New Mexico is split by a crack that is still opening
The Rio Grande did not carve its valley: it dropped into a tectonic hole that already existed. Under Albuquerque there are 7,350 metres of fill, the youngest lava is 5,400 years old and the ground above Socorro is still rising 2 millimetres a year.
Look at a map of New Mexico and you will see a line running north to south down the middle of the state. It is the Rio Grande. What almost nobody mentions is that the river did not choose that route: it dropped into a hole that was already there.
And the hole was not made by erosion. It was made by the continent starting to split.
01 A crack a thousand kilometres long and fifty wide
The Rio Grande rift is a north-trending continental rift zone. It extends from central Colorado to the state of Chihuahua, in Mexico, and separates the Colorado Plateau, to the west, from the interior of the North American craton, to the east.
It is not a single fracture: it is four linked basins, averaging about 50 kilometres across. The Española basin is about 120 kilometres north–south by 40 east–west; the San Luis, about 120 by 80.
The mechanism is pure shear: both sides pull apart evenly and slowly, and the lower crust and upper mantle stretch. Under the crack, the mantle is abnormally hot and partially molten to depths of about 400 kilometres.
02 Seven thousand three hundred metres of fill under Albuquerque
The Albuquerque basin is the largest of the three main ones: 160 kilometres north–south and 86 east–west at its widest point. It is also the oldest.
And it contains 7,350 metres of Palaeogene clastic sediments deposited on Precambrian basement. That is: between the city's surface and the ancient rock beneath there are over seven kilometres of sand, gravel and conglomerate shed from the surrounding ranges.
That fill is what makes the basin look like a plain. The tectonic hole is far deeper than the landscape: if you emptied the sediment out, the city would sit at the bottom of a seven-kilometre canyon.
And it explains the other feature of the place. The ranges flanking the basin, such as the Sandia above Albuquerque, are not folded mountain chains: they are the uplifted edges of the crack, with an abrupt face towards the valley and a gentle slope on the other side.
03 What comes up through the crack
As the lithosphere thins, the hot asthenosphere rises and comes into contact with the crust. That intrusion is responsible for practically all the volcanism associated with the rift, and in New Mexico that volcanism is abundant and visible.
The largest example is the Valles caldera, in the Jemez Mountains: a circular hole several kilometres across with its floor turned to grassland, ringed by the rim of what was the volcanic edifice. You drive across it without realising you are inside a crater.
And the basin fill is not only sediment: it alternates alluvial fans with mafic lava flows. The more alkaline lavas, by contrast, erupted outside the rift. The chemistry of the rock tells you which side of the crack you are on.
04 Lava from five thousand four hundred years ago
The proof that this is not ancient history is in the south of the state. The youngest eruptions in the rift region are those of the Valley of Fires, the Carrizozo Malpais, and they are about 5,400 years old.
Five thousand four hundred years is nothing. It is a flow that was hot when maize was already being farmed in the Americas and cities were rising in Mesopotamia. The rock is still black, rough and soil-free, because it has not had time to make any.
Walking along the edge of that flow the contrast is clear: on one side, scrub on pale earth; on the other, a field of blocks where only plants that find a crack with a little sediment in it can grow.
05 And it is still going: two millimetres a year
The figure that closes the argument is at Socorro, in the centre of the rift. Beneath the town, at 19 kilometres depth, there is a sill-like magma body that is inflating.
That chamber is responsible for anomalously high seismic activity in the area, including the largest rift-associated earthquakes in historic times: two events of approximately magnitude 5.8 in July and November 1906.
And geodetic measurements, on the ground and from space, show the surface above the magma body rising at about 2 millimetres a year. The crack that made the hole the river runs through is still working under your feet.
06 How to read the map
The first practical consequence is that in New Mexico the interesting roads are those crossing the crack, not those following it. Going east to west takes you from the uplifted edge to the basin floor and back up again, in under an hour.
The second is that almost everything that looks like an isolated feature is part of the same system: the Rio Grande gorge, the Valles caldera, the southern lava fields and the ranges flanking Albuquerque are the same phenomenon seen in four different phases.
And the third changes the scale of the trip. You are travelling along the edge of a continent pulling away from itself very slowly. Two millimetres a year is imperceptible, but it has been accumulating for thirty-five million years.
