It is not a circle

Almost everyone meets the rock cycle as a ring of arrows on a diagram, and almost everyone draws the wrong conclusion from it. The arrows look like a running order, so the cycle gets learned as a sequence: this becomes this, which becomes this, which becomes the first one again. ⚠️⚠️ THERE IS NO SEQUENCE. THE ROCK CYCLE HAS NO BEGINNING, NO PREFERRED DIRECTION, AND NOTHING IS OBLIGED TO COMPLETE IT. ⭐ THE ARROWS ARE NOT AN ORDER. THEY ARE AVAILABLE ROUTES. A rock at any point on that diagram has more than one thing that could happen to it next, and which one happens is not decided by the diagram. A rock can also skip most of it entirely. Something can be lifted up, worn down and laid back down again without ever going deep enough to be changed by heat and pressure, and it can do that over and over. ⚠️ THAT IS NOT AN EXCEPTION TO THE CYCLE. IT IS A PERFECTLY ORDINARY ROUTE THROUGH IT. So if the diagram does not decide, what does? ⭐⭐ WHERE THE ENERGY COMES FROM. AND THERE ARE ONLY TWO SOURCES, WHICH IS WHY YOUR SPECIFICATION PUTS ENERGY TRANSFER IN THE SAME LINE AS THE ROCK CYCLE. ⭐ ONE ENGINE IS UNDERNEATH: THE EARTH'S OWN INTERNAL HEAT. It melts rock, it cooks rock that is not melted, and it pushes rock upward. Everything that happens to a rock because it is deep is driven by this. ⭐ THE OTHER ENGINE IS ABOVE: THE SUN AND GRAVITY TOGETHER. The sun moves water and air about and drives the temperature changes that break rock apart; gravity takes the broken pieces downhill and stacks them up somewhere lower. Everything that happens to a rock because it is exposed is driven by this. ⚠️ EVERY SINGLE ARROW ON A ROCK CYCLE DIAGRAM BELONGS TO ONE ENGINE OR THE OTHER. NONE BELONGS TO BOTH. And that gives you the last part of this topic without a separate lesson on it. ⭐⭐ THE TWO ENGINES WORK AT WILDLY DIFFERENT SPEEDS. Which is why a rock can sit unchanged for an almost unimaginable stretch of time and then pass through two transformations comparatively quickly. ⚠️ THE CYCLE DID NOT SPEED UP. A DIFFERENT ENGINE TOOK OVER. One question to carry through everything that follows: which engine is driving this arrow, and how fast does that engine work?

Two engines, and where they hand over

Every arrow on a rock cycle diagram is driven by one of these. The third column is the part that decides which one has hold of a rock at any moment.

Tap what the two engines claim

Tap the TWO statements that follow from the cycle being routes rather than a sequence.

A rock at any point on the diagram has more than one thing that could happen to it next
A rock can be lifted, worn down and laid down again repeatedly without ever being changed by heat and pressure
A rock must pass through each stage of the cycle in turn before returning to the beginning
A long period in which a rock does not change means the cycle has stopped working on it

The rock that never went deep

A rock has been broken down, carried away, buried, pressed into solid rock, lifted up and broken down again, more than once, without ever being melted or cooked. A student says it has not completed the rock cycle. How would you correct that?

There is nothing to complete: that is an ordinary route through the cycle, driven entirely by the surface engine, and the rock was never within reach of the other one
It is an unusual case, so the cycle is a general rule with exceptions
The student is right, and the rock will eventually be melted to complete the cycle
It simply has not had long enough yet

Five processes, defined

Five terms, each defined by what it is. Which engine drives each one, and how fast, is the work of the steps that follow.

Match each route to what drove it

A cliff face splits along a crack after repeated freezing
Fragments are carried down a valley and dropped where the water slows
Layers of sediment are pressed together far below newer layers
Rock deep underground changes texture without ever melting
Rock that formed far below is now standing at the surface

The surface engine, using temperature change to break rock where it sits
The surface engine again, with gravity moving material downhill and the water simply carrying it
The weight of what lies above, which is the last thing the surface engine does before handing over
The engine underneath, supplying enough heat to change the rock but not enough to destroy it
The engine underneath again, and the moment the rock is handed to the other one

Two that follow from two engines

Select the TWO statements that follow from every arrow belonging to one engine or the other.

A rock that is unchanged for an immense period and then alters comparatively quickly has probably been handed from one engine to the other
A rock at the surface is out of reach of the deep engine no matter how long it stays there
Most arrows on a rock cycle diagram are driven by both engines working together
A rock that has not been melted has not really taken part in the cycle

Reading a rock cycle diagram properly

Half of this topic is being handed a diagram and asked what it shows. Here is how to take one apart so that it stops being a ring of arrows to memorise. ⭐⭐ FIRST, DO NOT LOOK FOR A STARTING POINT. THERE ISN'T ONE, AND LOOKING FOR ONE IS WHAT PRODUCES THE SEQUENCE MISREADING. ⭐ INSTEAD, LABEL EVERY ARROW WITH ITS ENGINE. Anything that happens because a rock is exposed belongs to the surface engine. Anything that happens because it is deep belongs to the engine underneath. Two arrows will be the handovers: one going up, one going down. THEN PICK ANY POINT AND ASK WHAT COULD HAPPEN NEXT. ⚠️ IF THERE IS MORE THAN ONE ARROW LEAVING IT, THE DIAGRAM IS TELLING YOU THAT THE NEXT STEP IS NOT DETERMINED, and saying so is worth more than reciting the loop. AND LOOK FOR THE SHORTCUTS. Most diagrams include a route that misses out most of the ring, and it is usually the one students ignore because it spoils the tidy circle. ⭐ IT IS THERE BECAUSE IT IS COMMON. ⚠️ WHEN A QUESTION ASKS ABOUT RATES, ANSWER WITH THE ENGINE RATHER THAN WITH A NUMBER. Surface processes can change a hillside noticeably within a single lifetime; the deep engine works far more slowly than that. ⭐ A COMPARISON YOU CAN DEFEND BEATS A FIGURE YOU CANNOT. ⚠️ AND DO NOT REACH FOR A NUMBER YOU ARE NOT SURE OF. A rate in millimetres per year, a temperature, a depth, an age: if you are not certain, describe the mechanism and the comparison instead. That is what is being marked. Two habits that cost marks on this topic. ⚠️ THE FIRST IS DESCRIBING THE DIAGRAM BACK. "The arrow goes from here to here" is reading aloud. Saying what drives that arrow, and what else could have happened instead, is analysis. ⚠️ AND THE SECOND IS TREATING THE CYCLE AS SOMETHING THAT MUST COMPLETE. Rocks are not working through a checklist. At every point there is more than one thing that might happen next, and the question is always which engine currently has hold of it. Where the internal engine gets its energy, and why crust moves at all, belongs to another part of your course.

One grain, several possible lives

Follow a single grain of hard mineral, about the size of a coarse sugar crystal, and watch how little of its future is decided at any point. IT BEGINS LOCKED INSIDE ROCK THAT COOLED FROM A MELT, FAR BELOW THE SURFACE. It could stay there indefinitely. Rock that deep is entirely out of reach of anything happening above it, and immense stretches of time can pass with nothing happening to this grain at all. ⚠️ THAT IS NOT THE CYCLE PAUSING. IT IS THE ENGINE UNDERNEATH HOLDING ON TO IT. SUPPOSE THE ROCK IS PUSHED SLOWLY TOWARDS THE SURFACE. ⭐ THIS IS THE HANDOVER, AND IT CHANGES EVERYTHING ABOUT WHAT CAN HAPPEN NEXT, because a rock that is exposed becomes available to a completely different set of processes. Now water gets into cracks and freezes, and the rock around our grain is broken apart where it stands. The grain is set loose. Rain moves it into a stream, the stream carries it down, and where the water slows it is dropped along with a great deal of other material. ⭐ NOTICE HOW FAST THIS PART IS BY COMPARISON. A HILLSIDE CAN VISIBLY CHANGE WITHIN A SINGLE LIFETIME, WHICH IS NOT TRUE OF ANYTHING THE OTHER ENGINE DOES. More material piles on top. The weight presses the grains together and water between them leaves behind material that cements them. Our grain is inside solid rock again. ⚠️⚠️ AND HERE IS THE POINT OF THE WHOLE EXAMPLE. WHAT HAPPENS NEXT IS NOT DETERMINED. If this rock is raised again before it is buried deeply, the grain is set loose a second time, moved, dropped and cemented once more. ⭐ IT CAN GO ROUND THAT SHORT LOOP AGAIN AND AGAIN, NEVER GOING DEEP, NEVER BEING COOKED, NEVER BEING MELTED. ⚠️ AND IT HAS NOT FAILED TO COMPLETE ANYTHING. THAT IS A ROUTE, AND A COMMON ONE. If instead it is buried under enough newer material, the engine underneath reaches it again. Heat and pressure change the rock around the grain while it stays solid. Push further still and it melts entirely, and the grain as a grain stops existing. ⭐ THREE VERY DIFFERENT FUTURES, ALL AVAILABLE FROM THE SAME POINT, AND THE DIAGRAM CANNOT TELL YOU WHICH. What decides it is whether the grain ends up exposed or buried, which is to say which engine has hold of it.

Order one route, not the cycle

These six events describe ONE possible route, not the order of the rock cycle, which has none. Put this particular journey into order.

The rock sits far below the surface, unreachable by anything happening above
It is pushed slowly upward until it is exposed
Repeated freezing breaks it apart where it stands
The loosened pieces are carried downhill and dropped where the water slows
More material piles on top and presses the pieces into solid rock
It is either raised again, or buried deeper, and nothing about the route so far decides which

Build the sentence that names the driver

This is what turns describing a diagram into explaining it. Assemble it.

The rock cycle run

Five questions on routes, engines and rates. Three lives.

Complete the rock cycle facts

The breakdown of rock where it lies, without being carried anywhere, is _____. The removal of loosened material from where it was broken is _____. The turning of loose sediment into solid rock by burial and compaction is _____. The raising of rock towards the surface is _____.

weathering erosion lithification uplift metamorphism deposition melting compaction

Three readings to correct

Three students working with a rock cycle diagram. In each case the observation is fine and the reasoning needs work.

Asked to describe a rock cycle diagram, a student writes out the ring of arrows in order, accurately, starting from molten rock. Why does it score poorly?
A student explains that a rock has been unchanged for an enormous period by saying the rock cycle "slowed down" during that time. How would you correct it?
Asked about rates, a student writes that rock is worn away at a particular number of millimetres each year, a figure they half-remember. What should they do instead?

Explain what the diagram is really showing

A classmate has learned the rock cycle as a fixed sequence starting with molten rock. Explain why that reading is wrong, and what the diagram actually shows.

Explain why the diagram has no starting point and why the arrows are routes rather than an order
Name the two sources of energy that drive the cycle, and say what each one acts on
Give one route a rock can take that misses out most of the diagram, and explain why it is ordinary rather than exceptional
Explain what the two handover arrows do, and why a rock's position decides which processes are available to it
Explain why a rock can be unchanged for an immense period and then alter comparatively quickly
Answer any point about rates by comparison rather than with a figure you cannot support

The Rock Cycle and Geological Rates

Get the method right under pressure

What you'll cover