GEOL 102 Lecture 2: Every Rock is a Record of History: Historical Approaches to Lithology

Every Rock is a Record of History: What Can It Tell Us?

The three main types of rock (igneous, metamorphic, sedimentary) all represent genetic classes. This means that you recognize most of them by their textures, which form throught clearly different processes. It is helpful therefore to think of the three types of rocks m as three ways of forming rock, rather than types of rock based on their chemistry.

Igneous – any rock cooled from a molten state
Metamorphic – any rock transformed due to intense heat and/or pressure
Sedimentary – any rock formed by bits of previously existing rock and/or organic material consolidated into a solid mass

These processes give rise to different textures because they influence the shape and the contacts between the mineral grains. If you can recognize the most common textures, you can classify most rocks into one or the other category without having to know any detailed chemistry.

What kind of event can each type of rock record?

Within igneous rocks:

Can tell if formed at depth (plutonic) due to intrusion between previously existing rocks or at surface (volcanic) due to extrusion onto surface:

Intrusive rocks cool slowly and form larger visible crystals (phaneritic texture).
Extrusive rocks cool quickly, and form microscopic crystals or glass (aphanitic texture). E.g.: obsidian (a glassy type of solidified lava).

Chemical composition can indicate the magma/lava source:

Primarily from mantle rock (very high in Mg and Fe, ultramafic)
Oceanic crust, or otherwise with very little mixing with crustal rock (high in Mg and Fe, basaltic or mafic)
Considerable mixture of mantle rock with continental rock, or formed by melted continental material (low in Mg and Fe, high in Si and Al, granitic or felsic)

Also, igneous rocks can be dated quite accurately by radiometric methods. This gives us absolute ages from which we can estimate the age of associated sedimentary rocks.

Within metamorphic rocks:

Determine whether due to contact metamorphism (e.g., baked region around igneous intrusions) or regional metamorphism. The latter may indicate large-scale tectonic events in the past.
Also, metamorphic grade (observed by particular combination of metamorphic minerals present) indicate the pressure/temperature conditions under which they formed: the best clue to previous depth at which these rocks formed. (For example, can use to see how deep the ancient Appalachians were).

Within sedimentary rocks:
MUCH useful information about the history of the planet's surface (configuration of land and oceans, major rivers, climate changes, height of sea level, appearance and disappearance of life forms):

Because sedimentary rocks form at the surface, record ancient surface conditions.
Sedimentary rocks also contain fossils (the remains of past life).
The type and structure of organic material preserved in them provides clues to the depth of burial of the rocks.

Major classes of sedimentary rocks:

1) Siliciclastic (aka clastic aka detrital)
2) “everything else” (carbonates, evaporites, coal, chert, etc.)

It is simplest to distinguish siliciclastic rocks from all others. Siliclastics are formed by bits of previously existing rocks cemented together. The chemical composition of the grains can be a clue to the source rock(s).

Source rock is broken down by mechanical and chemical weathering.
Mechanical weathering reduces brittle or soft minerals to smaller pieces, increasing the surface exposed to chemical reactions.
Chemical weathering eliminates most of the original minerals, which break down in the presence of oxygen or water.
Sediment is transported by water (or ice, or air)
During transport, sediment is sorted (winnowed out, largest and densest particles first) and rounded
Transported material is deposited in some environment
The particular environment will result in certain types of sedimentary structures.
These deposits will be in the form of layers (strata) of different grain size or composition, because of changes in the environment.

Carbonate rocks form by either abiotic chemical reactions (Ca and often some Mg combining with CO3 ions when their concentrations are sufficiently high in water) or by organisms (which pull these ions from the water to build their shell or skeleton). Some carbonate rocks are clastic or detrital, i.e. almost exclusively made up of shells and skeletons, intact or broken up.

Evaporites (rock salt and some rarer minerals) indicate presence of water and of arid conditions (allowing evaporation of a lot of water).