PART PART 5 (SEC 8 AND 9)

How do sedimentary rocks form?

Sedimentary rocks form at or near Earth's surface as weathered material accumulates, is transported, deposited, and turned into rock.

What does weathering do?

Weathering breaks down older rocks into fragments; transport (by water, wind, ice, or gravity) moves sediment until it settles.

What is lithification?

Lithification turns loose sediment into rock through compaction or cementation

When does bedding (layering) often develop?

Bedding (layering) often develops as flow conditions change and different grain sizes settle at different times.

What are the three sedimentary rock groups?

Clastic — made from broken pieces of pre-existing rocks.

Organic — forms from biological material (e.g. shells, plant remains).

Chemical — forms when dissolved material precipitates out of water.

Conglomerate – grain size?

Gravel-sized, > 2 mm

Sandstone – grain size?

Sand-sized, ~1/16–2 mm

Mudstone (incl. siltstone, shale) – grain size?

Finer than 1/16 mm

What do rounded grains often suggest?

Rounded grains often suggest transport by water, because collisions wear off sharp edges.

Most sedimentary rocks are detrital — derived from what?

Derived from the weathering of pre-existing rock — and the weathering process is dominated by just two end products: quartz sand and mud (clay)

Why does quartz survive weathering essentially unchanged?

Quartz is hard and chemically inert, so it survives weathering essentially unchanged. As grains are transported (e.g. down rivers, onto beaches), their edges get knocked off, rounding them — but they remain individual quartz grains rather than reacting chemically.

What happens to other common igneous minerals (feldspars, biotite, hornblende, olivine)?

They react chemically with rainwater as part of chemical weathering, and break down into clay or mud.

Why are quartz sand and mud/clay described as the most common minerals making up sedimentary rocks?

This is why quartz sand and mud/clay are described as the most common minerals making up sedimentary rocks — not the only ones, but the dominant two by far.

Limestone is commonly built from what?

Shell material (calcium carbonate, CaCO₃).

How does coal form?

Coal — forms when plant material is preserved in low-oxygen swampy settings and later buried.

How does rock salt form?

Rock salt — precipitates when saline water evaporates, leaving crystals behind.

How do cave stalactites and stalagmites form?

Cave stalactites and stalagmites form when dissolved limestone is re-precipitated underground (links to karst, Section 11).

What is metamorphism?

Any pre-existing rock can become metamorphic if heat and/or pressure change enough.

Does metamorphism involve melting?

Metamorphism does not involve melting — the rock stays solid, but recrystallises internally (texture and mineral arrangement change).

What can greater heat and pressure also make rocks do?

Greater heat and pressure can also make rocks deform and fold rather than simply fracture.

Contact metamorphism – Cause?

Heat near intruding magma

Regional metamorphism – Cause?

High heat AND high pressure over large areas

Contact metamorphism – Key feature?

No strong pressure-driven texture

Regional metamorphism – Key feature?

Commonly produces strong pressure-driven texture (foliation)

What happens if temperature and pressure rise too far?

If temperature and pressure rise too far, the rock crosses into melting and forms magma instead — the metamorphic–igneous limit.

Limestone → ?

Marble (stays calcite-dominated).

Sandstone → ?

Quartzite (stays quartz-dominated).

Mudstone + contact metamorphism → ?

Hornfels (hard, fine-grained).

Mudstone + regional metamorphism → ?

Develops new mica-rich textures as clay minerals change (see foliation series below).

How does foliation form?

Foliation forms when platy minerals grow or rotate under pressure, aligning perpendicular to the main stress direction.

Slate – description?

Low-grade foliation ("slaty cleavage"), splits along flat planes.

Schist – description?

Schistosity; visible mica grains give a sparkly, layered look.

Gneiss – description?

Gneissic layering; light and dark minerals separate into distinct bands (highest grade of this series)

What does chemical weathering do?

Chemical weathering alters minerals — often turning unstable igneous minerals into clay.

What does physical weathering do?

Physical weathering breaks rock mechanically through expansion, contraction, and abrasion.

What is regolith?

Regolith = the weathered geological material at Earth's surface. It comes in two forms: (1) loose weathered material that has been transported — e.g. sediment washed down streams or deposited on beaches — or (2) soil, where weathering happens to the bedrock in place, without the material being moved.

How does a soil profile develop?

A soil profile develops in situ from bedrock upward, with organic-rich material concentrated near the top.