What makes ocean waves, and why does the sea rise and fall as tides?
Describe how wind generates ocean waves and the parts of a wave, and explain that tides are caused by the gravitational pull of the Moon and Sun, including spring and neap tides (Virginia 2018 Earth Science SOL ES.10 and ES.11).
A SOL-level answer on waves and tides for the Virginia Earth Science EOC: how wind makes waves, the parts of a wave (crest, trough, wavelength, height) and what fetch controls, why tides are caused by the gravity of the Moon (and Sun), the daily pattern of two high and two low tides, and spring versus neap tides, with worked exam questions.
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What this topic is asking
Virginia Earth Science SOL standard ES.10 (with a link to the Earth-Moon-Sun content in ES.11) asks you to explain waves and tides. The EOC tests this with the parts of a wave, the role of wind and fetch, the cause of tides (the gravity of the Moon and Sun), the daily pattern of high and low tides, and the difference between spring and neap tides. Tides are a recurring item because they tie oceanography to astronomy.
How waves form and their parts
The size of waves depends on three factors: the wind speed, the length of time the wind blows, and the fetch, the distance of open water over which the wind blows. A storm with strong, long-lasting winds over a large fetch builds the biggest waves.
What tides are
The Moon's gravity pulls ocean water toward it, raising a bulge of water (high tide) on the side facing the Moon, and another bulge on the far side. As Earth rotates, a given coast passes through these bulges, giving most places two high tides and two low tides about every 24 hours and 50 minutes.
Spring and neap tides
"Spring" tides have nothing to do with the season; they refer to the water "springing" to a high range when the bodies align.
Try this
Q1. Name the three factors that determine the size of a wave. [2]
- Cue. Wind speed, the length of time the wind blows, and the fetch (distance of open water).
Q2. Explain why most coasts experience two high tides each day. [2]
- Cue. The Moon's gravity raises a water bulge on the near side and another on the far side of Earth; as Earth rotates, a coast passes through both bulges, giving two high tides.
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 marksWhat is the main cause of ocean tides? (A) wind blowing over the water. (B) the gravitational pull of the Moon and Sun. (C) earthquakes on the sea floor. (D) ocean currents.Show worked answer →
A 1-point multiple-choice item on tides.
The correct answer is B. Tides are the regular rise and fall of sea level caused mainly by the gravitational pull of the Moon (and, to a lesser extent, the Sun) on Earth's oceans. Wind (A) makes waves, earthquakes (C) can cause tsunamis but not the daily tides, and currents (D) are large-scale flows, not the tidal rise and fall.
The test rewards knowing the Moon's gravity (with the Sun's help) causes the tides.
VA Earth Science SOL 2024 (style)2 marksTwice a month the difference between high and low tide is especially large (spring tides). (a) Explain the alignment of the Sun, Earth and Moon that causes spring tides. (b) Explain why neap tides, with a smaller tidal range, occur at other times.Show worked answer →
A 2-point item on spring and neap tides.
(a) 1 point: spring tides occur when the Sun, Earth and Moon are in a line (at new moon and full moon), so the gravitational pulls of the Sun and Moon add together, giving the largest tidal range (highest highs and lowest lows).
(b) 1 point: neap tides occur when the Moon is at a right angle to the Sun relative to Earth (at the first and third quarter moons), so the Sun's pull partly cancels the Moon's, giving the smallest tidal range.
Markers reward the in-line alignment for spring tides in (a) and the right-angle alignment for neap tides in (b).
Related dot points
- Describe the features of the ocean floor (continental shelf, slope, abyssal plain, mid-ocean ridge, trench) and explain how temperature and salinity control seawater density (Virginia 2018 Earth Science SOL ES.10).
A SOL-level answer on the ocean for the Virginia Earth Science EOC: the features of the ocean floor and how they relate to plate tectonics, what salinity is and what changes it, how temperature and salinity control seawater density, and why this drives deep circulation, with worked exam questions.
- Explain surface currents (driven by wind and deflected by the Coriolis effect into gyres), deep density-driven circulation, and upwelling, and describe how currents transfer heat and affect climate (Virginia 2018 Earth Science SOL ES.10).
A SOL-level answer on ocean currents for the Virginia Earth Science EOC: wind-driven surface currents and gyres, the Coriolis effect, the difference between warm and cold currents, deep density-driven (thermohaline) circulation, upwelling and marine productivity, and how the Gulf Stream affects climate, with worked exam questions.
- Explain how Earth's rotation causes day and night, how the Moon's orbit causes its phases, and how the alignment of the Sun, Earth and Moon causes solar and lunar eclipses (Virginia 2018 Earth Science SOL ES.11).
A SOL-level answer on the Earth-Moon-Sun system for the Virginia Earth Science EOC: how rotation causes day and night, the cause and sequence of Moon phases, why eclipses happen only at certain alignments, the difference between a solar and a lunar eclipse, and why eclipses are not monthly, with worked exam questions.
- Describe estuaries and the Chesapeake Bay, including brackish water and its role as a nursery, and explain how watershed land use, runoff and sea-level rise affect coastal Virginia (Virginia 2018 Earth Science SOL ES.10 and human impact).
A SOL-level answer on the Chesapeake Bay for the Virginia Earth Science EOC: what an estuary is and why the Bay's brackish water makes it a nursery, the threats from nutrient runoff and eutrophication, the role of the watershed, sea-level rise and coastal flooding, and conservation, with worked exam questions.
- Explain the processes of the water cycle (evaporation, transpiration, condensation, precipitation, runoff, infiltration) and describe watersheds, groundwater and the water table (Virginia 2018 Earth Science SOL ES.9 and ES.10).
A SOL-level answer on the water cycle for the Virginia Earth Science EOC: the processes that move water (evaporation, transpiration, condensation, precipitation, runoff, infiltration), the energy that drives it, what a watershed and divide are, groundwater and the water table, and porosity and permeability, 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)