How Far Do the Continents Move in a Year?
― Hawaii Is Nearing Japan at the Speed Your Fingernails Grow
Continents and the sea floor ride on several huge slabs of rock, and they are still moving today. They travel a few centimetres a year, almost exactly as fast as your fingernails grow. That is too slow to notice in a lifetime, but kept up without a pause, it carries land across an ocean.
Have you ever looked at a world map and noticed that the eastern bulge of South America seems to fit the western dent of Africa, like pieces of a puzzle?
In fact, the two continents were once joined. They are thought to have drifted apart bit by bit, and the Atlantic Ocean formed in the gap.
So are the continents still moving today? And if they are, why do we feel nothing?
There are only two reasons we don't notice
The plates that carry the continents move roughly 2 to 10 centimetres a year. A fingernail grows about 4 centimetres a year. Watch it every day and you still never catch the moment it grows.
Houses, mountains and roads all move along with you. With no "fixed landmark" nearby, there is nothing to compare against. We could only measure the motion once we could use faraway objects in space as landmarks.
Yet this tiny movement never stops. Piled up over millions or tens of millions of years, it becomes a distance wide enough to span an ocean. Let's take it step by step.
As fast as a fingernail, but never resting
Earth's surface is covered by a dozen or so large slabs of rock. We call them plates. They are said to be tens of kilometres to about 100 kilometres thick, and they slide slowly over the softer layer beneath.
Figure 1 lines up how far things move in a year next to familiar things that grow. The Pacific Plate, which forms the sea floor east of Japan, moves about 8 centimetres a year. The Atlantic is widening by about 2.5 centimetres a year. One measurement says a nail grows about 3.5 millimetres a month, which comes to about 4 centimetres a year.
At 8 centimetres a year, an 80-year life covers just 6.4 metres. In a human lifetime that would not even take you across a classroom. No wonder we don't notice.
What if 8 centimetres a year goes on for tens of millions of years?
The Hawaiian Islands sit on the Pacific Plate. The plate heads roughly toward Japan, so Hawaii gets a little closer to Japan every year. Tokyo and Hawaii are about 6,200 kilometres apart.
Move the slider in Figure 2 to run time forward. After 10,000 years, Hawaii has moved only 0.8 kilometres. But after tens of millions of years, it has crossed most of the Pacific. A simple division says that covering 6,200 kilometres takes about 77.5 million years.
The same calculation works for the Atlantic. Take the North Atlantic to be about 5,000 kilometres wide, widening at 2.5 centimetres a year. The time it took to open comes to about 200 million years. The supercontinent that once held all the land together (Pangaea) is thought to have begun splitting about 200 million years ago. A plain calculation of speed × time roughly matches the ages we know from rock layers and fossils.
How do we measure motion we can't see?
The idea that continents move was published in 1912 by Germany's Alfred Wegener. But he could not explain what moves them, and for a long time the idea was not accepted. Only in the 1960s, when surveys of rock on the sea floor showed that the sea floor is slowly spreading, was it finally accepted.
Direct measurement became possible from the 1980s. Radio waves from objects billions of light-years away are picked up by two distant antennas, and that tells us the distance between the antennas to within a few millimetres. The objects are so far away that they serve as fixed landmarks. Methods that use radio signals from satellites are now widespread too. The Geospatial Information Authority of Japan runs about 1,300 GNSS reference stations across the country, and they keep recording, every day, which way and how many millimetres the Japanese archipelago has shifted.
The Hawaiian Islands form right above a hot spot deep underground, and are then carried northwest on the plate. So the island of Hawaii, which is erupting now, is the youngest, while Kauai to the northwest is thought to have formed about 5 million years ago. The two islands are about 500 kilometres apart. Divide 500 kilometres by 5 million years and you get about 10 centimetres a year. The line of islands itself works as a ruler that recorded the plate's speed.
The Pacific Plate dives beneath the Japanese archipelago at the trench east of Japan. As a result, many reference stations in eastern Japan are reported to be pushed west by a few centimetres a year in ordinary times. Japan sits on the front line where two plates collide.
Summary
Continents and the sea floor move roughly 2 to 10 centimetres a year, about as fast as a fingernail grows. Over a lifetime that adds up to only a few metres. But because it never stops, it crosses an ocean in tens of millions of years, and it made the Atlantic in 200 million.
The ground isn't slow. It just never rests.
Even at fingernail speed, 200 million years is enough to make an ocean.
For the records left on the sea floor, see the story of Earth's north and south poles swapping places. For seashell fossils on mountaintops, see the article on shell fossils in the mountains.
- Draw one thin line with a permanent marker near the base of your index fingernail. If it wears off, redraw it in the same spot.
- After a month, measure with a ruler how far the line has moved from the base. It should be about 3 to 4 millimetres.
- Multiply that length by 12 to get one year's worth. That is roughly how far the ground moves in a year. Save a year of clipped nails and you get a feel for that amount.
Nail growth is said to vary a little with age, season and finger. Try comparing your left and right hands, and your toenails too.
For those who want more ― terms, formulas and links to textbooksWe mark which level each part belongs to, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school "Earth Science Basics" and "Earth Science"
- High school+Advanced high school material, or textbook sidebar topics
- UniversityUniversity courses not taught in high school (geophysics, geodesy)
- ResearchNot yet taught as settled fact even at university; being investigated by researchers now
Junior highTerms: this phenomenon has names
- Plate: a slab of rock covering Earth's surface, tens to about 100 kilometres thick. There are a dozen or so.
- Trench: a deep groove where one plate dives beneath another. Trenches lie in the seas east and south of Japan.
- Mid-ocean ridge: an undersea mountain chain where plates pull apart and new plate forms. One runs north to south down the middle of the Atlantic.
Junior highHigh schoolChecking with a formula: how many years until Hawaii reaches Japan?
Plate speed can be treated as nearly constant, so distance travelled is found from "speed × time." Conversely, dividing distance by speed gives the time needed.
| In symbols | d ＝ v × t (so t ＝ d ÷ v) |
| In words | Distance travelled ＝ length moved per year × number of years |
| Where it comes from | It is the relation for motion at constant speed. A plate keeps moving in nearly the same direction at nearly the same speed for millions of years, so this formula gives a fair estimate. |
| d | Distance travelled (km) |
| v | Speed (about 8 cm a year; here converted to km per 10,000 years) |
| t | Time needed (units of 10,000 years) |
| Speed of the Pacific Plate | About 8 cm a year |
| Distance from Tokyo to Hawaii | About 6,200 km |
| Width of the North Atlantic, and its widening speed | About 5,000 km, about 2.5 cm a year |
| Length moved in 10,000 years (cm) | 8 × 10000 ＝ 80000 |
| Convert to km (1 km is 100,000 cm) | 80000 ÷ 100000 ＝ 0.8 |
| Time for Hawaii to arrive (×10,000 yr) | 6200 ÷ 0.8 ＝ 7750 |
| Length moved in a human life of 80 years (cm) | 8 × 80 ＝ 640 |
| Atlantic: width gained in 10,000 years (km) | 0.8 ÷ 3.2 ＝ 0.25 |
| Time for the Atlantic to open (×10,000 yr) | 5000 ÷ 0.25 ＝ 20000 |
By this calculation Hawaii reaches Japan in about 77.5 million years, or roughly 80 million. In one lifetime it moves 6.4 metres. The Atlantic opened in 20,000 × 10,000 years, about 200 million years, which closely matches the estimate for when the continents began to split (2.5 cm is one 3.2nd of 8 cm, so we divide 0.8 km by 3.2).
High schoolHigh school+The sea floor works like a tape recorder
High schoolIn Earth Science Basics, this is taught as plate tectonics. Rock born at a mid-ocean ridge records the direction of Earth's magnetic field of its time as it cools. Because the field flips every few hundred thousand years, a striped pattern of records lies mirrored on both sides of the ridge. Comparing the stripe widths with their ages gives the speed at which the sea floor spreads (the sea-floor spreading theory).
High school+When islands form over a fixed heat source beneath the plate (a hot spot), as in Hawaii, the line of islands shows the direction and speed of plate motion. The Emperor Seamount chain, running north from Hawaii, bends partway along, which is taken as evidence that the Pacific Plate changed direction. The method of measuring distance with radio waves from faraway objects is called very long baseline interferometry (VLBI).
UniversityPlates on a sphere: Euler poles and the forces that move plates
The motion of a plate on a sphere can be described as a rotation about an axis through Earth's centre. The point where this axis meets the surface is called the Euler pole. On the same plate, places 90 degrees from the Euler pole move fastest, and places near the pole move slowest. Of the forces that move plates, "slab pull" is thought to be the largest: the cold, heavy plate sinking at a trench pulls the rest along behind it. Ridge push at the mid-ocean ridges and drag from mantle convection add to it. Each plate's rotation rate is collected in geological models (such as MORVEL) built from sea-floor stripe patterns and topography.
📖 Derivations and further reading: Plate tectonics (Wikipedia, in Japanese) ／ Continental drift (Wikipedia, in Japanese)
ResearchWhat is still not clear
- When did plate motion begin? One view says it started about 4 billion years ago, another says much later. The question is not settled.
- Why does only Earth have it? Venus and Mars show no plate motion like Earth's. The presence of water and differences in internal heat are candidates, but the reason is unclear.
- The share of each driving force. How much pull, push and drag from below each contribute is still debated for each plate.
So even this article is an "explanation within what we know today." In particular, forecasts tens of millions of years ahead are simple estimates that assume speed and direction stay the same.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science "Changes in the Earth," Math "speed, time, distance" | The terms plate and trench, the speed × time calculation |
| High school | Earth Science Basics "plate tectonics" | Sea-floor stripe patterns, sea-floor spreading theory |
| High school+ | Earth Science "hot spots," "space geodesy" | Ages of the Hawaiian islands, surveying with radio waves from space objects |
| University | Geophysics, geodesy | Euler poles, slab pull |
| Research | Earth's evolution, comparative planetology | The start of plate motion, differences from other planets |
| ― | Links to daily life | Why the reference positions for map location data are corrected every few years |
- DeMets, C., Gordon, R. G., Argus, D. F. (2010) Geologically current plate motions. Geophysical Journal International
- Yaemsiri, S., Hou, N., Slining, M. M., He, K. (2010) Growth rate of human fingernails and toenails in healthy American young adults. Journal of the European Academy of Dermatology and Venereology
- Geospatial Information Authority of Japan (国土地理院), explanatory materials on the GNSS Earth Observation Network System (GEONET)
- National Astronomical Observatory of Japan (国立天文台), ed., Rika Nenpyo (理科年表, Chronological Scientific Tables), Maruzen Publishing (Earth science section: plate motion)
- Wegener, A., The Origin of Continents and Oceans (大陸と海洋の起源), Japanese translation by Akiho Miyashiro and Fumiko Shidō, Iwanami Bunko
* This article is a popular-science explanation for general readers. The numbers given are rough guides to help you understand how things work. Plate speeds differ by place and by how they are measured, and forecasts of future positions are simple estimates that assume speed and direction stay constant.