Exercises: The lithosphere and its materials

Define what the lithosphere is in your own words and explain how it differs from the geosphere.

Explain the difference between a rock and a mineral using the example of granite, which is made of quartz, feldspar, and mica.

Classify these materials as a rock or a mineral, justifying each answer: rock salt, granite, quartz, and marble.

What are the two basic types of materials of the lithosphere studied by geology? Explain how they are related to each other.

Classify these materials as a mineral or a rock, justifying each answer: slate, calcite, basalt, and common salt.

Does seawater form part of the lithosphere? Explain your answer and make clear which part of the ocean floor does belong to it.

Explain what a crystal is and name a mineral that forms crystals visible to the naked eye. Where could you find it?

Name three objects in your home made from materials that come from the lithosphere (for example, a window, a ceramic mug, or a metal frying pan) and state which geological material each one comes from.

Look at the schematic cross-section of the geosphere: the blue band marked with "?" highlights one of its regions. State which region it is and which layers form it, explain the relationship between the geosphere, lithosphere, crust, and mantle, and explain why we say that this region is "rigid".

Is a brick a rock? Is coal a rock? Explain both answers by looking at how each material is formed: whether it is natural or manufactured, and whether it is an aggregate of minerals.

On a field trip you find a dark stone with shiny crystals embedded in it. Explain what those crystals are likely to be, what the whole stone is called, and how they relate to one another.

Explain the difference between the crust and the lithosphere: which of the two is thicker, which part of the mantle the lithosphere includes, and why we say the lithosphere is the "rigid" layer.

The cement in pavements is made by firing a mixture of limestone and clay. Discuss whether cement is a rock and explain why we say it is a "product of the lithosphere" even though people manufacture it.

Granite shows three visible minerals (quartz, feldspar, and mica) and sandstone is made of quartz grains stuck together. Compare both rocks: in which one are the minerals easier to identify with the naked eye, and why? Which one do you think wears away sooner?

There are beaches with white, golden, and even black sand. Explain what these colours are due to (which rocks or minerals the sand comes from in each case) and reason what colour of sand you would expect on a beach near granite cliffs.

"A rock is like a sponge cake and minerals are its ingredients": explain what this analogy gets right and point out one important difference between baking a cake and forming a rock.

A student claims: "Since soil comes from the weathering and breakdown of crustal rocks, soil is simply crushed rock and belongs exclusively to the lithosphere."

1. a) Refute their claim by explaining why soil cannot be considered a rock, analyzing its composition and its structure.
2. b) Justify why geologists define soil as an "interface" in which four Earth subsystems interact (lithosphere, atmosphere, hydrosphere, and biosphere), naming a specific component contributed by each one.

At the headwaters of a mountain river, a granite massif outcrops (a rock composed approximately of 30% quartz, 50% feldspar, and 20% mica). However, when analyzing the fine sand of a beach located at the mouth of that river after traveling 350 km, it is found that 95% of the grains are exclusively quartz.

1. a) Interpret this change in composition and deduce what has happened to the feldspar and mica during transport compared to the resistance of quartz.
2. b) A bather fills a jar with that sand and says they are "taking a rock home as a souvenir." Reason whether the loose sand should be classified as a rock, as a collection of loose minerals, or as sediment, and explain what geological process would be missing for it to become a sedimentary rock.

A popular science documentary makes the following mistake when explaining the rock cycle: "A quartz grain from a granite located at a depth of 5 km can become part of a detrital sedimentary rock such as sandstone without the granite ever needing to reach the surface or come into contact with the atmosphere."

1. a) Identify and justify the scientific error in this statement, explaining why it is essential for the parent rock to outcrop at the surface to start the sedimentary cycle.
2. b) Reconstruct the logical sequence of geological stages (from weathering to diagenesis or lithification) that this quartz grain must undergo to end up integrated into a compact sandstone.

In the Geology laboratory you have four unknown solid samples (A, B, C, and D) and you have a copper coin (hardness 3), a glass plate (hardness 5.5), a magnifying glass, and a bottle of dilute hydrochloric acid. You obtain the following experimental data:

• Sample A: homogeneous appearance; the coin does not scratch it, but the glass scratches it; when struck, it breaks into fragments shaped like perfect cubes.

• Sample B: observed with the magnifying glass, it shows interlocking grains of three different colors; it easily scratches glass.

• Sample C: scratches glass; it is completely transparent and exhibits natural flat faces forming a pointed hexagonal prism.

• Sample D: uniform gray color; produces intense effervescence with bubbles when a drop of acid is poured onto it.

Answer, justifying your responses:

1. Determine which of the samples are definitely individual minerals and which are rocks.

2. Identify which mineral property is shown by the way sample A breaks and compare it with the faces of sample C.

3. Design a simple test to find out if sample B contains quartz.

A company sells transparent cubes online labeled as "pure laboratory mineral salt crystals". The company argues that their chemical composition (sodium chloride, NaCl) and their crystalline structure are exactly the same as those of natural halite extracted from mines.

1. a) Take on the role of an expert geologist and analyze whether this product strictly meets each of the requirements demanded by the scientific definition of a mineral.
2. b) Issue a reasoned verdict: is it correct to call it a mineral? Explain what difference exists between this synthetic crystal and a salt crystal formed on the shores of a coastal salt flat by the evaporation of seawater.

A craftsman finds two shiny black fragments: one is obsidian (natural volcanic glass formed by the instantaneous cooling of lava) and the other is a piece of glass from a broken bottle that someone threw into the countryside. The craftsman claims that both are minerals because they are solid, inorganic, and hard.

1. a) Refute his argument by explaining what characteristic of the internal microscopic structure both lack in order to be considered minerals.
2. b) Justify why the scientific community classifies obsidian as a rock, while bottle glass is completely excluded from the category of rock.

The town council of a mountain village where winters are very cold (with almost daily frosts) and it rains frequently is going to restore the facade of its town hall. The technical team must choose among three available types of rocks:

• Rock 1 (Limestone): medium porosity (5.8 %), moderate hardness (scratched by a steel knife), reacts and dissolves slowly with acidic waters.

• Rock 2 (Granite): almost zero porosity (0.3 %), very high hardness (scratches glass), inert to water and acids.

• Rock 3 (Highly porous sandstone): high porosity (19.2 %), absorbs a large amount of water in its voids, and has soft clayey cement.

1. a) Predict what would happen to the facade over the course of several winters if it were built with Rock 3, explaining the role played by water freezing inside the pores.
2. b) Justify which rock you should rule out if the area's rain contains dissolved acidic substances from urban pollution.
3. c) Argue which of the three rocks is the best technical choice to ensure that the building withstands the passage of decades without degrading.

To investigate how earthquakes originate in a lithosphere that is solid and rigid, you carry out the following experiment in the classroom: you rest a flat, rigid plastic ruler on the edge of a table, leaving half of it in the air. You push the free end downward with one finger little by little: the ruler bends slightly, resisting the force, until it suddenly snaps with a violent crack and you feel an instantaneous tremor on the table and in your hand.

1. a) Interpret the model: deduce what the continuous force of your finger, the elastic bending of the ruler, the snap of the break, and the tremor vibrating through the table represent in Earth's dynamics.
2. b) Based on this model, justify why the rigidity of rocks does not prevent earthquakes, but is precisely the essential condition for energy to accumulate and be released seismically instead of deforming smoothly and continuously.

Unit challenge. Have you ever thought about which part of the geosphere the objects in your own room come from? Become a geologist and prepare a brief technical report following these steps:

1. Explore the interior: Enter the earthquake viewer of the Irish National Seismic Network (INSN, Dublin Institute for Advanced Studies) or the European-Mediterranean Seismological Centre (EMSC) and locate a recent earthquake in Europe or the North Atlantic. Record its magnitude and depth (for example, $12.4\text{ km}$), and identify with justification in which layer of the geosphere it originated (crust or upper mantle).

2. Lithospheric audit at home: Look in your home for 3 objects made of geological materials (for example: table salt (halite), marble countertop, or granite floor). Classify each material as a rock or mineral and briefly describe its use.

3. Calculation and sustainability: Calculate what percentage of the Earth's mean radius ($6371\text{ km}$) the depth of the chosen earthquake represents, and write a reasoned proposal on the importance of recycling minerals to protect the lithosphere.

Assessment criteria:

• Correct identification of the layers of the geosphere and precision in the percentage calculation.

• Rigorous distinction between rocks and minerals in domestic environment items.

• Coherence and scientific foundation of the proposal for the sustainable use of lithospheric resources.