Earth Systems Science · Module 4

Why does one plate boundary pour out lava while another builds mountains?

Two boundaries a few hundred kilometres apart cause opposite disasters. Same heat inside. Different geometry and density.

Here is the science, math, and real data to build a boundary that behaves like the real one.

Start → Jump to my project Formula sheet
One ocean plate, two very different edges A cross-section, not to scale. On the left, a mid-ocean ridge: two plates pull apart, hot mantle rises, and runny lava erupts quietly, with only shallow earthquakes. The ocean plate moves right, thickening as it cools, until it bends down at a trench and sinks under a continent as a slab. Earthquakes follow the slab deeper and deeper. Where the top of the slab reaches about 100 kilometres deep, water released from it melts the mantle above, and an explosive volcano rises on the continent. Ridge: pulls apart quiet lava, shallow quakes trench arc volcano explosive continent ocean plate sinking slab quakes get deeper ≈100 km deep earthquake asthenosphere hot, solid rock that creeps not to scale
The same ocean plate is born at the ridge and dies at the trench. Each edge makes its own hazards.

Start here · The big picture

Moving plates melt rock, break rock, and raise rock

Four parts of Module 4, one system Four panels joined by arrows. Part 1, Plates, led by Ridge Runners: two plates move apart. Part 2, Volcanoes, led by Magma Makers: rock melts and erupts. Part 3, Earthquakes, led by Fault Line: a fault slips and sends out waves. Part 4, Mountains, led by Root Raisers: thick crust rises with a deep root. Under the panels: what moves, what melts, what breaks, what rises. Plates Part 1 · Ridge Runners Volcanoes Part 2 · Magma Makers Earthquakes Part 3 · Fault Line Mountains Part 4 · Root Raisers what moveswhat meltswhat breakswhat rises

Four parts, one driving question.

Your team leads one part and teaches it. The class needs all four to explain why volcanoes, earthquakes, and mountains happen where they do.

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Part 1 · Ridge Runners lead this part

Plates and the engine below

  • Say what a plate is and how thick it is.
  • Rank the forces that move plates.
  • Use magnetic stripes and GPS as evidence.
  • Name the three kinds of plate boundary.
  • Turn cm per year into km per million years.
Matching magnetic stripes on both sides of a spreading ridgeBands of rock form a mirror-image pattern on both sides of a dashed ridge axis. Arrows show the two plates moving apart; the rock at the ridge axis is the youngest. ridge axis: youngest rock

Part 1 · Key idea · The engine

A plate is the crust plus the top of the mantle

Ocean plate and continent plate, drawn to the same depth scale Two columns on one depth scale from 0 to 280 kilometres. Under the ocean: crust about 5 to 7 kilometres thick, then cold, rigid mantle rock down to about 100 kilometres; together they are the plate, or lithosphere. Under an old continent: crust about 30 to 40 kilometres thick, then rigid mantle down to about 200 kilometres. Below both lies the asthenosphere, hot solid rock that creeps. 050100150200250 km Ocean Old continent ↑ crust ≈ 5–7 km rigid mantle plate base ≈ 100 km crust ≈ 30–40 km rigid mantle plate base ≈ 200 km or more plate plate asthenosphere: hot, solid, creeps depth below the surface, drawn to scale

The lithosphere is the rigid crust plus the top of the mantle. That is the plate.

It averages about 100 km thick: nearly zero at ridges, 200 km or more under old continents.

Below it, the asthenosphere is hot, solid rock that creeps.

Part 1 · Key idea · The engine

The mantle is solid rock that flows very slowly

S waves cross the mantle but not the liquid outer core Earth cut in half, layers drawn to scale. An earthquake at the top sends S waves through the mantle to stations up to about 100 degrees away. No S waves reach the far side, because the outer core is liquid and S waves cannot travel through liquid. The mantle, which the S waves cross, must be solid. earthquake mantle: solid outer core liquid inner core: solid no S waves arrive on this side S waves layers to scale; paths simplified

S waves cannot travel through liquid, yet they cross the whole mantle. So the mantle is solid.

Over millions of years, hot solid rock still flows, like cold honey. Hot rock rises and cold rock sinks: convection.

Part 1 · Key idea · The engine

Sinking slabs pull the plates

Three forces on a plate, drawn by size A plate runs from a mid-ocean ridge on the left to a trench on the right, where its cold edge sinks as a slab. The largest arrow, slab pull, points down the sinking slab. A medium arrow, ridge push, points away from the high ridge. A small two-headed arrow under the plate is basal drag, which can help or resist. Faint loops in the mantle show that the sinking slab is part of the convection. slab pull strongest ridge push · second basal drag: helps or resists ridge trench cold plate the slab is the sinking, cold part of mantle convection

Slab pull: a cold, dense, sinking edge drags the rest of its plate. It is the strongest force.

Ridge push: the plate slides off the high, warm ridge. It comes second.

Plates are the cold top of mantle convection, not rafts riding on it.

Part 1 · Key idea · Density

Why does a 3.0 g/cm³ basalt plate sink into 3.3 mantle?

Rock densities against the hot mantle below a plate A dot plot of typical densities in grams per cubic centimetre. Continental crust, granite, 2.7. Ocean crust, basalt, 3.0. Mantle rock, peridotite, 3.3. Ocean crust deep in a subduction zone, eclogite, about 3.5. A dashed line at 3.3 marks the hot mantle under the plates. Rock left of the line floats; rock right of it sinks. A whole old ocean plate, mostly cold mantle rock, sits just right of the line, so it sinks. floats sinks Continental crustgranite Ocean crustbasalt Mantle rockperidotite Deep ocean crusteclogite Whole old platecold, mostly mantle 2.73.03.3≈3.5>3.3 2.62.83.03.23.43.6 density, g/cm³ · dashed line: hot mantle under the plates

Ocean crust alone would float: 3.0 is less than 3.3.

But a plate is mostly mantle rock, and cold rock is denser than the hot rock below. Old, cold plates sink.

Deep down, basalt turns into dense eclogite. Continents, at 2.7, never sink.

Part 1 · Key idea · The evidence

The sea floor records magnetic flips in matching stripes

Magnetic stripes, a mirror image across the ridge A map view of the sea floor with the ridge axis down the middle. Bands of rock magnetized in the normal direction alternate with bands magnetized in the reversed direction. The pattern on the left is a mirror image of the pattern on the right. A magnetometer trace above the map rises over normal bands and dips over reversed bands, the same on both sides. Rock is youngest at the ridge and older farther away. magnetometer reading normal reversed ridge: youngestmap view older ← same ages → older

As new basalt cools at the ridge, its magnetite locks in Earth's magnetic direction.

The field flips now and then; each magnetic reversal starts a new stripe.

Sea-floor spreading carries stripes away on both sides, so they match like a mirror.

Part 1 · Key idea · The evidence

GPS watches the plates move, a few centimetres a year

How fast real plate boundaries move A bar chart in centimetres per year. The Arctic Ridge spreads at less than 2.5. The Mid-Atlantic Ridge spreads at about 2.5. India moves toward Eurasia at 4.5. The East Pacific Rise near Easter Island spreads at more than 15. A dashed reference line at about 5 marks how fast fingernails grow. fingernails ≈ 5 Arctic Ridgespreading Mid-Atlantic Ridgespreading India → Eurasiacollision East Pacific Risenear Easter Island < 2.52.54.5> 15 0481216 speed, cm per year

GPS stations, bolted to rock, measure plates moving 1 to 15 cm a year: about as fast as fingernails grow.

Stripe ages give the same rates, averaged over millions of years. Two methods, one answer.

Part 1 · Key idea · Boundaries

Plates meet in three ways

Divergent, convergent, and transform boundaries Three panels. Divergent: two plates move apart and magma rises into the gap to make new crust. Convergent: two plates move together; the denser one bends down and sinks under the other, and a volcano grows on the top plate. Transform, seen from above: two plates slide past each other along a fault, and a road that crosses the fault is offset; no crust is made or destroyed. Divergent new crust forms Convergent one plate sinks Transform road offset, from above apart · together · past

Divergent boundary: plates pull apart, and new crust forms.

Convergent boundary: plates push together. One sinks (subduction), or both crumple.

Transform boundary: plates slide past each other. Crust is neither made nor destroyed.

Part 1 · Worked example · Practice Gym: classify a boundary

Name the boundary from its evidence

A two-question key for plate boundaries First ask how the plates move. Apart: a divergent boundary, with a ridge or rift, earthquakes shallower than 20 kilometres, and quiet basalt. Past each other: a transform, with offset features, shallow earthquakes, and no volcanoes. Together: then ask if at least one plate is oceanic. Yes: subduction, with a trench, earthquakes down to 300 to 660 kilometres, and a volcanic arc. No, two continents: collision, with mountains, no volcanic arc, and shallow to intermediate earthquakes. How do they move? aparttogetherpast Divergent ridge or riftquakes < 20 kmquiet basalt Is one plate oceanic? Transform offset featuresshallow quakesno volcanoes yesno Subduction trenchquakes to 300–660 kmvolcanic arc Collision: two continents mountains, deep rootno volcanic arcshallow–intermediate quakes
Plates
oceanic and oceanic
Motion
together
Clue
quakes deepen along a dipping plane to 500 km
  1. Motion firstTogether means convergent.
  2. Plate pairAt least one oceanic plate, so one sinks: subduction.
  3. Test the clueQuakes at 500 km need a sinking slab. It fits.
  4. Predict two featuresA deep-sea trench, and a volcanic island arc above the slab's 100 km depth.
Answer

Ocean–ocean convergent boundary with subduction. Check: only subduction gives quakes deeper than about 300 km.

Part 1 · Worked example · Ridge Runners' math

Turn centimetres per year into kilometres per million years

One conversion chain, from a plate's yearly step to a million years A chain of four boxes. 5 centimetres per year, times 1,000,000 years, gives 5,000,000 centimetres. Divide by 100 to get 50,000 metres. Divide by 1,000 to get 50 kilometres. So 5 centimetres per year is 50 kilometres per million years. Below, a bar shows that over 100 million years the plate moves 5,000 kilometres, the width of an ocean. 5 cm per year 5,000,000 cm 50,000 m 50 km per Myr × 1,000,000 years÷ 100 cm per m÷ 1,000 100 Myr at 5 cm/yr = 5,000 km thousands of km: a whole ocean basin
Plate speed
5 cm/yr
Time
1 Myr = 1,000,000 years
Units
100 cm = 1 m · 1,000 m = 1 km
  1. Distance in cm5 cm/yr × 1,000,000 yr = 5,000,000 cm
  2. To metres5,000,000 cm ÷ 100 = 50,000 m
  3. To kilometres50,000 m ÷ 1,000 = 50 km
  4. Shortcut1 cm/yr = 10 km/Myr, so 5 × 10 = 50 km/Myr.
Answer

5 cm/yr = 50 km per million years. Check: over 100 Myr that is 5,000 km, an ocean's width.

Conversion rule: Module 4 Practice Gym, “Plate rates: cm/yr to km/Myr.” For comparison, the Mid-Atlantic Ridge spreads about 2.5 cm/yr: USGS.

Part 1 · Worked example · Practice Gym: spreading rate

How fast is the ridge spreading? Read the stripes

Full rate and half rate across a ridge A ridge axis runs down the middle. The two matching 24-million-year-old stripes sit 1,200 kilometres from the axis on each side, so they are 2,400 kilometres apart. Each plate moves away from the axis at the half rate; the two stripes separate at the full rate. ridge axis 24 Myr 24 Myr 1,200 km · half 1,200 km · half stripe to stripe: 2,400 km · full
Stripe age
24 Myr
Stripe to stripe
2,400 km
  1. DeclareStripe to stripe uses the full rate (both plates).
  2. Full rate2,400 km ÷ 24 Myr = 100 km/Myr
  3. Convert100 km/Myr ÷ 10 = 10 cm/yr
  4. Half rateEach plate: 10 ÷ 2 = 5 cm/yr, so the stripe is 1,200 km from the axis.
Answer

Full rate 10 cm/yr; half rate 5 cm/yr. Check: 10 × 10 × 24 = 2,400 km, and 10 cm/yr is inside GPS's 1 to 15.

Example and rule d = 10 × R × t: Module 4 Practice Gym, “Seafloor-spreading rate, distance, age.”

Part 1 · Common mistakes

Three plate mistakes that cost points

An S wave crossing solid mantle under a plate S waves cross: solid
✗ Mistake

“The plates float on an ocean of magma.”

✓ Fix

The mantle is solid; S waves cross it. Melt forms only in small pockets.

A sinking slab pulling its plate slab pull
✗ Mistake

“Convection currents carry the plates like a conveyor belt.”

✓ Fix

Slab pull drives plates most. They are the cold top of the convection.

From the axis use the half rate; stripe to stripe use the full rate half half full = stripe to stripe
✗ Mistake

“Distance from the ridge axis uses the full rate.”

✓ Fix

From the axis, use the half rate. Stripe to stripe, use the full rate.

Part 1 · Check yourself

Two quick checks

Matching 10-Myr-old stripes sit 600 km apart across a ridge. What is the full spreading rate?

Show the answer

B, 6 cm/yr. 600 km ÷ 10 Myr = 60 km/Myr, and 60 ÷ 10 = 6 cm/yr. A is the half rate, one plate's speed. C skips the ÷ 10 conversion. D divides by 100 instead of 10.

Which force does the most to move the plates?

Show the answer

B, slab pull. A cold, dense slab sinking at a trench drags its plate. A, ridge push, is real but second. C is the conveyor-belt myth: basal drag can help or resist. D, Earth's spin, does not drive plates.

Part 2 · Magma Makers lead this part

Volcanoes: where rock melts

  • Explain the two ways solid mantle melts.
  • Predict where a volcano sits above a sinking slab.
  • Link silica to sticky magma and explosions.
  • Find a plate's speed from a hot-spot chain.
A volcano fed by a magma chamberA cone-shaped volcano with a narrow pipe rising from a magma chamber below it to the vent, where gas rises.

Part 2 · Key idea · Where rock melts

Drop the pressure, and hot rock starts to melt

Rising mantle crosses its melting curve A graph with temperature increasing to the right and depth increasing downward. The melting curve of dry mantle rock slopes down to the right: deeper rock needs a higher temperature to melt. Rock to the right of the curve is partly molten. Under an old plate, the temperature line stays left of the curve, so the rock stays solid. Under a ridge, hot rock rises almost straight up: it cools only a little while its pressure drops a lot, so it crosses the melting curve and starts to melt. temperature, hotter → depth ↓ (pressure rises) partly molten solid melting curve, dry rock under an old plate rising under a ridge melting starts shapes only, no scale

Most of the mantle stays solid: it is hot, but squeezed.

Under a ridge, hot rock rises. Its pressure drops fast while it barely cools, so it crosses the melting curve: decompression melting.

Most of Earth's magma forms this way, as runny basalt.

Part 2 · Key idea · Where rock melts

Add water, and the mantle melts at a lower temperature

Water from a sinking slab melts the mantle above it A cross-section drawn to scale, with a slab dipping 35 degrees from the trench. The slab carries water in its minerals. Where the top of the slab reaches about 100 kilometres deep, the water is driven out and rises into the hot mantle wedge above. The wet wedge melts, and blobs of magma rise to an arc volcano directly above that point. The slab itself is the coldest rock in the picture. 100 km deep trench arc volcano cold slab carries water wet mantle wedge melts here water leaves the slab overriding plate to scale, dip 35°

A sinking slab carries water locked in its minerals.

Near 100 km deep, heat drives the water up into the hot mantle wedge. Water lowers the rock's melting point, like salt on ice: flux melting.

The slab itself stays cold.

Part 2 · Worked example · The Boundary Works lab model

Where does the volcano go? Find the 100 km point

Slab dip sets how far the volcano sits from the trench Drawn to scale, with a 50 kilometre scale bar. Three slabs start at the same trench and dip at 30, 45, and 60 degrees. Each crosses the 100 kilometre depth line at a different place, and the volcano sits directly above that point: 173 kilometres from the trench for a 30 degree slab, 100 kilometres for 45 degrees, and 58 kilometres for 60 degrees. 60°: 58 km45°: 100 km30°: 173 km trench 100 km deep 30°45°60° 50 km
Slab dip
30°
Water leaves at
100 km deep
Lab rule
distance = 100 km ÷ tan(dip)
  1. Tangenttan 30° = 0.577
  2. Distance100 km ÷ 0.577 = 173 km from the trench
  3. Steeper: 45°100 km ÷ 1.000 = 100 km
  4. Steeper still: 60°100 km ÷ 1.732 = 58 km
Answer

173 km from the trench at 30°. Check: a steeper slab brings the volcano closer, but the depth stays 100 km.

Classroom model: The Boundary Works lab, Arc–trench distance readout (Investigation 2). Real arcs sit near or far depending on slab dip: USGS.

Part 2 · Key idea · Magma's personality

More silica makes stickier magma

Magma types by silica content A silica scale from 45 to 80 percent. Basalt has up to 52 percent silica, andesite 52 to 62, dacite 62 to 72, and rhyolite more than 72. Viscosity, or stickiness, rises from basalt to rhyolite. Kilauea's basalt is about 52 percent silica. Mount St. Helens' 1980 dacite is about 64 percent. basaltandesitedaciterhyolite 4552627280 % silica (SiO₂) by weight Kīlauea basalt ≈ 52% Mount St. Helens 1980 dacite ≈ 64% runny, like syrup stiff, like asphalt viscosity rises

Silica (SiO₂) builds chains in the melt that tangle and resist flow.

Viscosity is how strongly a fluid resists flowing.

Basalt has the least silica and runs. Rhyolite has the most and barely moves.

Part 2 · Key idea · Magma's personality

Sticky magma traps gas, so it can explode

Bubbles escape runny magma but stay trapped in sticky magma Left: in runny Kilauea basalt, with about 0.5 percent water, bubbles rise and escape and lava pours out. Middle: in sticky Mount St. Helens dacite, with about 4 percent water, many small bubbles stay trapped until the magma shatters into ash. Right: Mount St. Helens stood 2,950 metres high before May 18, 1980; afterward, the highest point of its crater rim was 2,550 metres, about 400 metres lower. Runny basalt ≈ 0.5% water gas escapes · lava flows Sticky dacite ≈ 4% water gas trapped · shatters to ash 2,950 m 2,550 m St. Helens, 1980 ≈ 400 m gone before (dashed) · after

Dissolved water turns to gas bubbles as magma rises.

In runny basalt, bubbles escape and lava pours out quietly.

In sticky magma, bubbles stay trapped until the magma shatters into ash.

In 1980, Mount St. Helens lost about 400 m of its top.

Part 2 · Key idea · Hot spots

A hot spot stays nearly still while the plate slides over it

The Hawaiian chain grows older away from the hot spot A cross-section, not to scale. A plume of hot rock rises under the Pacific Plate. The active volcano, the Island of Hawaii, sits over it; its oldest exposed rocks are less than 0.7 million years old. The plate moves to the left, toward the northwest, carrying older volcanoes away. Kauai's oldest rocks are about 5.5 million years old. Farther along, the oldest volcanoes are eroded and have sunk below sea level. plate moves Island of Hawaii active < 0.7 Myr Kauai ≈ 5.5 Myr older, eroded, sinking hot spot nearly still Pacific Plate volcanoes get older this way not to scale

A hot spot is a long-lived source of melt beneath a plate.

The plate slides over it, leaving a chain of volcanoes that get older in the direction the plate moves.

Hot spots drift too, but slowly compared with plates.

Part 2 · Worked example · Practice Gym: hot-spot track

How old is that island? Use the hot-spot track

An island 1,800 kilometres down a hot-spot track A line of volcanoes runs left from an active volcano over the hot spot. Tick marks every 450 kilometres reach an extinct island 1,800 kilometres away. The plate moves left at 9 centimetres per year. The island's age is the unknown. 04509001,3501,800 km hot spot island: age? plate: 9 cm/yr
Distance
1,800 km from the active hot spot
Plate speed
9 cm/yr
  1. Convert9 cm/yr × 10 = 90 km/Myr
  2. Set upage = distance ÷ speed
  3. Divide1,800 km ÷ 90 km/Myr = 20 Myr
Answer

About 20 million years old. Check: 90 km/Myr × 20 Myr = 1,800 km, and farther islands are older.

Example and rule t = d ÷ (10 × v): Module 4 Practice Gym, “Hotspot-track ages and plate velocity.”

Part 2 · Key idea · Features and hazards

The magma decides the volcano's shape and its dangers

A shield volcano and a stratovolcano, with their main hazards Left: a broad, gently sloping shield volcano built from runny basalt, like Mauna Loa, with a lava flow running down its side. Right: a steep stratovolcano built from layers of sticky lava and ash, with an ash cloud above it, a pyroclastic flow of hot rock and gas racing down one side, and a lahar, a volcanic mudflow, filling a valley below. Shield volcano runny basalt · gentle slopes lava flow Stratovolcano sticky magma · steep layers ash pyroclastic flow lahar shapes only, not to scale

Runny basalt builds broad shield volcanoes. Their main hazard is lava flows.

Sticky magma builds steep stratovolcanoes. They throw ash, send pyroclastic flows of hot rock and gas downhill, and start lahars (volcanic mudflows).

Part 2 · Common mistakes

Three volcano mistakes to avoid

Water rising from a cold slab into the mantle above cold slab
✗ Mistake

“Friction melts the sinking slab.”

✓ Fix

The slab is the coldest rock there. Its water melts the mantle above it.

A huge quiet shield beside a small explosive cone huge, quiet
✗ Mistake

“A bigger volcano is more explosive.”

✓ Fix

Silica and gas decide. Huge Mauna Loa erupts quietly.

Steep and gentle slabs reach 100 km at different distances same 100 km depth
✗ Mistake

“The volcano is always the same distance from the trench.”

✓ Fix

The depth is fixed, about 100 km. A steeper slab puts the volcano closer.

Part 2 · Check yourself

Two quick checks

An island sits 600 km from its hot spot on a plate moving 8 cm/yr. How old is it?

Show the answer

B, 7.5 Myr. 8 cm/yr = 80 km/Myr, and 600 ÷ 80 = 7.5. A skips the × 10 (600 ÷ 8). C divides by 800. D multiplies instead of dividing (600 × 8 ÷ 100).

Where two continents collide, why is there no volcanic arc?

Show the answer

B. Flux melting needs a sinking, water-bearing slab, and continents do not sink. A: magma does rise through thick crust in the Andes. C: the mantle is hot; it lacks water or a pressure drop. D: weight does not stop magma.

Part 3 · Fault Line lead this part

Earthquakes: when the frame slips

  • Explain why a locked fault slips all at once.
  • Tell the focus from the epicentre.
  • Find distance from the S–P delay, and locate with three stations.
  • Use depth to identify a boundary.
  • Compare magnitudes on a log scale.
A seismogram: P waves, then S wavesA seismogram trace: small P-wave wiggles arrive first, then larger S-wave wiggles. The gap between the two arrivals is labelled the S–P delay. P S S–P delay

Part 3 · Key idea · Why faults slip

A locked fault bends, stores energy, then snaps back

Elastic rebound, seen from above Three map views of a road crossing a fault. 1: the road is straight and the fault is locked. 2: over decades the two sides move in opposite directions; friction holds the fault, so the road bends into an S shape as the rock stores energy. 3: friction loses; in seconds the rock snaps straight again on each side, and the road is now broken and offset at the fault. 1 · Locked straight road 2 · Bending for decades energy stored offset 3 · Snap back earthquake, in seconds map view, from above

A fault is a break where blocks of rock slip past each other.

Friction locks it while the rock on both sides bends for decades, storing energy.

When friction loses, the rock snaps back in seconds: elastic rebound.

Part 3 · Key idea · Faults

Three kinds of fault match three kinds of boundary

Normal, thrust, and strike-slip faults Three panels. Normal fault, side view: pulling apart lets the block above the sloping fault drop down, so its rock layers sit lower; common at spreading ridges. Thrust fault, side view: squeezing pushes the block above the fault up and over, so its layers sit higher; common at subduction zones and in mountains. Strike-slip fault, seen from above: the blocks slide sideways past each other and a fence across the fault is offset; common at transform boundaries. Normal Thrust Strike-slip pull apartspreading ridges squeezetrenches, mountains slide pasttransforms · from above

Normal fault: pulling apart drops one block down.

Thrust fault: squeezing shoves one block up and over.

Strike-slip fault: blocks slide sideways past each other.

Part 3 · Key idea · Finding the earthquake

The focus is underground; the epicentre is the spot above it

Focus and epicentre of the 1989 Loma Prieta earthquake A side view drawn to scale, with depth marked in kilometres. A fault slopes steeply down into the ground. The rupture starts at the focus, 17.2 kilometres deep, the depth USGS lists for the 1989 Loma Prieta earthquake. Seismic waves spread out from the focus in rings. The epicentre is the point on the surface straight above the focus. 0510152025 km epicentre on the surface, straight above focus 17.2 km deep fault drawn to scale Loma Prieta, 1989

The focus is where the rupture starts, underground.

The epicentre (epicenter) is the point on the surface straight above it.

Earthquake catalogues list both: the epicentre and the depth.

Part 3 · Key idea · Seismic waves

P waves arrive first; S waves follow

How P and S waves move rock, and how they show on a seismogram Top: a P wave squeezes and stretches rock along the direction it travels, about 6.0 kilometres per second in the crust, and it can cross solids and liquids. Middle: an S wave shakes rock side to side, across the direction it travels, about 3.5 kilometres per second, through solids only. Bottom: a seismogram shows the P wave arriving first, the S wave later, and the gap between them, the S minus P delay. P wave · push–pull ≈ 6.0 km/s · solids and liquids travel rock moves along the path S wave · side to side ≈ 3.5 km/s · solids only travel rock moves across the path P S S–P delay surface waves

P waves push and pull the rock. They are the fastest, so they arrive first.

S waves shake the rock side to side. They are slower and cannot cross liquid.

A Slinky shows both.

Part 3 · Worked example · Practice Gym: S–P distance

How far away was the earthquake? Use the S–P delay

Travel-time graph: the S–P gap grows with distance Travel time against distance from the earthquake. The P line rises slowly at 6.0 kilometres per second; the S line rises faster at 3.5 kilometres per second. The gap between them grows with distance. At 252 kilometres, P arrives after 42 seconds and S after 72 seconds, a gap of 30 seconds. 0100200300400 0306090120 s distance from the earthquake, km P · 6.0 km/s S · 3.5 km/s 30 s gap P at 42 s S at 72 s 252 km
P speed
6.0 km/s
S speed
3.5 km/s
S–P delay
30 s
  1. S time per km1 ÷ 3.5 = 0.2857 s/km
  2. P time per km1 ÷ 6.0 = 0.1667 s/km
  3. Delay per km0.2857 − 0.1667 = 0.1190 s/km
  4. Distance30 s ÷ 0.1190 s/km = 252 km
Answer

About 252 km, or 8.4 km per second of delay. Check: P takes 252 ÷ 6.0 = 42 s, S takes 72 s, and 72 − 42 = 30 s.

Classroom model: Module 4 Practice Gym, “Epicenter distance from S–P lag” (Vp = 6.0, Vs = 3.5 km/s, ≈ 8.4 km per second of delay). Fault Line's locator cards round this to 8 km per second. Seismologists use measured travel-time tables: USGS.

Part 3 · Key idea · Finding the earthquake

One station gives a distance. Three stations find the epicentre

Three distance circles meet at one epicentre A map drawn to scale, with a 100 kilometre scale bar. Station A recorded a 30 second S minus P delay, so the earthquake was 252 kilometres away: a circle of that radius. Station B: 20 seconds, 168 kilometres. Station C: 15 seconds, 126 kilometres. Circles B and C cross at two points, so two stations are not enough. All three circles meet at only one point: the epicentre. B and C cross here too epicentre A: 30 s → 252 km B: 20 s → 168 km C: 15 s → 126 km 100 km

One S–P delay gives only a distance. The quake could be anywhere on a circle around that station.

Two circles usually cross twice. A third circle picks the one point: the epicentre.

Part 3 · Key idea · Where earthquakes happen

Earthquake depth is a fingerprint of the boundary

Focal depths of four real earthquakes Depth from 0 to 700 kilometres, with USGS depth classes shaded: shallow 0 to 70, intermediate 70 to 300, deep 300 to 700. Nepal 2015, magnitude 7.8, a collision: 8.2 kilometres deep. Loma Prieta 1989, magnitude 6.9, transform: 17.2 kilometres. Nisqually 2001, magnitude 6.8, inside the sinking Juan de Fuca slab: 51.8 kilometres. Fiji 2018, magnitude 8.2, inside the old Pacific slab: 600 kilometres. 0100200300400500600700 8 km17 km52 km600 km Nepal 2015collision Loma Prietatransform Nisqually 2001in a slab Fiji 2018in an old slab intermediate 70–300 kmdeep 300–700 km shallow 0–70 km depth below the surface, km (to scale)

Ridges and transforms keep earthquakes shallow. Collisions add some intermediate ones.

Only a sinking slab carries earthquakes down to about 700 km.

So deep earthquakes are a fingerprint of subduction.

Part 3 · Worked example · Practice Gym: magnitude on a log scale

How much bigger is a magnitude 7 than a magnitude 5?

Each magnitude step: 10 times the wave height, about 32 times the energy Two bar charts on a logarithmic axis from 1 to 1,000, compared with a magnitude 5. Wave height: magnitude 6 is 10 times, magnitude 7 is 100 times. Energy: magnitude 6 is about 32 times, magnitude 7 is about 1,000 times. Wave height M 5M 6M 7 110100 Energy M 5M 6M 7 1≈ 32≈ 1,000 1101001,000 times a magnitude 5 · log scale
Larger
M 7.0
Smaller
M 5.0
  1. DifferenceΔM = 7.0 − 5.0 = 2
  2. Wave height10ΔM = 10² = 100 times
  3. Energy101.5 × ΔM = 10³ = 1,000 times
  4. Rule-of-thumb checkAbout 32 per step: 32 × 32 = 1,024 ≈ 1,000
Answer

Waves 100 times taller, about 1,000 times the energy. Not “2 more.”

Per step: 10 times the amplitude, about 32 times the energy, USGS. Formula 101.5 ΔM: Module 4 Practice Gym, “Magnitude arithmetic on a log scale.”

Part 3 · Key idea · Risk

One magnitude, many intensities

Shaking weakens with distance and grows on soft ground A map view. Rings around the epicentre show shaking getting weaker with distance: strong, moderate, light, then barely felt. Two buildings sit the same distance from the epicentre. The one on firm rock stands. The one on soft, wet fill shakes harder and cracks. The rings are an illustration, not measured values. epicentre strongmoderatelightbarely felt firm rock: stands soft, wet fill: cracks illustration, not to scale

Magnitude: one number for the earthquake's size.

Intensity: how hard the ground shakes at one place. It falls with distance and is worse on soft ground.

Risk also depends on how we build.

Part 3 · Common mistakes

Three earthquake mistakes to avoid

Magnitude steps multiply 6 = 2 × 3 ✗ 10 × 10 × 10 ✓
✗ Mistake

“A magnitude 6 is twice a magnitude 3.”

✓ Fix

Three steps: 1,000 times taller waves and about 32,000 times the energy.

One station: only a circle ? ?
✗ Mistake

“One station tells you where the earthquake was.”

✓ Fix

It gives only a distance: a circle. You need three stations.

Focus below, epicentre above epicentre focus
✗ Mistake

“The epicentre is where the earthquake started.”

✓ Fix

It started at the focus, underground. The epicentre is the spot above it.

Part 3 · Check yourself

Two quick checks

With the Practice Gym speeds (P 6.0, S 3.5 km/s), S arrives 20 s after P. How far away is the epicentre?

Show the answer

C, 168 km. 20 s ÷ 0.119 s/km = 168 km, or 8.4 × 20. A uses only the S speed (20 × 3.5). B uses only the P speed (20 × 6.0). D divides 20 by 8.4.

How many times more energy does a magnitude 8.0 release than a magnitude 6.0?

Show the answer

C. ΔM = 2, so energy = 101.5 × 2 = 1,000, or 32 × 32. A subtracts the magnitudes. B is the wave-height ratio, not energy. D adds 32 + 32 instead of multiplying.

Part 4 · Root Raisers lead this part

Mountains: raising the roof

  • Name the two settings where big ranges rise.
  • Explain isostasy with the iceberg idea.
  • Calculate the root under a range.
  • Read folds and thrust faults as squeezing.
  • Explain why erosion makes a range rebound.
A mountain range floats on a deep rootA mountain range stands above the surrounding crust, and a much deeper root of the same crust bulges down beneath it. root

Part 4 · Key idea · Settings

Two ways to build a range: an arc or a collision

An Andes-type arc range and a Himalaya-type collision range Left: an ocean plate sinks under a continent. Water from the slab melts the mantle, and a chain of volcanoes builds a range on the thick continental crust, like the Andes. Right: two continents meet. Neither can sink, so the crust crumples, stacks up, and thickens into a high range with a deep root and no volcanoes, like the Himalayas, where old sea-floor rocks have been pushed up high. Ocean under continent ocean plate continent volcanic arc range like the Andes Continent meets continent deep root no volcanoes collision range like the Himalayas

Ocean meets continent: water from the slab feeds a volcanic arc range, like the Andes.

Continent meets continent: neither sinks, so the crust crumples and thickens in a collision, like the Himalayas. No volcanoes.

Part 4 · Key idea · Isostasy

Thick, light crust floats high, with a deep root

Thicker crust stands higher and reaches deeper Three columns of crust floating on mantle, drawn to scale, using The Boundary Works lab densities of 2.87 for crust and 3.3 for mantle. Crust 35 kilometres thick stands at the normal level. Crust 50 kilometres thick stands about 2.0 kilometres higher and has a root about 13 kilometres deep. Crust 70 kilometres thick stands about 4.6 kilometres higher with a root about 30 kilometres deep. Most of each extra thickness is below, like an iceberg. 35 km50 km70 km normal level 2.0 km higher 4.6 km higher root 13 km root 30 km base of normal crust mantle (denser, solid, slowly flows)

Light crust floats on denser mantle, like an iceberg on water.

Thicken the crust and it rides higher, but most of the extra thickness sinks as a crustal root.

This floating balance is isostasy.

Continents have deep roots, like icebergs: USGS, What is a tectonic plate? Column numbers: classroom model, The Boundary Works lab (crust 2.87, mantle 3.3; collision thickens crust from 35 to 70 km).

Part 4 · Worked example · Practice Gym: isostatic root

How deep is the root under a 5 km range?

A mountain column balanced against a normal column Drawn to scale. A normal crust column floats with its top at the normal level. Beside it, the mountain column stands 5.0 kilometres higher and has a root 28 kilometres deeper. At the depth of the root's base, both columns weigh the same: the extra rock on top is balanced by light crust replacing denser mantle in the root. h = 5.0 km r = 28 km normal crust mantle 3.3 crust 2.8 root normal level equal weight at this depth
Peak height h
5.0 km
Crust ρc
2.8 g/cm³
Mantle ρm
3.3 g/cm³
Gym rule
r = h × ρc ÷ (ρm − ρc)
  1. Why it worksThe extra rock on top (h × ρc) balances the root's lift, r × (ρm − ρc).
  2. Density gap3.3 − 2.8 = 0.5
  3. Top5.0 × 2.8 = 14.0
  4. Rootr = 14.0 ÷ 0.5 = 28 km
Answer

The root is about 28 km deep. Check: g/cm³ cancels, leaving km, and the root is 5.6 times the height.

Example and rule: Module 4 Practice Gym, “Isostatic root beneath a mountain range” (a classroom model of Airy isostasy).

Part 4 · Worked example · The Boundary Works lab model

The lab's rule: how high does 30 km of thickening stand?

Thickening splits into a little height and a lot of root Drawn to scale, with a 20 kilometre scale bar. Before: crust 35 kilometres thick at the normal level. After a collision adds 30 kilometres: crust 65 kilometres thick, standing 3.9 kilometres higher with a root 26.1 kilometres deep. About 13 percent of the thickening shows as height and 87 percent goes into the root. 35 km before 65 km after a collision 3.9 km up 26.1 km root base of the old crust 20 km · to scale 13% up, 87% down
Thickening
30 km (35 → 65 km)
ρc, ρm
2.87 and 3.3 g/cm³
Lab rule
h = thickening × (ρm − ρc) ÷ ρm
  1. Density gap3.3 − 2.87 = 0.43
  2. Fraction up0.43 ÷ 3.3 = 0.130
  3. Height30 km × 0.130 = 3.9 km
  4. Root30 − 3.9 = 26.1 km
Answer

3.9 km higher, with a 26.1 km root. Check: the Practice Gym rule agrees: 3.91 × 2.87 ÷ 0.43 = 26.1 km.

Classroom model: The Boundary Works lab, Airy peak elevation readout (h = Δt × 0.13 with ρc 2.87, ρm 3.3). Same balance as the Practice Gym rule on the previous slide.

Part 4 · Key idea · Stress, folds, and faults

Squeezing folds rock layers, then breaks them

Folded layers cut by a thrust fault A side view of rock layers squeezed from both sides. The layers bend into an arch, an anticline, and a trough, a syncline. On the right, the rock has broken along a thrust fault that slopes down to the right, and the block above the fault has slid up and over, stacking the layers higher. squeeze squeeze anticline (arch) syncline(trough) thrust fault stacked higher

Stress is a push or pull on rock.

Squeezing bends layers into folds: arches called anticlines and troughs called synclines.

Squeeze harder and the rock breaks. A thrust fault stacks one slab on another.

Part 4 · Key idea · Uplift meets erosion

Erosion lowers the peaks, and the root lifts the range back up

Removing rock from the top makes the range float up Two floating crust blocks, drawn in true proportion. Before: a block with a high top and a deep root. After erosion removes a layer from the top, the lighter block floats upward, so its root rises by most of the removed thickness, and its surface ends up only a little lower than before. With The Boundary Works lab densities, eroding 1,000 metres evenly lowers the surface by only about 130 metres. Before After erosion root rises ≈ 87% of it old surface drops only ≈ 13% layer to erode true proportions, lab densities 2.87 and 3.3 mantle

Rain, rivers, and ice strip rock off the top: erosion.

Lighter now, the range floats back up, so its surface drops far less than the rock removed. Rising land is uplift.

Over millions of years, deep rock reaches the surface.

Roots and floating crust: USGS, What is a tectonic plate? Rebound fraction from the classroom model's densities, The Boundary Works lab: drop = removed × (3.3 − 2.87) ÷ 3.3 ≈ 0.13.

Part 4 · Worked example · Plate-motion history

How far does India push into Asia in 10 million years?

Distance grows steadily at 45 kilometres per million years A straight line on a distance against time graph. At 45 millimetres per year, the same as 45 kilometres per million years, India moves 225 kilometres in 5 million years and 450 kilometres in 10 million years, if the rate stays the same. 02.557.510 Myr 0200400 km 225 km 450 km slope = 45 km/Myr time from now, if the rate holds
Rate
45 mm/yr (India toward Eurasia)
Time
10 Myr
  1. mm to cm45 mm/yr ÷ 10 = 4.5 cm/yr
  2. To km per Myr4.5 cm/yr × 10 = 45 km/Myr
  3. Distance45 km/Myr × 10 Myr = 450 km
Answer

About 450 km, if the rate holds. Check: 1 mm/yr = 1 km/Myr, so 45 mm/yr is 45 km each million years.

Part 4 · Key idea · Mountains and climate

A high range wrings out the rain and leaves a dry side

Rain on the near side, a rain shadow on the far side A cross-section, not to scale, from south on the left to north on the right. Moist air moves north and is forced up the Himalayas. It cools, forms clouds, and rains on the southern slopes. Air sinking down the far side warms and dries, leaving the high Tibetan Plateau in a rain shadow. moist air rises, cools, rains sinks, warms, dries rain shadow dry Tibetan Plateau south north not to scale

A high range forces moist air up. It cools and rains on the near side.

Air sinking down the far side warms and dries: a rain shadow.

Fossils show Tibet was mild and wet 105 million years ago. Uplift and a move north dried it.

Part 4 · Common mistakes

Three mountain mistakes to avoid

Most of a range's thickness is a root below root
✗ Mistake

“Mountains are rock piled on top of the crust.”

✓ Fix

They are thick crust floating high. Most of it is a root below.

Two continents meet with no volcano no volcano
✗ Mistake

“Volcanoes built the Himalayas.”

✓ Fix

Two continents collided. With no sinking slab, there is no volcanic arc.

After erosion the surface drops only a little −13%
✗ Mistake

“Erode 1 km off the top, and the range is 1 km lower.”

✓ Fix

The root floats it back up. With the lab's densities, it drops about 0.13 km.

Part 4 · Check yourself

Two quick checks

A range stands 3.0 km high. Crust is 2.7 g/cm³ and mantle 3.3 g/cm³. How deep is its root?

Show the answer

C, 13.5 km. r = 3.0 × 2.7 ÷ (3.3 − 2.7) = 8.1 ÷ 0.6 = 13.5 km. A multiplies the height by the gap (3.0 × 0.6). B divides by the mantle density instead of the gap. D puts the mantle density on top; 16.5 km is also the whole extra thickness, height plus root.

Erosion attacks the Himalayas every year. Why do they still rise?

Show the answer

A. USGS measures India converging at 45 mm/yr, and the range rises more than 1 cm a year. B: a collision has no volcanic arc. C: the mantle is solid. D: rivers and ice still erode the range.

Part 5 · Every team · Build a Plate Boundary

Use it in your project

“Can you build a boundary that behaves the way the real one does, and does your model fail where the real Earth does?”

  • Film your model working, then breaking.
  • Split the boundary into five problems.
  • Test predictions against real catalogue data.
  • Find the slides your team's build needs.
Open the project Project Library
The four Build a Plate Boundary deliverablesFour cards: a model video, a subproblem log drawn as a tree of boxes, a prediction table with a tick and a cross, and a field journal. model video subproblem log prediction table ✓ ✗ field journal

Use it · Build a Plate Boundary · Model demonstration video

Two minutes: show it working, then break it on purpose

A storyboard for the two-minute model video Four frames along a two-minute timeline. 0:00 to 0:20: name the boundary type and the plate pair. 0:20 to 1:00: run the model at a stated rate and show the features appear. 1:00 to 1:30: point to where earthquakes, volcanoes, and high ground would be, and why. 1:30 to 2:00: the stress test: push the model past what it was built for and narrate what breaks and why. 0:000:201:001:302:00 1 · Name it boundary typeand plate pair(O–O, O–C, C–C) 2 · Run it motion at astated ratefeatures appear 3 · Point out where quakes,volcanoes, andhigh ground go 4 · Stress test push it pastits design; saywhat breaks, why
Turn in
Model demonstration video · Studio Turn-In, one video file
Do
Run your model, then push it past what it was built for. Narrate what breaks and why the real Earth does or does not.
Data
Your build tests, with units
Slides
Boundary types · What moves plates · Rates

Use it · Build a Plate Boundary · Design rationale and subproblem log

Split the boundary into five problems before you build

One boundary, five solvable subproblems A tree. At the top: your boundary, its type and plate pair. Below it, five subproblems: plate motion, density contrast, earthquake depths, volcanism, and surface shape, each with the slide idea it needs: slab pull, who sinks, depth as a fingerprint, flux or decompression melting, and isostasy. Under all five: for each, write how your build solves it, the material limit that shaped it, and what it leaves out. Your boundary + plate pair 1 · Platemotion 2 · Densitycontrast 3 · Quakedepths 4 · Volcanism(or none) 5 · Surfaceshape slab pull,rate who sinksand why depth is afingerprint flux ordecompression trench, arc,root For each: how you solved it · the material limit · what it leaves out Write the split before you build, not after.
Turn in
Design rationale and subproblem log · Assessment Center, typed
Do
Name your boundary, then log five subproblems: how each is solved, the limit that shaped it, and what the model leaves out.
Slides
Slab pull · Who sinks · Depth · Melting · Isostasy

Use it · Build a Plate Boundary · Prediction and verification table

Predict, then check your model against a real boundary

One worked row for an ocean–ocean subduction model. Your table has all three rows for your own boundary.
FeatureOur model predictsReal boundary, same typeCatalogue value and sourceMatch?
Earthquake depth rangeFoci reach ≈ 600 km along the slabTonga–FijiM 8.2 at 600 km deep, 2018 (USGS catalogue)Yes
Volcanic styleyour predictionyour boundaryyour source and dateyes or no
Topographic signatureyour predictionyour boundaryyour source and dateyes or no
Turn in
Prediction and verification table · Assessment Center, typed
Do
Name one mismatch and explain its physical cause. “Our materials were wrong” is not a cause.
Data
USGS catalogue · NOAA relief
Slides
Depth · Volcano style · Ranges

Use it · Build a Plate Boundary · Field journal

Your journal: record the failures, not only the wins

The seven parts of one journal entry A journal page with seven labelled parts: 1 header, 2 today's question, 3 raw record, 4 method log, 5 sense-making in progress, 6 dead ends, and 7 handoff. Dead ends is highlighted: describe what failed and why. 1 · Header: date, day, role 2 · Today's question 3 · Raw record, with units 4 · Method log 5 · Sense-making 7 · Handoff 6 · Dead ends: what failed, and why
Turn in
Field journal · Note Studio, one note titled with the project name
Do
One seven-part entry per project day, plus a closing retrospective.
Focus
A model that collapsed on day 3 and was rebuilt on day 4 is strong evidence. Record why it failed.
Slides
Plates · Volcanoes · Quakes · Mountains

Use it · Build a Plate Boundary · Your team's build

The slides your build and teach-back need

Ridge Runners · spreading-ridge machine

Matching stripes, a transform offset, a cold-water tub.

Magma Makers · volcano lab bench

Salted ice, syrup and gas, a hot-spot dot track.

Fault Line · earthquake station

A stick-slip fault, an epicentre locator, a shake table.

Root Raisers · mountain workshop

A float tank and a squeeze box.

The Boundary WorksLab readouts are model evidence: “When we changed ___, the ___ changed.” Open the lab

Use it · The rest of the year

Module 4 comes back in every project

Part 6 · Reference · Formula sheet

Every Module 4 formula on one page

Rate conversion

1 cm/yr = 10 km/Myr

1 mm/yr = 1 km/Myr. Distance = rate × time. Example

Spreading (Practice Gym)

d = 10 × R × t

R full rate (cm/yr), t in Myr, d stripe to stripe (km). From the axis, use R ÷ 2. Example

Hot-spot track (Practice Gym)

t = d ÷ (10 × v)

d in km, v in cm/yr, t in Myr. Islands age in the direction the plate moves. Example

S–P distance (Practice Gym)

d = Δt ÷ (1/Vs − 1/Vp) ≈ 8.4 × Δt

Vp 6.0, Vs 3.5 km/s. Locator cards round to 8. One station gives a circle; use three. Example

Magnitude

amplitude × 10ΔM · energy × 101.5 ΔM

Per whole step: 10 times the waves, about 32 times the energy (USGS). Example

Isostatic root (Practice Gym)

r = h × ρc ÷ (ρm − ρc)

h peak height, r root depth, both km. Erode it, and it rebounds up. Example

Airy height (Boundary Works lab)

h = Δt × (ρm − ρc) ÷ ρm ≈ 0.13 Δt

ρc 2.87, ρm 3.3. Root = Δt − h. Erode e evenly: surface drops ≈ 0.13e. Example

Arc position (Boundary Works lab)

x = 100 km ÷ tan(dip)

The arc sits above the slab's 100 km depth. Slab tip depth = 10 × rate × sin(dip) × t, at most 660 km. Example

Boundary key (Practice Gym)

motion first, then plate pair

Apart: divergent. Past: transform. Together with an ocean plate: subduction. Two continents: collision. Example

Depth classes (USGS)

0–70 · 70–300 · 300–700 km

Shallow, intermediate, deep. Below about 70 km, only inside sinking slabs. Slide

Rock and magma numbers

Density (g/cm³): granite 2.7 · basalt 3.0 · peridotite 3.3 · eclogite ≈ 3.5. Silica: basalt ≤ 52% · andesite 52–62 · dacite 62–72 · rhyolite > 72%. Density · Silica

Real rates (USGS)

Mid-Atlantic Ridge 2.5 · India toward Eurasia 4.5 · East Pacific Rise near Easter Island > 15 cm/yr. GPS range 1–15 cm/yr. Slide

Part 6 · Reference · Vocabulary

Module 4 words, in plain language

Lithosphere
Rigid crust plus uppermost mantle: the plate.
Asthenosphere
Hot, solid mantle below the plates that creeps slowly.
Convection
Heat moved by hot material rising and cold material sinking.
Slab pull
A sinking plate edge dragging its plate. The strongest force.
Ridge push
A plate sliding off a high ridge. Second strongest.
Eclogite
Dense rock (≈ 3.5 g/cm³) that basalt becomes deep in a slab.
Magnetic reversal
A flip of Earth's magnetic field, recorded in new rock.
Sea-floor spreading
New ocean crust forming at a ridge and moving away.
Half rate
One plate's speed away from the ridge axis.
Divergent boundary
Plates pull apart; new crust forms.
Convergent boundary
Plates push together; one sinks or both crumple.
Transform boundary
Plates slide past; crust neither made nor destroyed.
Subduction
One plate sinking beneath another into the mantle.
Magma
Melted rock underground.
Decompression melting
Hot rock melting as its pressure drops.
Flux melting
Rock melting because added water lowers its melting point.
Mantle wedge
The mantle between a sinking slab and the plate above.
Silica
SiO₂, the main ingredient of magma; more makes it stickier.
Viscosity
How strongly a fluid resists flowing.
Hot spot
A long-lived source of melt beneath a plate.
Shield volcano
Broad, gentle volcano built from runny basalt.
Stratovolcano
Steep, layered volcano built from sticky magma and ash.
Pyroclastic flow
A fast avalanche of hot rock and gas.
Lahar
A volcanic mudflow.
Volcanic arc
A chain of volcanoes above a sinking slab.
Fault
A break where blocks of rock slip past each other.
Elastic rebound
Bent rock snapping back when a locked fault slips.
Normal fault
Pulling apart drops one block down.
Thrust fault
Squeezing pushes one block up and over another.
Strike-slip fault
Blocks slide sideways past each other.
Focus
Where an earthquake's rupture starts, underground.
Epicentre (epicenter)
The surface point straight above the focus.
P wave
Fast push–pull seismic wave; crosses solids and liquids.
S wave
Slower side-to-side seismic wave; solids only.
Magnitude
One number for an earthquake's size, on a log scale.
Intensity
How strongly the ground shakes at one place.
Collision
Two continents meeting; neither sinks, so crust thickens.
Isostasy
Crust floating in balance on the denser mantle.
Crustal root
Thick crust reaching down under a mountain range.
Stress
A push or pull on rock.
Fold · anticline · syncline
A bend in rock layers: an arch (anticline) or a trough (syncline).
Erosion
Rain, rivers, and ice carrying rock away.
Uplift
A rise of the land surface.
Rain shadow
The dry side of a mountain range.

More wordsLook up any term in the Ascent Lexicon.

Part 6 · Reference · Trustworthy data

Real data for your project, from the people who measure it

Cite itWrite the source, the link, your search settings, and the date. Catalogues update.

Part 6 · Ready for your project?

Three last checks

An island 1,200 km down a hot-spot track is 15 Myr old. How fast is the plate?

Show the answer

B. 1,200 km ÷ 15 Myr = 80 km/Myr = 8 cm/yr. A skips the ÷ 10. C halves it, but a hot-spot track has no half rate. D divides by 100.

Two continents collide. Which pair of features do you expect?

Show the answer

B. Neither continent sinks, so there is no deep slab and no water to flux-melt the mantle. A describes subduction. C: collisions shake a lot. D describes a spreading ridge.

With the lab's densities (2.87, 3.3), erosion removes 2 km evenly from a range. How much lower is its surface?

Show the answer

C. Drop = 2 km × (3.3 − 2.87) ÷ 3.3 = 2 × 0.13 = 0.26 km. A ignores rebound. B is how far the root rises. D: some drop always remains.

You made it through Module 4

Now build a boundary that fails where the Earth does

Practice the math, test ideas in The Boundary Works, then build.

Build a Plate Boundary Practice Gym · Module 4 The Boundary Works lab Module 4 page
A classroom plate-boundary model on a trayTwo blocks of model material meet on a tray. An arrow pushes one block toward the other, a small volcano sits on the far block, and a jagged crack marks where the model breaks. find where it breaks