris.

Correct answer: Lahar

Lahars have the consistency of wet concrete and can roar down river valleys at high speeds, burying entire towns miles away from the volcano. They can trigger during an eruption due to melting snow caps, or days later due to heavy rain soaking loose ash deposits. Pyroclastic surges are dry, gas-driven hazards.

Correct answer: Convergent plate boundaries

The Ring of Fire is dominated by convergent boundaries where oceanic plates sink beneath continental or younger oceanic plates into the mantle. This subduction forces water out of the descending plate, lowering the melting point of the overlying mantle rock and generating the silica-rich magma that fuels highly explosive stratovolcanoes.

Correct answer: The silica content and viscosity of the magma

Viscosity is a fluid's resistance to flow. Magma with high silica content (felsic/rhyolitic) is highly viscous and traps volcanic gases, building up immense pressure until it shatters explosively. Low-silica (mafic/basaltic) magma is runny, allowing gases to escape easily, resulting in quiet, flowing lava eruptions.

Correct answer: Smooth, billowy, or rope-like surfaces

Pahoehoe is a Hawaiian term for basaltic lava flows that possess a smooth, ropey texture, which forms when the surface skin cools while the molten lava underneath continues to advance. 'Aa' lava, by contrast, is cooler and slower, breaking into sharp, blocky, clinkery masses as it moves.

Correct answer: Transform boundary

Transform boundaries, such as the San Andreas Fault in California, involve tectonic plates grinding horizontally past one another. Because these boundaries do not involve plates separating to vent mantle material or subducting to melt crust, no magma is generated. This absence of magma makes volcanism rare, though shallow, violent earthquakes are frequent.

Correct answer: Seismograph

A seismograph detects and records the arrival times and amplitudes of seismic waves, providing the data needed to locate an earthquake's epicenter and calculate its magnitude. Tiltmeters are used to track the swelling of volcanoes as magma moves upward, while barometers measure atmospheric pressure.

Correct answer: Mantle plume or hot spot

The Hawaiian Islands are a classic example of intraplate volcanism fueled by a stationary hot spot, which is a plume of hot mantle material rising to the crust. As the Pacific plate moves slowly over this hotspot, a chain of volcanoes is built in sequence, leaving older, extinct islands down-drift and creating new ones over the plume.

Correct answer: How rocks store strain along a locked fault and snap back when the fault breaks

The elastic rebound theory describes how tectonic forces slowly warp and stress rocks along a locked fault line, storing elastic energy much like a stretched rubber band. When the friction holding the fault together is finally overcome, the rocks snap back into place, releasing that stored energy instantaneously as seismic waves.

Correct answer: Tsunami

A tsunami is a series of massive ocean waves triggered when a submarine earthquake forces a large volume of water upward or downward. These waves travel across the deep ocean at speeds matching jet airliners, growing dangerously tall when they slow down in shallow coastal waters. Storm surges are weather-driven and unrelated to tectonic events.

Correct answer: Sulfur dioxide

Sulfur dioxide (SO2) reacts with water vapor, oxygen, and dust in the atmosphere to create fine sulfate aerosols and sulfuric acid. This interaction creates 'vog', a volcanic smog that poses health risks to breathing passages and causes acid rain that damages vegetation. Carbon dioxide is also vented in large volumes but does not directly cause acid rain or vog.

Correct answer: 3

The time delay between the arrival of the fast P-wave and the slower S-wave reveals the distance to the epicenter, but not the direction. By drawing distance circles around three separate stations, the unique point where all three circles intersect pinpoints the exact epicenter, a process known as triangulation.

Correct answer: Volcanic ash

Volcanic ash consists of tiny, jagged particles less than 2 millimeters in size that are heavy, abrasive, and do not dissolve in water. When airborne, ash clouds can travel thousands of miles, disrupting air travel by melting inside jet engines and threatening human lung health when inhaled. Volcanic bombs and lapilli are larger fragments that land closer to the eruption site.

Correct answer: Normal fault

Normal faults occur when the crust is being pulled apart or extended by tensional forces, causing rock blocks to drop downward along the fault plane. Reverse and thrust faults are produced by compressional forces pushing rocks together, while strike-slip faults involve horizontal sliding along vertical fractures.