Rocks and the rock cycle

Name the three main types of rocks based on their origin and give an example of a rock of each type.

Classify these rocks as igneous, sedimentary, or metamorphic: granite, sandstone, marble, basalt, limestone, and slate.

Classify these rocks as igneous, sedimentary, or metamorphic, justifying one of them: basalt, limestone, marble, and pumice.

Find a rock in your surroundings (on a building facade, a sidewalk, a wall, or a riverbed), describe its appearance, and say which type you think it is (igneous, sedimentary, or metamorphic) with one reason.

Mini-investigation: choose a rock visible in a building, monument, or wall in your town. Observe whether it has crystals, layers, grains, or remains of organisms; deduce which type of rock it is, justifying each clue, and then check it with a field guide or in a reliable source.

The diagram represents the rock cycle, but one of the processes is marked with "?". State the name of that process, explain what it consists of, and describe the full cycle starting from magma.

Explain what magma is, where it is found, and which type of rock forms when it cools and solidifies.

Explain how each type of rock (igneous, sedimentary, and metamorphic) is formed, stating the main process responsible: cooling of magma, compaction of sediments, or heat and pressure.

A classmate makes the following statement: "The rock cycle is a closed, mandatory loop: an igneous rock must always be eroded to yield a sedimentary one, then be compressed into a metamorphic one, and finally melt into magma; there are no shortcuts." Prove that their statement is incorrect by designing two real alternative geological pathways in which stages are "skipped":

1. a) Explain how a basalt newly cooled on the surface can be transformed directly into a metamorphic rock without ever becoming a sedimentary rock.
2. b) Explain how a metamorphic rock located at depth can directly remelt into magma without first becoming a sedimentary rock.

Justify in each case what physical conditions (pressure, temperature, or burial) make each of these direct transformations possible.

Two students are debating the driving engines of the Earth. Emma claims: "If solar energy stopped reaching the Earth, both the water cycle and the rock cycle would immediately stop completely." Carlos refutes her: "Even if the Sun were to go out, essential parts of the rock cycle would remain active for millions of years, whereas the water cycle would indeed be paralyzed."

1. a) Assess who is right and justify your answer by explaining what the two energy sources driving the rock cycle are, compared to the energy source that powers the water cycle.
2. b) Predict which stages of the rock cycle would stop if the Earth's surface were frozen with no liquid hydrosphere or active atmosphere, and which stages of the rock cycle would continue operating thanks to Earth's internal heat.

Granite shows visible crystals of several minerals, whereas obsidian is a glass without crystals, even though both are igneous rocks. Explain what difference in the cooling process produces such a different appearance.

Granite forms when magma cools slowly inside the crust, yet today we can touch granite at the surface of many mountains. Explain how it has come to be exposed: what has happened to the rocks that were above it.

Pumice is an igneous rock full of tiny holes that floats on water. Explain how those holes formed (gases from the lava trapped as it cooled) and why they make it so light.

At the top of a mountain, at an altitude of 1800 meters, a geologist discovers a layer of conglomerate made up of rounded granite pebbles. However, no granite outcrops on that mountain; the nearest granite massif is 60 km away in another mountain range.

1. a) Interpret what information the rounded shape of the granite pebbles provides about the transport medium and the distance traveled, comparing it to what a breccia (rock with angular fragments) would indicate.
2. b) Granite is a plutonic igneous rock formed by the slow cooling of magma several kilometers deep. Reconstruct, with justification, the geological history of that material: how could it reach the surface, break down, and end up lithified inside a conglomerate?
3. c) If the sediments that form this conglomerate were deposited in a low-lying area (sedimentary basin), how do you justify that today they are found at the summit of a high-altitude mountain?

What is a sediment? Give three examples of different sediments and explain which type of rock they can end up forming.

Describe which process turns a pile of loose sediments (sand, mud) into a hard, compact sedimentary rock.

A hiker finds a rock with embedded shell remains. What type of rock is it almost certainly? Explain why it is practically impossible to find fossils in an igneous rock.

Many limestones formed from the remains of shells and skeletons of marine organisms. Explain what a layer of limestone located at the top of a mountain tells us about the past of that place.

On an exam, a student writes: "Metamorphic rocks like slate are found at such great depth and pressure that it is impossible for them to transform into sedimentary rocks without first melting into magma and being expelled by a volcano."

1. a) Refute the student's statement by justifying what geological processes (tectonic uplift and erosion of overlying layers) allow a rock formed deep inside the crust to become directly exposed to weathering.
2. b) Design a step-by-step sequence describing how the minerals of that slate exposed on a mountainside can end up transformed into a new sedimentary rock (such as a sandstone) in a marine basin, indicating the external geological agents involved in each phase and demonstrating why it is not necessary to pass through the magma state.

Research which type of rock dominates the landscape of your area (limestone, granite, slate, sandstone...) and write a summary explaining how that rock influences the landscape: landforms, soil type, vegetation, or crops. Include a simple diagram and cite your sources.

Metamorphism transforms a rock without melting it completely. Name the two factors responsible for this transformation and explain why the resulting marble is harder and more compact than the limestone it comes from.

River pebbles are round. Explain which processes (erosion and transport) have gradually rounded them and what can happen to them if they are buried under more sediments for millions of years.

In the geology lab, there are two unlabeled samples of metamorphic rocks (Sample A and Sample B). It is known that one comes from the metamorphism of a limestone and the other from a clay. To identify them, you carry out two tests:

1. You pour a few drops of hydrochloric acid on both: Sample A produces immediate effervescence with carbon dioxide bubbles, while Sample B does not react.

2. With a hand lens, you observe their structure: Sample B presents thin, oriented parallel sheets (foliation) that separate easily, while Sample A shows regular interlocking crystals without sheets.

1. a) Reason out which metamorphic rock each one is (marble or slate) and what original sedimentary rock it came from.
2. b) Justify why two rocks subjected to high pressures and temperatures give rise to such different appearances: what role does the initial mineral composition of the parent rock play during metamorphism?

On an expedition through a volcanic island, samples are collected along the bed of a ravine that descends from a volcano to the sea:

• Sample 1 (at the summit): large, dark, angular basalt blocks with small degassing cavities.

• Sample 2 (in the middle course): basalt pebbles of about 3 cm, with smoothed edges and rounded shapes.

• Sample 3 (on the seashore): loose black sand formed by tiny mineral grains of basalt.

• Sample 4 (obtained through a borehole 25 meters beneath the beach): a compact, cohesive rock formed by grains of black sand strongly cemented together.

1. a) Interpret what external geological processes have caused the progressive change in size and shape of the material between samples 1, 2, and 3.
2. b) Predict what underground physical-chemical processes (compaction and cementation) must have occurred to transform the loose sand of sample 3 into the consolidated rock of sample 4, naming the overall process that has taken place.