How do earthquakes and volcanoes form, and how do we locate and measure them?
Explain how earthquakes and volcanoes form at plate boundaries, locate an earthquake epicenter using P-wave and S-wave arrival times, and relate volcano type to magma composition (Virginia 2018 Earth Science SOL ES.7).
A SOL-level answer on earthquakes and volcanoes for the Virginia Earth Science EOC: focus versus epicenter, the difference between magnitude and intensity, locating an epicenter from P-wave and S-wave lag at three stations, why earthquakes and volcanoes cluster at plate boundaries, and how magma composition controls eruption style, with worked exam questions.
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What this topic is asking
Virginia Earth Science SOL standard ES.7 includes the earthquakes and volcanoes that plate motion produces. The EOC tests this with the focus-versus-epicenter distinction, the magnitude-versus-intensity distinction, the triangulation method for locating an epicenter using P-wave and S-wave timing, and the link between magma composition and how violently a volcano erupts. These items often pair with a map of global earthquakes and volcanoes (which trace the plate boundaries) or a travel-time graph.
Focus and epicenter
Magnitude versus intensity
A heavily tested distinction:
- Magnitude is a single measure of the energy released, calculated from seismograph recordings (the Richter scale and the moment-magnitude scale). It does not change with where you are.
- Intensity is a measure of the effects and shaking felt at a particular place, which decreases with distance from the epicenter and depends on the local ground.
So one earthquake has one magnitude but many intensities, strongest near the epicenter and weaker farther away.
Locating the epicenter
Why earthquakes and volcanoes cluster at boundaries
Map the world's earthquakes and volcanoes and they trace the plate boundaries, because that is where plates grind, collide and pull apart. The Pacific Ring of Fire is the belt of subduction-zone volcanoes and earthquakes around the Pacific. This clustering is one of the strongest pieces of evidence for plate tectonics.
Volcanoes and magma composition
Volcanoes form where magma rises to the surface, chiefly at subduction zones (convergent boundaries), at mid-ocean ridges and rifts (divergent boundaries), and at hot spots. How a volcano erupts depends on the magma:
- Felsic (high-silica) magma is thick and sticky (high viscosity) and traps gases, so pressure builds and the eruption is explosive (Mount St. Helens), often building steep composite (stratovolcano) cones.
- Mafic (low-silica) magma is thin and runny (low viscosity), so gas escapes easily and the eruption is gentle, with flowing lava building broad shield volcanoes (Hawaii).
Try this
Q1. Explain the difference between the focus and the epicenter of an earthquake. [2]
- Cue. The focus is where the energy is released below the surface (on the fault); the epicenter is the point on the surface directly above the focus.
Q2. Explain why a volcano with thick, high-silica magma erupts more explosively than one with runny, low-silica magma. [2]
- Cue. Thick (high-viscosity) felsic magma traps gas so pressure builds and releases violently; runny (low-viscosity) mafic magma lets gas escape, giving gentle, flowing eruptions.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
VA Earth Science SOL 2023 (style)1 marksHow many seismograph stations are needed to locate the epicenter of an earthquake? (A) one. (B) two. (C) three. (D) four.Show worked answer →
A 1-point multiple-choice item on locating an epicenter.
The correct answer is C. Each station's P-wave and S-wave arrival lag gives the distance to the epicenter, which is drawn as a circle of that radius around the station. One circle gives only a distance, two circles intersect at two points, but three circles intersect at a single point, so three stations are needed to pin down the epicenter (triangulation).
The test rewards knowing that three stations and triangulation locate the epicenter.
VA Earth Science SOL 2024 (style)2 marksAt a seismograph station, the P-waves from an earthquake arrive several minutes before the S-waves. (a) Explain why the P-waves arrive first. (b) Explain how the size of the time gap between them helps find the distance to the epicenter.Show worked answer →
A 2-point item on seismic-wave timing.
(a) 1 point: P-waves travel faster than S-waves, so for the same distance the P-waves arrive first.
(b) 1 point: the farther the station is from the epicenter, the more the faster P-waves pull ahead of the slower S-waves, so a larger P-S time gap means a greater distance; reading the gap on a travel-time graph gives the distance.
Markers reward the speed difference in (a) and linking a larger time gap to a greater distance in (b).
Related dot points
- Describe the compositional and physical layers of Earth's interior (crust, mantle, outer core, inner core; lithosphere and asthenosphere) and explain how seismic waves provide the evidence (Virginia 2018 Earth Science SOL ES.7).
A SOL-level answer on Earth's interior for the Virginia Earth Science EOC: the crust, mantle, outer core and inner core, the lithosphere and asthenosphere, the difference between continental and oceanic crust, and how P-waves and S-waves and the shadow zone reveal that the outer core is liquid, with worked exam questions.
- Explain plate tectonic theory: the evidence for moving plates, mantle convection as the driving force, the features and motions at divergent, convergent and transform boundaries, and Virginia's geologic provinces (Virginia 2018 Earth Science SOL ES.7).
A SOL-level answer on plate tectonics for the Virginia Earth Science EOC: the evidence from continental fit, fossils and seafloor spreading, mantle convection as the driving force, the features at divergent, convergent and transform boundaries, hot spots, and Virginia's geologic provinces from the Coastal Plain to the Appalachian Plateau, with worked exam questions.
- Classify igneous, sedimentary and metamorphic rocks by how they form and explain the rock cycle, including how cooling rate, lithification, and heat and pressure transform rock (Virginia 2018 Earth Science SOL ES.5).
A SOL-level answer on rocks for the Virginia Earth Science EOC: how igneous, sedimentary and metamorphic rocks form, the link between cooling rate and crystal size, clastic versus chemical sediment, foliated versus nonfoliated metamorphic rock, and how the rock cycle transforms one type into another, with worked exam questions.
- Organize, analyze and interpret data using tables and graphs (line, bar, scatter), identify trends and the relationship between variables, and calculate the rate of change and percent (Virginia 2018 Earth Science SOL ES.1).
A SOL-level answer on data and graphs for the Virginia Earth Science EOC: choosing the right graph type, putting the independent variable on the x-axis, reading and describing trends, interpolating and extrapolating, calculating rate of change and percent deviation, and what a gradient on a map means, with worked exam questions.
- Describe air masses and the weather at cold, warm, stationary and occluded fronts, and explain how thunderstorms, hurricanes and tornadoes form (Virginia 2018 Earth Science SOL ES.9).
A SOL-level answer on weather systems for the Virginia Earth Science EOC: how air masses get their properties from their source region, the weather at cold, warm, stationary and occluded fronts, and how thunderstorms, hurricanes and tornadoes form, including their hazards in Virginia, with worked exam questions.
Sources & how we know this
- 2018 Science Standards of Learning (Earth Science) — Virginia Department of Education (2018)
- SOL Practice Items (All Subjects) — Virginia Department of Education (2024)