Earth Science-Sedimentary Rocks

Clastic Rock

A clast is a fragments/ chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks that are composed of rock fragments. CLASTIC ROCKS ARE MEASURED BY THE SIZE OF THEIR FRAGMENTS

Bioclastic Rock

An organic rock that is composed of something that was ONCE LIVING! usually derived from shell fragments (eg. limestone)

Burial

As layers are piled one upon another, the sediments beneath are buried, sometimes by hundreds of metres of sediment above. The weight of these layers compacts (squashes down) the sediment grains. (see squashed layers of sedimentary rock in pic)

Crystalline/Chemical Sedimentary Rocks

Chemical sedimentary rocks are crystalline, that is made of crystals interlocked with each other. Conceptually, that is the same texture we see in igneous and metamorphic rocks, but NOT in clastic sedimentary rocks. Chemical sedimentary rocks form because of the precipitation of crystals from aqueous (water) solutions and from chemical residues. When crystals grow out of a solution (for instance, halite, or table salt, out of water) they can only use the space they have available, often interfering with each other's growth pattern. Some crystals form because of a simple variation in the concentration of ions in presence of water (for instance, calcite and aragonite), while others require evaporation of water (for instance, halite, or gypsum), and others still form as chemical residues (for instance, some iron oxides).

Deposition

Deposition is the geological process in which sediments, soil, and rocks are added (DEPOSITED) to a landform or land mass. Wind, ice, and water, as well as sediment flowing via gravity, transport previously eroded sediment and build LAYERS of sedimentary rock.

Erosion

Erosion is the action of exogenic processes (such as water flow or wind) which remove soil and rock from one location on the Earth's crust, then transport it to another location where it is deposited.

Sedimentary Rock

Sedimentary rocks are types of rock that are formed by the deposition of material (sediments) at the Earth's surface and within bodies of water. This deposition occurs from natural agents such as wind/rain/ice.

Sediments

Sediments are a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice, and/or by the force of gravity acting on the particles.

Compaction

This happens when sediments are deeply buried, placing them under pressure because of the weight of overlying layers. This squashes the grains together more tightly.

Weathering

Weathering is the breaking down of rocks, soil and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters.

Cementation

this is where new minerals stick the grains together - just as cement (from a bag) binds sand grains in a bricklayer's mortar. If you look carefully at the microscope photo, you can see mineral crystals that have grown around the sediment grains and bonded them together.