How Does One Rock Become Another?: Going Further 6–8

Three kinds of rock

In Grade 7 you sorted rocks into three kinds, by how they form. Here they are again, with rocks you can find in Nepal:

Kind	How it forms	Examples	In Nepal
igneous	molten rock (magma) cools and hardens	granite, basalt, pumice, obsidian	granite on Everest, Makalu and Manaslu
sedimentary	pieces of other rocks, or shells, are laid down in layers, buried and hardened	sandstone, shale, conglomerate, limestone	sandstone and conglomerate in the Churia hills; limestone at Chobhar and Godawari
metamorphic	a rock is changed by heat and pressure, without melting	marble, slate, schist, gneiss, quartzite	marble at Godawari; slate roofs in the hills; schist and gneiss in the High Himalaya

Every rock is made of minerals (you met them in Grade 8: What a mineral is), and a rock’s minerals and the way its grains fit together hold clues to how it formed.

In an igneous rock, the grains are crystals that grew as the magma cooled. Magma that cools slowly, kilometers underground, has thousands of years to grow big crystals you can see, as in granite. Lava that cools in hours or days at the surface makes tiny crystals (basalt), or none at all: obsidian is a natural glass.
A sedimentary rock is made of grains or shells stuck together, usually in layers, and it may hold fossils.
In a metamorphic rock, the minerals have grown again in the solid rock. Squeezed from one side, the new flat crystals line up, so that slate splits into thin sheets and gneiss has bands of light and dark minerals.

Everest shows all three kinds, one above the other.

Figure 23.1 The rocks of Everest’s summit pyramid, simplified, with heights to scale. Limestone with sea-lily fossils at the top; the Yellow Band of marble; schist; and at the bottom gneiss, threaded with sheets of pale granite that melted out of the buried rock about 20 million years ago. The layers were laid down in order on a sea floor, then buried, changed, partly melted, and lifted.

A climber in a red suit on a steep snow slope, clipped to a rope, crossing a band of brownish-yellow rock that runs diagonally across the snow under a deep blue sky — **Figure 23.2** A climber crossing the Yellow Band, high on Everest. The yellowish-brown rock under the snow is marble, a limestone changed by heat and pressure. Photo: Lloyd Smith, CC BY-SA 3.0

A rock has big crystals of three minerals, all fitted together, with no layers. How did it most likely form?

Pick an answer to see why.

Any rock can become any other

Pick up a pebble from the bed of the Trishuli or the Bagmati. It is round and smooth, but it didn’t start that way. It was broken off a cliff higher up, then rolled and knocked along the riverbed until its corners were worn away. One day it may be ground into sand, carried to the plains, and buried. That is one step of a journey that never ends. Here are all the steps, the processes that change rock:

Weathering: rock at the surface is broken up by frost, heat and cold, plant roots and water, and some of its minerals are dissolved or rotted into clay (Grade 6).
Erosion: the pieces, called sediment, are carried away by rivers, glaciers, wind and landslides, always downhill in the end.
Deposition: where the water or wind slows down, it drops its load: on riverbeds, plains, lake floors and above all the sea floor. Dissolved minerals reach the sea too, where sea animals use them to build shells.
Compaction and cementation: as more layers pile on top, the weight squeezes the sediment, and minerals crystallize between the grains and glue them together. Loose sand becomes sandstone, mud becomes shale, and heaps of shells become limestone. This takes thousands to millions of years.
Metamorphism: buried deeper, or caught where two plates push together, the rock is heated to a few hundred degrees and squeezed. Its minerals change, without melting: shale becomes slate, then schist, then gneiss; limestone becomes marble; sandstone becomes quartzite.
Melting: deeper and hotter still, any rock begins to melt, and becomes magma.
Cooling: magma is less dense than the solid rock around it, so it rises. It cools and crystallizes into igneous rock, slowly underground (granite) or fast at the surface after a volcano erupts (basalt).
Uplift: the pushing of the Earth’s plates lifts rock that formed deep down, or on the sea floor, up into mountains, where weathering starts on it again.

Because each step can follow almost any other, these processes don’t make a neat circle. Granite can be weathered into sand without ever being metamorphosed; a sedimentary rock can be pushed deep and melted without being metamorphosed first; a metamorphic rock can be squeezed again into another metamorphic rock. Geologists call the whole web the rock cycle: Earth’s materials go round and round it, changing their form but never being used up.

Figure 23.3 A model of the rock cycle, as a slice through the land from the mountains to the sea. Amber arrows are the steps driven by the Sun’s energy (through rain, rivers, ice and wind); red arrows are the steps driven by the Earth’s internal heat; the gray arrow is burial, by the weight of the layers above. Not to scale: the deep part is squeezed up, and the real distance from the mountains to the sea is a thousand kilometers.

The rock cycle is about matter, and matter is not made or destroyed. The atoms in a grain of quartz in Everest’s granite may have been in a sandstone before, and in a granite before that. A calcium atom in the summit limestone was in a sea lily’s skeleton, and before that dissolved in sea water, and before that in a rock worn away on some other land. When that limestone is worn away, its calcium will wash down the Dudh Koshi to the sea, and may end up in a shell again.

Start with a granite and choose what happens to it next. Each step shows the rock it becomes, and whether the energy for that step comes from the Sun or from the Earth’s internal heat. Try to turn the granite into the marble of Everest’s Yellow Band. How many steps does it take, and how many different routes are there?

Steps: 0
Driven by the Sun: 0
By Earth’s heat: 0
By gravity: 0

Your rock is a granite, deep under a mountain. Choose what happens to it next.

A geologist finds a marble on a mountain. What must have happened to it?

Pick an answer to see why.

What drives the cycle: two engines

Nothing changes without energy. Lifting a mountain, grinding a pebble, melting rock: each needs energy, and the rock cycle has two sources of it.

The Sun

Sunlight warms the land and the sea. It evaporates water, which rises as vapor, comes down as rain and snow on the mountains, and runs back to the sea in rivers. The Sun lifts the water; then gravity pulls it downhill, and the moving water carries rock with it. Glaciers are snow that the Sun lifted, too. The Sun heats the air unevenly, which makes the winds. Its daily warming and nightly cooling crack rock and freeze and melt the water in its cracks. And it feeds the plants whose roots split rock. So weathering, erosion and deposition all run on energy from the Sun Grade 9: Grade 9: The Sun, the ultimate source of energy.

The Earth’s internal heat

The inside of the Earth is hot. In Nepal, you can feel it in the hot springs of the villages called Tatopani (“hot water”), and miners know it: the deeper a mine goes, the hotter its rock. In the upper crust, the temperature rises by roughly 25 °C for every kilometer you go down. The heat has two sources. Some is left over from the Earth’s violent birth, when rocks crashed together and the whole planet melted Grade 8: Grade 8: A young, violent Earth. The rest, about half, is made all the time by radioactive atoms of uranium, thorium and potassium in the rock, as they decay.

This heat escapes slowly towards the surface. On its way it makes the mantle flow, slowly, which moves the plates of the Earth’s crust. So metamorphism, melting, volcanoes and uplift all run on the Earth’s internal heat. (How it moves the plates is the topic Do Continents Really Move?.)

Gravity joins in on both sides: it pulls water, ice and landslides downhill, and it is the weight of new layers that squeezes buried sediment into rock.

Worked example

How deep is hot enough?

The temperature at the surface is 20 °C, and it rises by 25 °C for every kilometer down.

a.What is the temperature 10 km down?
b.Wet rock in the crust starts to melt at about 700 °C. Roughly how deep is that?

Answer

a.The rise is 25 °C/km × 10 km = 250 °C, so the temperature is 20 + 250 = 270 °C.
b.The rock must warm by 700 − 20 = 680 °C. At 25 °C per kilometer, that takes 680 ÷ 25 ≈ 27 km. Under most of the world, the crust is about 35 km thick, so melting this deep is rare. Under the Himalaya and Tibet, where India has pushed under Asia, the crust is up to about 70 km thick, and some of the rock of the High Himalaya did melt, at about 700 °C. That is where the granite on Everest and Makalu came from: it melted out of buried sedimentary and metamorphic rock.

Which engine is bigger? The heat coming out of the Earth is spread over its whole surface: on average about 0.09 watts through every square meter. Sunlight absorbed by the Earth, averaged over day and night and the whole globe, is about 240 watts per square meter: some 2,700 times as much.

So why does the small engine matter at all? Because the Sun’s energy works only at the surface. Sunlight warms only the top few meters of the ground, and most of its energy goes back into space as heat within days. It can wear rock away, but it can’t melt rock, or lift a mountain. The Earth’s heat works at depth, everywhere, without stopping, for millions of years. Without it, the Sun’s engine would wear the continents down flat into the sea, and the cycle would stop.

Which of these is driven mainly by the Earth’s internal heat?

Pick an answer to see why.

Slow and sudden

You might never see a mountain rise or a valley deepen: most of the rock cycle is far too slow. Rivers in the Himalaya cut down into their beds by a few millimeters a year, up to about a centimeter in the fastest places. Mountains rise at about the same pace. In a human lifetime, that adds up to less than a meter. But the Earth has had millions of lifetimes.

Try it

How fast did the summit rise?

The limestone at the top of Everest formed on a sea floor and now stands about 8.8 km above the sea. India began to collide with Asia about 50 million years ago. If the rock had risen at a steady rate since then, what would the rate be, in millimeters a year?

Hint

Change 8.8 km into millimeters, then divide by the number of years.

Answer

8.8 km = 8,800,000 mm.
$\text{rate} = \frac{8{,}800{,}000\ \text{mm}}{50{,}000{,}000\ \text{years}} = 0.18\ \text{mm/year}$
About 0.2 mm a year, the thickness of two sheets of paper. It is an average: the rise has been faster at some times than at others, and today GPS shows parts of the Himalaya rising by a few millimeters a year, while rain and rivers wear the tops down.

A river cuts down into the rock of a high, flat land. Choose how fast it cuts, then move through time and watch its valley deepen. The valley is drawn to scale: a kilometer across is as long as a kilometer down.

The river cuts down, each year:

Time: 100,000 years

Depth of the gorge: 500 m
Width at the top: 1.4 km
Cut in a human life (80 years): 40 cm

A deep valley, hundreds of meters down. Nobody alive would have seen it change.

Not everything is slow. Some of the biggest changes happen in seconds or minutes:

What happens	An example	How long it takes	What it changes
an earthquake	Gorkha earthquake, 25 April 2015	under a minute	the land under Kathmandu and around it, about 100 km across, rose by up to about 1 m
a landslide	Jure, Sindhupalchok, 2 August 2014	a few minutes	about 5.5 million m³ of rock fell, burying a village and damming the Sun Koshi into a lake 3 km long
a glacial lake bursting	Dig Tsho, Khumbu, 4 August 1985	a few hours	an ice avalanche fell into a lake; the lake burst through its wall of stones, and its flood tore down the valley, sweeping away a nearly finished hydropower plant, bridges, houses and fields
a meteorite hitting the ground	Lonar crater, Maharashtra, India	seconds	made a crater 1.8 km wide and 150 m deep in basalt

A huge gray slope of loose rock and mud spread across a green hillside under a cloudy sky, with trees at its edges — **Figure 23.4** The Jure landslide in Sindhupalchok, seen after it fell on 2 August 2014. Heavy monsoon rain set it off at about half past two in the morning, and in a few minutes the slope was gone. Photo: Ascii002, CC BY-SA 3.0

A satellite view of farmland in purple, green and pink, with a round dark lake in a pale ring near the center — **Figure 23.5** Lonar crater in India, from a satellite, in false colors. A meteorite blasted it out of the basalt of the Deccan in a few seconds. How long ago is still debated: dates range from tens of thousands of years to about 570,000 years. Photo: NASA, Public domain

Sudden events and slow ones work together. An earthquake shakes the slopes; the next monsoon’s rain sets off landslides on them; the landslides drop rock into the rivers; and the rivers spend years carrying it away, a little at a time.

Things also happen on every size. A grain of sand is a fraction of a millimeter across; a landslide is hundreds of meters; a river’s valley is tens of kilometers long; and the Himalaya stretch about 2,400 km. Big, slow changes are made of countless small, quick ones: each grain moved, each crack opened.

Figure 23.6 How long some of the Earth’s changes take, and how big they are, roughly. Both scales stretch: each line up the side is ten times the size of the one below, and the times run from a second to tens of millions of years. The sudden events are at the left, the slow processes at the right; the slowest is also the biggest.

Which of these changes Earth’s surface fastest?

Pick an answer to see why.

A story from science

The lake that became a valley

The old story of the Kathmandu Valley, in the Swayambhu Purana, says that the valley was once a lake, with a lotus flower shining on it where Swayambhu now stands. The bodhisattva Manjushri came from the north, cut through the hills at Chobhar with his sword, and let the water out, and the lake bed became the valley where people settled.

Geologists, drilling into the valley floor, have found that part of it is true. There was a lake. As the Mahabharat hills to the south rose, perhaps about a million years ago, they dammed the rivers, and the valley filled with water. For hundreds of thousands of years, the rivers dropped their mud into it, and the lake floor filled with fine, dark clay: its name in the drill records is the Kalimati (“black earth”) formation, like the place in Kathmandu. The lake drained in stages, as the Bagmati cut down through the limestone at Chobhar; earthquakes may have helped. When the last of it went, geologists still debate: some studies find it nearly gone about 40,000 years ago, others find some of it lasting until about 10,000 years ago. Much of the clay that the valley’s potters and brick kilns dig today was laid on the floor of that lake.

So the story has the place right, Chobhar, and the result right. What took a sword stroke in the story took a river many thousands of years.

Looking along a narrow gorge with steep, gray rock walls and trees on top, with a fast, foaming river at the bottom, under an arched concrete bridge — **Figure 23.7** The Bagmati leaving the Kathmandu Valley through the Chobhar gorge, cut into limestone. This is the gorge of the Manjushri story. Photo: Shadow Ayush, CC BY-SA 4.0

Reading the land

Water, ice and wind each shape the land in their own way, and the shapes they leave behind are evidence. A geologist walking up a valley in Nepal can read what made it, the way you read footprints.

Rivers

A river cuts down into its bed, while landslides and rain on the slopes on either side keep wearing them back. So a river valley is a V, often a deep and narrow one: a gorge. Rivers in Nepal’s mountains cut some of the deepest gorges on Earth, like the Kali Gandaki’s, between Dhaulagiri and Annapurna. Where a river comes out of the hills and slows down, it drops its stones and sand in a wide fan; a river that later cuts down into its old bed leaves its old floor as a flat step on the valley side, a terrace, a good place for a village or rice fields.

A narrow valley between steep, wooded mountainsides with gray cliffs, a blue-green river running over a bed of gray boulders, and a road being built on the right — **Figure 23.8** The Kali Gandaki gorge near Rakhu Bhagawati, Myagdi: a V-shaped valley, with the river at its bottom on a bed of boulders it has brought down. Photo: Bijay Chaurasia, CC BY-SA 4.0

Glaciers

A glacier is a river of ice, fed by snow. It flows downhill slowly, from a few meters to a few hundred meters a year, and it is often hundreds of meters thick, so it fills its valley. Rocks frozen into its bottom and sides scrape and grind the rock beneath like sandpaper. So a glacier wears away the sides of its valley as well as the floor, and turns a river’s V into a U, with a wide, flat floor and steep sides. The rocks it carries are dropped where the ice melts, in long ridges of jumbled stones of every size, called moraines.

Figure 23.9 Left: a valley cut by a river, a V. Right: a valley carved by a glacier, a U, with moraines along its sides where the ice dropped the stones it carried. Long after the ice has melted, the shape shows that a glacier was there.

A wide valley between gray, snow-streaked mountains, with a broad, rounded floor of gravel, seen from a dirt road above it — **Figure 23.10** A U-shaped valley in Ladakh, in the Indian Himalaya. The glacier that carved it has melted back to the snowy peaks at its head. Photo: DanHobley, CC BY-SA 3.0

A wide glacier covered in gray stones, with patches of bare ice, between a long, steep-sided ridge of gray stones and snowy mountains — **Figure 23.11** A glacier near Everest, seen from Kala Patthar, its ice mostly buried under the rocks it carries. Beyond it runs a long, steep-sided ridge of stones, a moraine, dropped by the ice when the glacier was thicker. Photo: Jone Jones, CC BY-SA 3.0

Many valleys in the Himalaya, like the Khumbu and Langtang, are U-shaped in their upper parts, where the glaciers reached in the last ice age, and V-shaped lower down, where only rivers have worked. Their moraines show how far down the ice once came.

Wind

Wind can carry only small grains, sand and dust, so in a wet, green country like Nepal it does little. In dry places with little to hold the ground, it does much more. In the Thar Desert of India and Pakistan, it piles sand into dunes that creep across the land. And in northern China, dust blown from the deserts over the last 2.6 million years has piled into the Loess Plateau, a land of fine yellow soil, in places more than 100 m deep. Wind-blown sand also scours rock, like sandpaper, close to the ground.

In a valley far from any ice today, you find a wide, flat floor with steep sides, and long ridges of jumbled stones of every size, from sand to boulders, along its sides. What made it?

Pick an answer to see why.

Make a rock cycle

Activity

A rock cycle in crayons

You need: three or four old wax crayons of different colors, a pencil sharpener or a grater, a sheet of aluminium foil, a heavy book, a steel bowl and hot water from a kettle. Ask an adult to help with the hot water.

Weathering: shave each crayon into a small heap of shavings, a different color for each heap. These are your sediments.
Sedimentary rock: on the foil, make a layer of one color, then another on top, then a third. Fold the foil over and press it under the heavy book, or stand on it. Unwrap it: you have a crumbly, layered “rock”, with each grain still visible.
Metamorphic rock: wrap it again, and float the foil packet on the hot water for a few seconds at a time, until the wax softens but doesn’t melt. Take it out and press it hard from the sides, or fold it. Unwrap it again.
Igneous rock: make a little open boat of foil, put some of your rock in it, and float it on the hot water until the wax melts. Take it out and let it cool.
Weathering again: shave your new rock into the next set of sediments. You have gone round the cycle.

What you should see

The pressed layers stick together, but you can still see the separate shavings, like the grains in sandstone: that is compaction and cementation. Warmed and squeezed, the layers bend and smear, and the grains merge into a harder, denser block with wavy bands of color, like the bands of a gneiss, though the wax never melted: that is metamorphism. Melted, the colors mix into one, and cool into a smooth solid with no layers or grains: an igneous rock. The model is good at showing that the same material can take every form. Its weak point: the energy. Real rock is heated by the Earth’s internal heat, kilometers down, over millions of years, not by a kettle in minutes.

Activity

How fast does a river round a stone?

You need: a few pieces of white chalk (or soft, broken pieces of brick), a plastic bottle or jar with a lid, water, and a kitchen scale if you have one.

Break the chalk into pieces about 2 cm long with sharp corners. Keep one piece aside to compare. Weigh the rest if you can.
Put them in the bottle, half fill it with water, and close the lid.
Shake it hard, 100 times. Take the pieces out, look at their corners, and weigh them. Look at the water.
Repeat, until you have shaken them 500 times.

What you should see

The corners wear away first, and the pieces get rounder and smaller with every hundred shakes, while the water turns milky with fine chalk. If you weighed them, plot the mass against the number of shakes: the pieces lose mass fastest at first, while they still have sharp corners. This is how a river rounds its pebbles, and the water carries away the fine dust as mud. A rounded pebble has traveled a long way; a sharp-cornered one has only just been broken off. Real rock is far harder than chalk, so a river takes many kilometers to do what your bottle did in minutes.

In short

Rocks are igneous (from cooled magma), sedimentary (from sediment or shells, buried and hardened) or metamorphic (changed by heat and pressure without melting). Any of them can be any age.
The rock cycle: weathering, erosion, deposition, compaction and cementation, metamorphism, melting, cooling and uplift. Any rock can become any other; Earth’s matter is recycled, never used up.
Two engines drive it. The Sun’s energy, through water, ice and wind, with gravity, drives weathering, erosion and deposition at the surface. The Earth’s internal heat, left over from its birth and made by radioactive decay, drives metamorphism, melting, volcanoes and uplift, deep down.
Rock gets hotter by roughly 25 °C per kilometer down; limestone turns to marble at a few hundred degrees, and wet rock starts to melt at about 700 °C.
Most changes are slow, a few millimeters a year, but over millions of years they build mountains and cut gorges. Others are sudden: earthquakes, landslides, floods, meteorite impacts.
Landforms are evidence: a V-shaped valley was cut by a river, a U-shaped one with moraines by a glacier; sea-floor fossils on a mountain show it was lifted.

Questions

Which process turns sand into sandstone?

Pick an answer to see why.

Where does the energy come from that lifts limestone from the sea floor to the top of Everest?

Pick an answer to see why.

a.

Describe one possible journey for a grain of quartz, starting in the granite of Everest and ending in a metamorphic rock. For each step, name the process and the source of the energy that drives it.

Answer

One possible journey:

The granite is lifted up as the Himalaya rise (uplift; the Earth’s internal heat, which moves the plates).
Frost and water break the granite up, freeing the quartz grain (weathering; the Sun’s energy, which drives the weather).
Glaciers and rivers carry it down to the Bay of Bengal (erosion; the Sun, which lifted the water, and gravity, which pulls it downhill).
It settles on the sea floor with other sand (deposition; gravity, which pulls the grain down once the water slows).
More layers bury it, and the sand is pressed and cemented into sandstone (compaction and cementation; gravity, the weight of the layers above).
The sandstone is pushed deep down where plates collide, and heated and squeezed into quartzite (metamorphism; the Earth’s internal heat).

Other routes are just as good, as long as each step is a real process with its energy source: for example, the grain could stay in the granite while it is buried and squeezed into a gneiss.

b.

Sea lily fossils are found in the limestone at the top of Everest. Explain what this tells us, and give the evidence for each step of your explanation.

Answer

Sea lilies lived only in the sea, and limestone forms from shells and skeletons heaped up on a sea floor. So this rock formed on the floor of a sea, at or below sea level.
The rock is now 8,849 m above the sea, so it has been lifted nearly 9 km.
Nearby, lower on the mountain, the same kind of rock has been changed into marble (the Yellow Band), and there is granite that formed from melted rock: evidence that the rocks of the Himalaya were pushed deep and heated, as well as lifted.
What could do the lifting? The plates: India is still moving into Asia, at about 4 cm a year, and GPS shows the Himalaya still rising by a few millimeters a year.

So the fossils show that the top of the world was once a sea floor, and that the Earth’s internal heat has lifted it, slowly, over tens of millions of years.

c.

A river cuts down at 4 mm a year.

a.How deep a gorge can it cut in 500,000 years, in meters?
b.How much does it cut in a human lifetime of 80 years?
c.Use your answers to explain why we don’t notice the land changing.

Answer

a.4 mm/year × 500,000 years = 2,000,000 mm = 2,000 m, 2 km.
b.4 mm/year × 80 years = 320 mm, about 32 cm, and spread over 80 years.
c.In a year the change is a few millimeters, smaller than the bumps on the riverbed, so nobody can see it happening. But the Earth is so old that even a slow change adds up to kilometers: 500,000 years is only about a hundredth of the time the Himalaya have been rising.

d.

Sort these into sudden (seconds to days) and slow (thousands to millions of years), and say for each whether the Sun’s energy or the Earth’s internal heat drives it:

a.a landslide in the monsoon
b.a glacier carving a U-shaped valley
c.an earthquake
d.marble forming from limestone
e.a flood when a glacial lake bursts

Answer

a.Landslide: sudden; the Sun (the monsoon rain, which the Sun’s heat lifted from the sea, soaks the slope), with gravity.
b.Glacier carving a valley: slow; the Sun (it lifted the water that fell as snow), with gravity.
c.Earthquake: sudden; the Earth’s internal heat (it moves the plates, which build up strain until the rock snaps).
d.Marble forming: slow; the Earth’s internal heat.
e.Glacial lake flood: sudden; the Sun (it melted the ice and lifted the water), with gravity.

e.

Draw your own model of the rock cycle: a slice through the land from a mountain to the sea, like Figure 23.3. Show the three kinds of rock, sediment and magma, and arrows for at least six processes, colored by the energy that drives them. Then write one thing your model doesn’t show well.

Answer

A good model has: a mountain with weathering at the top; a river carrying sediment to the sea (erosion) and dropping it (deposition); layers on the sea floor becoming sedimentary rock (compaction and cementation); rock deep below being heated and squeezed (metamorphism), and deeper still melting into magma; magma rising and cooling into igneous rock, underground or from a volcano; and uplift lifting rock back up into the mountain. Sun-driven arrows (weathering, erosion, deposition) in one color, internal-heat-driven arrows (metamorphism, melting, uplift, volcanoes) in another.
Things a model like this doesn’t show well (any one is fine): the time each step takes, from minutes (a landslide) to millions of years (uplift); the sizes, since the deep part is squeezed up and the land shortened; that the steps don’t happen in a fixed order, and any rock can skip steps; or that the cycle happens in many places at once.

Think about it

A planet with one engine

Imagine the Earth’s internal heat ran out, as the Moon’s has. What would happen to the rock cycle, and to the Himalaya, over hundreds of millions of years? And if instead the Sun went out, but the inside stayed hot?

Answer

With no internal heat, the plates would stop and no new mountains would rise. No rock would be melted or metamorphosed. But the Sun would keep driving rain, rivers, ice and wind, so the Himalaya and every other mountain would be worn down, and their rock carried into the sea as sediment. In a few hundred million years the continents would be worn nearly flat, low land by shallow seas, and the cycle would be stuck at its sedimentary half.
With no Sun (leaving aside that everything would freeze and die), there would be no rain, no rivers and no wind: no weathering or erosion. Plates would still move, mountains would still rise and volcanoes erupt, but nothing would wear the mountains down, and no sediment would be made. The cycle would be stuck at its other half. It needs both engines.

Where to go next

How Do Rocks Tell Time? The layers that the rock cycle lays down, read like the pages of a book, to put the Earth’s history in order.
Do Continents Really Move? The internal heat moves the plates. The clues that the continents were once joined, and how they move.
What Does Water Do to Rock? In the Grades 9–12 book: how frost, acid rain and rivers break rock down and carry it away, in detail, with the caves of Pokhara.
Why Is Everest Still Rising? In the Grades 9–12 book: what is inside the Earth, how we know, and how the heat inside lifts the Himalaya.