The Hawaiian islands are arranged by age, and the youngest is still growing

One detail of the map of Hawaii looks decorative and is in fact a clock: the eight main islands line up along an almost perfect diagonal running northwest to southeast. That is not an aesthetic coincidence. It is the record of a movement.

Travel that diagonal from one end to the other and you are travelling five million years, in the order in which they happened.

01 A six-thousand-kilometre conveyor belt

The mechanism is disconcertingly simple. At one particular point in the Earth's mantle, beneath the central Pacific, sits a source of magma that has stayed in the same place for tens of millions of years. The oceanic plate above it is not still: it moves northwest.

The result is that the magma punches through the plate, builds a volcano and, in time, that volcano drifts away from its supply and goes out. Behind it another starts forming. Repeated over millions of years, the process leaves a row of volcanoes that get older the further northwest you go.

The full chain is some 6,200 kilometres long, and the islands above water are only its final stretch; the rest are sunken seamounts. One precision is due: the classic model describes a motionless hotspot, and there is now research suggesting the hotspot itself has shifted over time. The age progression, in any case, is well established.

02 How you see an island's age

What matters for a traveller is that this age is not a laboratory figure: it is visible to the naked eye. As soon as a volcano stops growing, rain, surf and landslides start working on it, and every million years leaves a recognisable mark.

At the old end of the archipelago, Kauai's rock is about 5.5 million years old and the result is plain: canyons over a thousand metres deep, cliffs fluted like church organs, wide valleys with deep soil and not one active volcano. The island has spent five million years coming apart.

That erosion also manufactures what tourism calls scenery: the ravines, the tall waterfalls, the pale sand produced by the breakdown of coral and rock. All of it is accumulated time, which is why none of it exists at the other end of the line.

03 The young end: rock still unused

To the southeast, Hawaii Island has rock 700,000 years old or less, and in some stretches days old. There the landscape is exactly the opposite: black surfaces, smooth or jagged, without ravines, without soil and with barely any vegetation. It is not a desert: it is an island that has not yet had time to erode.

That contrast even explains the colour of the sand. On the young island the beaches are black, because the sand is lava ground down by the surf and has not yet had time to mix with anything else. On the old ones it is pale, because they have spent millions of years receiving carbonate from the reefs.

04 The tallest mountain in the world

The hotspot has built, in total, around 750,000 cubic kilometres of lava. Stood upright, that produces a figure hardly anyone keeps in mind: measured from its base on the sea floor, Mauna Kea reaches 10,203 metres, more than Everest.

The difference is that only a little over four of those ten kilometres show. The rest is under water, and the weight of such a structure also presses down the crust it rests on. These islands are not peaks above the sea: they are the tips of enormous mountains built up from the floor of the Pacific.

That detail explains something that surprises new arrivals. On the young island you go from sea level to four thousand metres in a few tens of kilometres of road, crossing wet forest, dry scrub and an ash desert where there can be snow.

05 The next island is already under construction

If the mechanism is a conveyor belt, something must be forming behind. And there is: about 35 kilometres southeast of Hawaii Island a submarine volcano is growing, Kamaʻehuakanaloa, still in the stage before shield building.

It will not emerge on any timescale that matters to anyone alive: it has tens of thousands of years to go at the very least. But it is there, and knowing it closes the argument. The row of islands does not end at Hawaii; it ends, for now, at a seamount unnamed on most maps.

06 What this changes for the traveller

The first consequence is that the islands are not variants of the same place. Changing island is not changing beach: it is changing geological age, and with it the relief, the soil, the colour of the sand and even the amount of fresh water available. Choosing an island is the important decision of the trip.

The second is that direction means something. Travelling the archipelago southeast to northwest — from bare lava to eroded canyons — is travelling the process in order, and each island explains the next. Doing it the other way works just as well as long as you know what you are looking at.

The third is about timescale. Almost everything impressive here is very recent in geological terms: a Kauai cliff is younger than the present Mediterranean, and a flow on Hawaii Island may be younger than you are. It is one of the few places on the planet where the landscape can be dated by looking at it.