What if a mountain wore away?

A mountain is made of hard rock. It looks like it will stay the same forever.

But what if it slowly wore away? Where would all that rock go?

Mountains seem to be the most lasting things on Earth. Yet every mountain is slowly being broken apart and carried away, a little each year, too slowly for us to notice.

What if you could watch a whole mountain wear away? In this thought experiment, you can follow three paths: how rain, ice and wind break rock apart, how rivers carry the pieces off, and how new mountains rise to take the old ones' place.

Geologists separate two processes. Weathering is the breaking down or dissolving of rock where it sits; erosion is the transport of the resulting material by water, wind or ice. Their opposite, deposition, builds up land where the material settles.

This thought experiment speeds up time to follow one mountain's rock from summit to sea. It asks which processes do the work, where the material ends up, and why Earth still has mountains at all. The physical processes are well documented; the imagined journey of a single mountain is an illustration, labeled as an idea where it goes beyond measurements.

Water gets into cracks in rock. At night it freezes into ice and pushes the crack wider.

Tree roots grow into cracks too. Little by little, the rock breaks into pieces.

Water is a powerful rock breaker. It seeps into cracks, and when it freezes it expands, working like a wedge that pushes the crack wider. When the ice melts, the water carries tiny bits away. This is called frost wedging.

Plants help too: a seed sprouting in a crack grows roots that slowly split the rock. Some rock is dissolved by weak acids in rainwater, and in dry places, windblown sand can blast rock smooth. Glaciers grind rock as they slide downhill.

Mechanical weathering includes frost wedging, in which water freezes and expands in cracks; exfoliation, in which outer sheets of rock flake away, as at Yosemite's Half Dome; and root wedging by plants. Chemical weathering changes rock's makeup: carbonic acid dissolves limestone, and iron-bearing minerals oxidize.

Erosion then removes the debris. National Geographic Education identifies liquid water as the major agent, from raindrop splash, which can throw soil particles up to 0.6 meters, to rivers and floods. Wind abrades soft rock in dry regions, and glaciers scrape, grind and transport rock; ice sheets can erode as much as half a centimeter a year.

Let's pretend we can watch a mountain for millions of years, very fast.

The sharp top might get rounder and lower. Its rock might roll into rivers and travel all the way to the sea. This is only pretend.

Imagine a time-lapse camera pointed at a mountain for millions of years. Its sharp peaks might crumble and round off. Rockfalls and landslides might send boulders tumbling down. Streams might cut deeper valleys, and the whole mountain might get lower and lower.

The rock would not disappear. It might become sand and mud carried by rivers, spread over farm fields in floods, or piled up as new land where a river meets the sea. These are ideas, built from what scientists measure today.

Accelerating time would reveal a sequence: summits fracture and round off, mass wasting such as rockslides delivers debris downslope, and streams incise valleys while carrying sediment away. Relief might fall steadily unless uplift continued.

The mass would be redistributed, not destroyed. Fine sediment might settle on floodplains, build deltas, or reach the ocean floor. Old ranges show where this leads: the Appalachians, NPS notes, probably once had peaks as high as the Himalayas or the Alps, and have been worn to modest heights over 300 million years. Rates in the imagined time-lapse would depend on climate, rock type and vegetation.

When it rains a lot, rivers turn brown. The brown is mud and sand from the land.

The river carries it far away and drops it where the water slows down.

Have you ever seen a river turn brown after a storm? That color is sediment: bits of rock and soil being carried along. Fast water can lift and carry bigger pieces than slow water, so rivers carry the most sediment during storms.

Where the water slows down, the sediment settles. It can spread over the land beside the river and make rich soil for farming, or pile up at the river's mouth to make a delta.

USGS explains that fast-moving water can suspend and move larger particles than slow water, so rivers look muddiest in storms; more than half of a year's sediment may move during a single storm period. Material settles where velocity drops.

That deposition builds landforms. Floodplain sediment is often mineral-rich farmland; deltas, National Geographic Education notes, are made almost entirely of eroded river sediment, and China's Yellow River is named for the windblown loess it carries. Dams interrupt the journey: reservoirs trap sediment because their water moves too slowly to keep it suspended, and they slowly fill with mud.

If mountains keep wearing away, why are there still tall mountains?

Because giant pieces of Earth's crust push new mountains up, very slowly!

If mountains keep wearing away, why does Earth still have tall ones? Because the plates that make up Earth's outer shell keep moving. Where they push together, they lift new mountains. The Himalayas are still rising more than 1 centimeter a year.

Water can also carve down as land rises. The Grand Canyon is about 446 kilometers long and about 1.6 kilometers deep. It formed as the land rose and the Colorado River cut deeper and deeper over about 5 million years.

Mountains persist because uplift competes with erosion. USGS reports that the Himalayas, formed as India collided with Eurasia 40 to 50 million years ago, still rise by more than 1 cm a year. The Hawaiian Islands show the opposite phase: carried off their hotspot, the volcanoes age and erode, and Kauai's 5.5-million-year-old rocks are deeply eroded.

Uplift can also drive erosion. National Geographic Education describes how, over about 5 million years, tectonic uplift helped the Colorado River cut into the Colorado Plateau, forming the Grand Canyon: about 446 km long, up to 29 km wide and around 1.6 km deep.