EARTHSCI LESSON 5

Earth’s Crust Composition

Approximately 85% oxygen and silicon; together form silicon-oxygen tetrahedron, the basic building block of silicate minerals.

Silicates

Termed as common rock-forming minerals.

Mode of Formation

Rocks classified based on how they formed; physical and chemical properties depend on mode of formation.

Igneous Rocks

Formed from solidification of molten rock (magma or lava).

Magma

Molten rock beneath Earth’s surface.

Lava

Molten rock extruded to surface through volcano or fissure.

Igneous Rock Textures: Phaneritic

Large interlocking crystals; slow cooling.

Igneous Rock Textures: Porphyritic

Two-stage crystallization: large crystals (phenocrysts) form before extrusion, fine-grained groundmass forms at surface.

Igneous Rock Textures: Aphanitic

Fine-grained; crystals not visible; rapid cooling prevents large crystal growth.

Igneous Rock Textures: Vesicular

Voids from trapped gas bubbles during rapid cooling.

Igneous Rock Textures: Glassy

Non-crystalline; rapid quenching.

Plutonic (Intrusive) Igneous Rocks

Formed beneath surface; slow cooling; coarse-grained phaneritic texture. Examples: granite, diorite, gabbro.

Volcanic (Extrusive) Igneous Rocks

Formed at or near surface; fast cooling; aphanitic, porphyritic, or vesicular textures. Examples: rhyolite, andesite, basalt.

Classification by Silica Content: Felsic

65% silica; light-colored.

Classification by Silica Content: Intermediate

55–65% silica; medium gray.

Classification by Silica Content: Mafic

45–55% silica; dark-colored.

Classification by Silica Content: Ultramafic

Granite vs Rhyolite

Same composition; granite = phaneritic; rhyolite = aphanitic/porphyritic.

Diorite vs Andesite

Same composition; diorite = coarse-grained; andesite = fine-grained.

Gabbro vs Basalt

Same composition; gabbro = coarse crystals; basalt = fine crystals.

Sedimentary Rocks

Formed at or near Earth’s surface from sediments.

Sediments

Fragments of pre-existing rocks or organic matter.

Sedimentary Processes

Weathering, erosion, transport, deposition, compaction, cementation.

Lithification

Process of sediments hardening into sedimentary rocks.

Diagenesis

Collective processes of lithification: compaction, cementation, recrystallization, chemical changes.

Sedimentary Features: Fossil Assemblages

Remains and traces of organisms preserved in rock.

Sedimentary Features: Stratification

Layering (strata). >1cm = bedding, <1cm = lamination; indicates changes in deposition.

Types of Sedimentary Rocks

Clastic and Non-Clastic.

Clastic Sedimentary Rocks

Made of broken fragments (clasts) of old rocks.

Clastic Components: Grains

Sand-sized or larger fragments.

Clastic Components: Matrix

Fine-grained (clay to silt).

Clastic Components: Cement

Minerals binding grains and matrix.

Clastic Classification

Based on particle size.

Rudaceous Rocks

50% clasts, >2mm diameter, mainly rock fragments.

Arenaceous Rocks

50% sediments, 0.063–2mm, high quartz possible.

Argillaceous Rocks

50% sediments, <0.063mm, mainly clay and some quartz.

Non-Clastic Sedimentary Rocks

Formed from evaporation, precipitation, or organic matter lithification.

Non-Clastic: Evaporites

From evaporation leaving minerals. Examples: halite, gypsum, dolostone.

Non-Clastic: Precipitates

Minerals crystallize from supersaturated water. Example: limestone.

Non-Clastic: Bioclastic

Formed from compacted organic matter.

Metamorphic Rocks

Formed below surface via metamorphism: recrystallization under pressure/temperature.

Types of Metamorphism

Contact and Regional.

Contact Metamorphism

Magma intrudes, heating surrounding rocks. Produces non-foliated rocks (e.g., hornfels).

Regional Metamorphism

High pressure and recrystallization during mountain-building (orogeny). Produces foliated rocks (schist, gneiss). Marble (non-foliated) also possible.

Foliation

Alignment/banding of mineral grains under pressure.

Protolith

Parent rock of metamorphic rock.