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Plates pull apart, sink, collide or slide past at a few centimeters a year; each margin brings its own earthquakes and volcanoes, and slow speeds build oceans over millions of years.
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By the end of this lesson you will be able to sort plate margins by their movement and hazards, and turn plate speeds into distances and times.
You met plates and their boundaries in Grade 5, where earthquakes and volcanoes line up along them. This lesson measures how fast plates move, turns their slow speeds into distances over millions of years, and sorts the hazards each kind of margin brings.
| Term | What it means |
|---|---|
| Plate | A rigid slab of the Earth's outer shell, moving a few centimeters a year. |
| Divergent margin | Where two plates pull apart and new crust forms. |
| Subduction | One plate sinking beneath another into the mantle. |
| Collision margin | Where two continents push together and build mountains. |
| Transform margin | Where two plates slide past each other sideways. |
| Hotspot | A fixed plume of hot rock that melts through a plate moving over it. |
Plates move a few centimeters a year, and what happens at their edges depends on how they move.
To turn a speed into a distance, multiply centimeters a year by years, then divide by 100,000 for kilometers: $1$ cm a year for a million years is $10$ km.
Another way: picture
Picture your fingernails. They grow about three or four centimeters a year, too slowly to watch. Now picture them growing for a million years: they would reach thirty or forty kilometers. Plates move at that speed, and they have been moving for hundreds of millions of years.
Another way: steps
Plate movement creates relief over long periods. Uplift exposes rock to weathering: physical breakup or chemical alteration in place. Erosion removes material, transport moves it, and deposition stores it when the transporting flow loses capacity. These processes can operate simultaneously in different parts of a landscape; a mountain does not have to finish rising before rivers erode it.
Trace an invented mountain-to-coast sequence. Convergence helps raise a range. Water entering cracks freezes and expands under suitable conditions, loosening fragments. Runoff carries some fragments into a river; finer sediment travels farther and some settles on a floodplain or at the coast. Dissolved material also moves with water and is not visible as grains. Finding sediment at the coast is evidence of transport and storage, not proof that the coast itself is a plate boundary.
Distinguish mechanisms using evidence. A fractured outcrop with fragments still beneath it supports weathering in place. An eroded bank and muddy floodwater support removal and transport. Layered sand downstream supports deposition but does not uniquely identify the sediment's original rock. Check rock type, transport direction and relative age before joining these observations into an explanation. Tectonic timescales and single flood events belong in one system but should not be treated as equally rapid.
Each kind of margin brings its own hazards.
| Margin | Movement | Earthquakes | Volcanoes | American example |
|---|---|---|---|---|
| divergent | apart | yes, shallow | yes, gentle | Iceland (not American) |
| subduction | one sinks | yes, largest | yes, explosive | Cascades, Alaska |
| collision | together | yes | few | Himalayas (not American) |
| transform | past | yes | no | San Andreas Fault |
Every margin has earthquakes. Volcanoes need rock to melt, which happens where plates pull apart or one sinks beneath another.
Along the Mid-Atlantic Ridge, the North American plate and the Eurasian and African plates pull apart about two to three centimeters a year. Magma rises into the gap and cools into new ocean floor.
Over about two hundred million years, that spreading opened the whole Atlantic Ocean. Iceland sits on the ridge above sea level, so you can walk across the gap between two plates there.
Off the coast of Oregon, Washington and northern California, the small Juan de Fuca plate sinks beneath North America at the Cascadia subduction zone. As it sinks, water released from it melts the rock above, and the magma rises to feed the Cascade volcanoes: Mount Rainier, Mount Hood, and Mount St. Helens, which erupted explosively in 1980.
Subduction zones produce the largest earthquakes on Earth. The 1964 Alaska earthquake, magnitude 9.2, the largest ever recorded in North America, struck at one.
When two continents meet, neither sinks, because continental rock is too light. Instead the crust crumples and thickens into mountains. India's collision with Asia, still going on, has raised the Himalayas, with Mount Everest the highest peak on Earth.
The Appalachian Mountains in the eastern United States were raised by an ancient collision hundreds of millions of years ago, and have been worn down since.
Along the San Andreas Fault in California, the Pacific plate slides northwest past the North American plate. Most of the fault is locked: strain builds for decades or centuries, then releases in an earthquake that jerks the two sides meters past each other, as in San Francisco in 1906.
A few stretches creep steadily, slowly bending fences, curbs and walls built across them. Because no plate sinks, there are no volcanoes along the fault.
Some volcanoes sit far from any margin, over hotspots: plumes of hot rock rising from deep in the mantle. As a plate moves over a hotspot, a chain of volcanoes forms, each carried away and going extinct as a new one rises over the plume.
The Hawaiian Islands formed this way. The Big Island sits over the hotspot and is still erupting; islands to the northwest are older and farther away. An island's age and distance give the Pacific plate's speed, about seven to nine centimeters a year.
A kilometer is $1000$ meters and a meter is $100$ centimeters, so a kilometer is 100,000 centimeters. A plate moving $5$ cm a year for a million years moves 5,000,000 cm, which is $50$ km.
A handy shortcut: at $1$ cm a year, a plate moves $10$ km every million years. So $2.5$ cm a year is $25$ km per million years.
Most earthquakes happen at plate margins, but not all. The New Madrid seismic zone in the middle of the country, near where Missouri, Kentucky, Tennessee and Arkansas meet, produced a series of strong earthquakes in 1811 and 1812, far from any active margin.
These intraplate earthquakes happen along old weaknesses in the crust, and because they are rare, the buildings near them are often not designed for shaking.
Checking an answer. Plate speeds should come out between about one and ten centimeters a year. Distances over a million years should be tens of kilometers, not thousands.
Multiplying a rate by years is allowed because plates move at nearly steady speeds over millions of years, even though faults move in jerks. Dividing by one hundred thousand is allowed because that is exactly how many centimeters make a kilometer.
Using a hotspot chain to find a speed is allowed because the hotspot stays roughly fixed, so an island's distance from it records how far the plate carried it.
Knowing a region's margin tells its people what to prepare for. Along the Cascadia subduction zone, towns plan for a great earthquake and a tsunami. In California, building codes require structures to withstand strong shaking. Near the Cascade volcanoes, towns map where mudflows would run.
The next lesson measures earthquakes themselves, and asks why the same size of earthquake kills far more people in some places than in others.
The most common slip is dividing by one thousand instead of one hundred thousand when turning centimeters into kilometers. Another is thinking plates move too slowly to matter.
A third is expecting volcanoes at every margin, when transform and collision margins rarely have them. A fourth is thinking earthquakes happen only at margins.
The Hawaiian Islands stretch northwest from the Big Island, where Kīlauea and Mauna Loa still erupt, through Maui, Oʻahu and Kauaʻi, to small atolls and then undersea mountains thousands of kilometers away. The islands get older the farther they are from the Big Island: Kauaʻi is about five million years old.
Geologists explain the pattern with a hotspot. A plume of hot rock rises beneath the Pacific plate, which moves northwest over it, so each volcano is carried away and goes extinct while a new one grows over the plume. Dividing each island's distance from the hotspot by its age gives the plate's speed, about seven to nine centimeters a year.
A new volcano, Kamaʻehuakanaloa, formerly called Lōʻihi, is already growing on the sea floor southeast of the Big Island. In tens of thousands of years it may rise above the waves as the chain's newest island.
Off the Pacific Northwest coast, the Juan de Fuca plate sinks beneath North America along the Cascadia subduction zone, about 1,000 kilometers long. Geologists found drowned forests and layers of tsunami sand showing that its last great earthquake, around magnitude 9, struck in January 1700; Japanese records of a tsunami that arrived with no local earthquake pin down the date.
The zone has been locked since then, storing strain as the plates keep moving a few centimeters a year. Scientists estimate a significant chance of another great earthquake in the coming decades, with shaking lasting minutes and a tsunami reaching the coast within about fifteen to thirty minutes.
Coastal towns in Oregon and Washington have built tsunami evacuation routes and, in places, vertical evacuation towers. Schools practice drills, and a warning system called ShakeAlert sends alerts to phones seconds before shaking arrives.
A few centimeters a year sounds like nothing, and it is easy to think plates move too slowly to shape the world. But over millions of years those centimeters become oceans, mountain ranges and chains of islands, and the strain they build is released in sudden earthquakes.
It is also tempting to expect volcanoes wherever the ground shakes. Every margin has earthquakes, but volcanoes need melting rock, which happens where plates pull apart or one sinks beneath another, not where they slide past or crumple together.
A plate moves $3.5$ cm a year. Find its movement in a million years, in centimeters.
$3.5 \times 1000000 = 3500000$
Rate times years.
Convert to kilometers.
$\dfrac{3500000}{100000} = 35\ \text{km}$
One hundred thousand centimeters a kilometer.
Check with the shortcut.
$3.5 \times 10 = 35$
Ten kilometers per million years per centimeter.
Find it in ten million years.
$350\ \text{km}$
Ten times as far.
An island $2400$ km from a hotspot is $30$ million years old. Convert the distance.
$2400 \times 100000 = 240000000\ \text{cm}$
Centimeters.
Convert the age.
$30 \times 1000000 = 30000000\ \text{years}$
Years.
Divide the two numbers.
$\dfrac{240000000}{30000000} = 8\ \text{cm a year}$
The plate's speed.
Check with the shortcut.
$\dfrac{2400}{30} = 80\ \text{km per million years}$
Eighty divided by ten is eight.
Say which way the plate moved.
$\text{away from the older island}$
Toward the newer ones.
A ridge spreads $2.5$ cm a year. Find its widening in a million years.
$2.5 \times 10 = 25\ \text{km}$
The shortcut.
Find its widening in $4$ million years.
$25 \times 4 = 100\ \text{km}$
Per million times millions.
Find how long it takes to open $5000$ km.
$\dfrac{5000}{25} = 200\ \text{million years}$
Width over widening.
Compare with the Atlantic.
$\text{about the same}$
It opened over about two hundred million years.
Name the margin.
$\text{divergent}$
Plates pulling apart.
Name its hazards.
$\text{shallow earthquakes, gentle volcanoes}$
Runny lava.
Multiply for centimeters.
$4 \times 500000 = 2000000$
Rate times years.
Divide for kilometers.
$\dfrac{2000000}{100000} = 20$
Kilometers.
Check with the shortcut.
A plate moves about $2.5$ cm a year. How far does it move in $2000000$ years, in kilometers?
Complete the worked solution: an ocean widens at $4$ cm a year along its mid-ocean ridge. Find how many kilometers it widens in a million years, and in $6$ million years.
Find the widening in a million years.
$\text{rate} \times \text{ten} =$ k
Kilometers.
Find the widening over the whole span.
$\text{per million} \times \text{millions} =$ w
Kilometers.
Say where the new floor comes from.
$\text{magma rising at the ridge}$
A divergent margin.
Say what the Atlantic shows.
$\text{an ocean opened this way}$
Over about two hundred million years.
Match each kind of plate margin to what happens there.
| plates pull apart and new crust forms | an ocean plate sinks, feeding explosive volcanoes | two continents push up great mountain ranges | plates slide past each other in sudden jerks | |
|---|---|---|---|---|
| a divergent margin, such as the Mid-Atlantic Ridge | ||||
| a subduction margin, such as the Cascades | ||||
| a collision margin, such as the Himalayas | ||||
| a transform margin, such as the San Andreas Fault |
For each kind of plate margin, say whether it usually has earthquakes and whether it usually has volcanoes.
| Earthquakes? | Volcanoes? | |
|---|---|---|
| divergent margin | ||
| subduction margin | ||
| collision margin | ||
| transform margin |
A fence built straight across a creeping stretch of a transform fault is already offset by $8$ cm, and the fault slips about $3$ cm a year. Write the offset, in centimeters, as a function of the years $t$ from now.
Answer:
A chain of volcanic islands formed over a fixed hotspot as a plate moved across it. An island now $700$ km from the hotspot is $10$ million years old. How fast has the plate moved, in centimeters a year?
Answer: cm a year
Los Angeles sits on the Pacific plate, sliding northwest past San Francisco on the North American plate. Suppose the two cities are $500$ km apart along the San Andreas Fault and the plates slide past each other $4$ cm a year. In how many millions of years would they be side by side?
Answer: million years
A survey follows 4 kilometers from an uplifting range to a coast. Cracked bedrock lies upslope, loose fragments enter a stream, and layered sand accumulates at its mouth. Mark all supported sentences in a linked process interpretation, including what cannot be inferred about plate boundaries.
This task has no paper form; do it on a device.
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
A fence built straight across a creeping stretch of a transform fault is already offset by $8$ cm, and the fault slips about $3$ cm a year. Write the offset, in centimeters, as a function of the years $t$ from now.
Answer:
You can explain plate margins. Explain why the San Andreas Fault has earthquakes but no volcanoes.
24. Your turn: a plate moves $4$ cm a year for $500000$ years. How far does it move, in kilometers?, step 3
$4 \times 10 \times 0.5 = 20$
Half a million years.