Formation of Rocks

Question

Step-by-step solution

1. Step 1

   Igneous rocks form when magma (underground) or lava (surface) cools and solidifies. Slow cooling underground produces large crystals (e.g. granite); fast cooling at the surface produces small crystals (e.g. basalt).

2. Step 2

   Sedimentary rocks form from layers of sediment (rock fragments, shells, organic matter) deposited by wind or water. Over millions of years, sediments are compacted and cemented together. Examples: limestone (shells/coral), sandstone (sand grains), shale (clay/mud).

3. Step 3

   Metamorphic rocks form when existing rocks are changed by intense heat and/or pressure deep underground, without melting. Examples: marble (from limestone) and slate (from shale).

Question

Step-by-step solution

1. Step 1

   Weathering — breakdown of rocks at the surface by physical (freeze-thaw), chemical (acid rain) or biological (plant roots) processes, producing small rock fragments.

2. Step 2

   Erosion and transportation — weathered rock fragments are carried away by wind, water, ice or gravity to a new location.

3. Step 3

   Sedimentation/deposition and compaction/cementation — fragments settle, layers build up, lower layers are compressed and mineral cements bind them into sedimentary rock.

4. Step 4

   Crystallisation (igneous) / heat and pressure (metamorphic) — magma cools to form igneous rock; or rocks subjected to heat and pressure recrystallise as metamorphic rock. Melting links all three types back to magma.

Question

Step-by-step solution

1. Step 1

   Permeability is the ability of water to pass through the pore spaces (gaps) of a rock or soil.

2. Step 2

   Permeable rocks: limestone and sandstone — water can pass through pore spaces or cracks (e.g. through limestone via solution). Most sedimentary rocks are permeable.

3. Step 3

   Impermeable rocks: granite (igneous), marble (metamorphic), shale (sedimentary, very fine grains with almost no pore space) — water cannot pass through and runs off the surface.

4. Step 4

   Water that percolates through permeable rock accumulates in aquifers — underground water stores that can be tapped by wells, an important source of potable water in many regions.

Question

Step-by-step solution

1. Step 1

   Sample A (8 mm crystals): Intrusive cooling — magma solidified slowly deep underground. Sample B (0.1 mm crystals): Extrusive cooling — lava solidified rapidly at the surface.

2. Step 2

   Crystal growth explanation: Crystals form as atoms arrange into a regular lattice structure as the melt cools. When cooling is slow (intrusive), atoms have sufficient time and energy to migrate and join existing crystal structures, allowing large, well-formed crystals to grow. When cooling is rapid (extrusive), atoms are 'locked into place' very quickly before they can organise into large crystals — resulting in very small crystals or even an amorphous (glassy) texture.

3. Step 3

   Named rocks: Sample A → granite (coarse-grained, intrusive). Sample B → basalt (fine-grained, extrusive). Note: if Sample B had cooled even faster (e.g. quenched in water), it could produce obsidian (volcanic glass — no crystals at all).

Question

Step-by-step solution

1. Step 1

   Non-renewable argument: The rock cycle operates over geological timescales — millions to hundreds of millions of years per complete cycle. Human extraction removes rocks, ores and minerals far faster than geological processes can replenish them. For example, the copper ore deposits being mined today formed over tens of millions of years by hydrothermal activity. At current extraction rates, known copper reserves could be exhausted within ~60 years.

2. Step 2

   Rare counterargument (technically renewable): Geologically, new igneous, sedimentary and metamorphic rocks are continuously forming — volcanism creates new basalt at mid-ocean ridges, rivers deposit new sediments, and new metamorphic zones develop at plate boundaries. On a timescale of millions of years, the rock cycle does replenish mineral deposits. Some sedimentary minerals (e.g. beach sand, some limestone) accumulate relatively quickly compared to metallic ores.

3. Step 3

   Evaluation: Despite the technical renewal argument, on any meaningful human timescale (decades to centuries), rocks, ores and minerals are effectively non-renewable. No mining engineer can wait 50 million years for new ore to form. This justifies the classification as non-renewable and reinforces the case for recycling, reduced consumption, and sustainable management. The exception might be building aggregates (sand, gravel) where some natural replenishment via river systems and coastal erosion occurs on century timescales.

Question

Step-by-step solution

1. Step 1

   Prediction: The limestone adjacent to the granite intrusion will be transformed into marble through contact metamorphism.

2. Step 2

   Processes: The granite pluton (solidified from magma) is extremely hot as it intrudes. Heat conducts into the surrounding limestone, causing the calcite crystals to recrystallise into the interlocking mosaic of larger calcite crystals characteristic of marble. The limestone does not melt — it undergoes solid-state recrystallisation. This is called contact (thermal) metamorphism.

3. Step 3

   Limited extent of the metamorphic zone: The intensity of metamorphism decreases with distance from the intrusion as the heat dissipates. Very close to the intrusion: highest temperature → most complete recrystallisation (high-grade marble). Further away: lower temperature → less complete recrystallisation → transitional zone. Beyond the thermal influence: unaltered limestone. This graded zone is called a metamorphic aureole.