Clastic Rocks and Geomorphology

What is a clastic rock?

Sedimentary rocks made of fragments of pre-existing rocks

Silicaclastic rocks? how do they form?

Rocks rich in quartz and clay, originating from exposed, weathered, and eroded continental crust

Lithic fragments

Eroded pieces of other rocks, reduced to sand grains in sedimentary rocks

what is the matrix?

Fine-grained material surrounding clasts in clastic rocks, different from cement formed by chemical precipitate

Quartz

Abundant, stable, and resistant to weathering, making it common in clastic sediments

Clay

End product of feldspar weathering, acting as a matrix between clasts in sedimentary rocks

Feldspar

Constitutes 60% of the Earth's crust, weathers out to form clays and iron ions in clastic rocks

Mica

Originates from metamorphic rocks, persists as small flakes easily transported, used as a sustainable glitter

Iron oxides

Unstable mafic minerals decaying to clay and iron ions, produced by oxidizing iron in pore spaces

Pyrite

Formed in oxygen-reducing environments by sulfur-reducing bacteria, indicating high organic productivity

Bioclastic material

Fragments of shells/skeletons, mostly carbonate materials, common in carbonate rocks such as limestone

Volcaniclastic material

Material expelled from a volcano as ash or bombs, mostly lithic clasts, includes materials taken up through indirect transport

Wacke

Unsorted clastic rock with varied grain size, angular clasts, and mixed mineralogy clasts in a muddy matrix

Arkose

Clastic rock with a high percentage of unstable minerals such as feldspar, texturally and compositionally immature

Lith-arenite

Clastic rock composed of unstable minerals, poorly sorted, and with angular clasts, texturally and compositionally immature

Quartz-arenite

Clastic rock with a high percentage of quartz-dominated mineral grains, texturally and compositionally very mature

Compositional maturity

Presence of only stable end products (quartz/clay) in a clastic rock, indicating compositional maturity

Matrix vs. cement

Matrix is fine-grained material between clasts, different from cement which is a chemically precipitated material sticking grains together

Porosity

Empty spaces in rock filled with water, oil, or gas, important for permeability

Intergranular porosity

Porosity between grains in a rock

Intragranular porosity

Porosity within grains, such as microcracks or fossil chambers

Weathering

Breaking down of preexisting rocks through physical and chemical means, producing solid fragments and dissolved ions

Erosion

Removal of solid material from its source by gravity, water, ice, or wind

Mechanical weathering

Physical breakage and disintegration of rock, involving processes like salt wedging, frost wedging, and root weathering

Chemical weathering

Chemical reactions promoting decomposition of rocks, including hydration, hydrolysis, dissolution, oxidation, and biochemical weathering

Differential weathering

Highlighting compositional differences in rocks due to varying rates of weathering

Sediment supply and accommodation space

Necessary for deposition, generated by subsidence, rising sea level, and sediment supply

Depositional environments

Locations where sediments are deposited, such as mountains, rivers, deserts, coastal areas, continental shelf, ocean trenches, continental slope, and abyssal plains

What are some surface processes?

-processes that act on or close to the surface of the Earth.

-tectonics that shape the surface

-weathering and erosion

What are the three major types of rock?

What is their source material/forming process

IGNEOUS

• melting of rock in deep crust/mantle

• formed through crystallisation/ solidification of magma

• from volcanoes

SEDIMENTARY

• formed by weathering and erosion of rock at the earths surface

• from deposition, burial and lithification

METAMORPHIC

• rocks transformed under high pressure and temp

• deformation and recrystallisation

outline the rock cycle from initial magma.

-Magma is derived from melting of the asthenosphere (upper mantle) or the crust. this can be intrusive and extrusive.

-Intrusive igneous rocks can be uplifted to the Earth’s surface

-Weathering and erosion of the Earth’s surface breaks rocks down into clasts (sediment) and ions

-Sediment and ions are transported away from the source and deposited in low-energy environments

-Sediments are buried at depth and lithified (i.e., turned to stone) to form sedimentary rocks

-Sedimentary rocks may be uplifted back to the surface where they are recycled by weathering and erosion into new sediments

-Alternatively, as sedimentary rocks are further, increasing temperatures (heat) and overburden pressure turns then into metamorphic rocks

-Additional burial may be due to further deposition of sediment above, or may be due to tectonic stacking of geological units

-Metamorphic rocks may be uplifted back to the surface where they are cycled by weathering and erosion into new sediments

-Further heating can cause metamorphic rocks to melt, producing magma. Melting is usually not complete (i.e., partial melting) as different minerals have different melting temperatures

-This means the components of all sedimentary rocks have been recycled many times.

zircon can be used in radioactive dating. outline its potential evolution.

1)      A zircon crystal forms as part of igneous rock in continental rift. In an area of extension the rock is extruded. Thinning and melt occurs.

2)      The igneous rock is eroded to produce sand near the volcano.

3)      The sand is buried and lithified to form a sandstone, and then uplifted and exposed at the Earth’s surface

4)     The sandstone is eroded. The zircon crystal forms part of a new sediment which is washed out to sea, being deposited in a submarine fan.

5)      Eventually, the ocean closes and the submarine fan containing the zircon is subducted

6)      During subduction, the sedimentary rock containing the zircon is metamorphosed with increasing temperatures and pressures

7)      Continued collision produces a mountain range and the new metamorphic rock, containing the zircon, is uplifted

8)      Now exposed at the Earth’s surface, the metamorphic rock is eroded to form a new sediment, containing the zircon, which then travels down a river to a beach.

give three ways sedimentary rocks are important

-contains abundance of economic minerals and materials

-hydrocarbons usually found in porous sedimentary rocks called reservoir rocks

-coal is contained within sedimentary sequences

-depositional environments and processes are useful for understanding geological history

-fossils more common in these and can be used to date rock beds

what melts to form magma? what mineral is close to melting point?

partial melting of asthenosphere (upper mantle)

olivine

what is solidified magma with the earth called? give examples

intrusive bodies

sills, dikes

what is the term for magma that reaches the earth surface?

extrusive rock

what can cause intrusive bodies to become weathered?

uplift from tectonics

where does sediment move from and to?

from an area of net erosion to a low energy environment of net deposition.

what can cause additional burial of sediments?

-further deposition above

-tectonic stacking of geological units

why is melting often not complete?

partial melting is due to different minerals having different melting points. pressure can also affect them differently.

what is the wilson cycle?

the opening and closing of oceanic basins due to the cyclical processes of subduction and divergence of plate boundaries.

what can the study of current environments and sedimentary rock tell us about past environments?

full range of modern environments existed in the past.

some modern rocks have no modern example eg banded ironstones.

Sedimentary rocks are classified according to their dominant formation process. what are the 4 main categories? give an example of each.

-siliciclastic (conglomerate, sandstone and mudstone)

-biogenic, biochemical and organic (limestone, chert, oil)

-chemical (evaporites, ironstone)

-volcaniclastic (ignimbrites, tuffs)

what is the difference between matrix and cement?

matrix is finer-grained material between clasts and cement hold sediment together

What are typical areas of net deposition?

-coastal environments (deltas, lagoons, tidal flats)

-open marine environments (shallow continental shelves)

what is a sedimentary facies?

body of sedimentary rock with features that distinguish it from other facies. it can be unique due to depositional process, lithology, grain size or fossil content.

what is the difference between lithofacies and biofacies?

lithofacies are defined on sedimentary characteristics

biofacies rely on paleontological differences

in what ways does geology control geomorphology?

• plate tectonics

• volcanoes

• mineral based eg hardness

• structures eg unconformity, folds

what is typical geomorphology at convergent plate boundaries?

continent-continent collision can cause mountain ranges (Himalayas)

uplift

metamorphism due to pressure

continent-oceanic and oceanic-oceanic can have deep trenches and magmatic arcs with strata volcanoes

areas of extension in backarc basin. this is due to the subducting slab retreating and increasing its angle.

what is the typical geomorphology at divergent margins?

basins and topographic lows.

steep cliffs either side

what is typical geomorphology of transform conservative boundaries?

pull apart basins and transpression mountains

how does stable tectonic settings impact geomorphology? example?

No uplift so the location has extended period of net erosion

peneplains are formed from this. They are the penultimate stage of fluvial erosion during times of stability. normally form near sea level

what test is used for hardness?

Mohs Hardness test

what features are formed due to rock hardness?

waterfalls

monument valley with structures in deserts

coves

what is the concept of uniformitarianism?

the idea the present is the key to the past. geomorphological structures such as mountains would have formed over millions of years. This went against the ideas of catastrophises