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Geology | rocks

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What Are rocks?

Rocks are the relatively hard naturally occurring material that form the Earth's crust. The building blocks of rocks are minerals; these are solid chemical compounds that occur naturally on Earth. Rock can consist of a single mineral or of several minerals. Some rocks are made from interlocking mineral crystals that fit tightly together. Others are made up from shattered fragments (grains) of older rocks and minerals which have been naturally cemented back together.  There are three main types of rock, classified by how they are formed: 

  • igneous. (like basalts and granites)

  • sedimentary (like limestone and shale)

  • metamorphic (like marble and slate)

The rock cycle

Rocks and their constituent minerals are important.  They contain a record of Earth's history.  We can decipher the history of our planetary home if we understand the processes that form rocks.

Our language is littered with metaphors about the durability and longevity of rocks - they are perceived as fixed and unchanging. It could be not further from the truth.  Rocks are changing slowly and gradually over time.

The three main rock types: sedimentary, igneous and metamorphic rocks have all been subjected to physical changes such as melting, cooling, eroding, compacting, or deforming that occur in the rock cycle over many millions of years.  

 

It is mind numbing to consider that the Earth is 4.5 Ga and some of the rocks you see in exposures today, have been round the rock cycle multiple times.

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SEDIMENTARY

(Limestone)

METAMORPHIC

(Garnet Amphibolite)

IGNEOUS

(Rhyolite)

surface Processes

Igneous Processes

Igneous rocks, formed at the earth's surface are the result of extrusion of molten rock material from the upper mantle or crust onto the land surface. This hot melt is known as magma and when it breaks through to the surface it is called lava.

Extrusion can be effusive, producing copious amounts of high-temperature, low-viscosity lava; or it can be  dangerously explosive.

Sedimentary Processes

Weathering is a process that occurs to in situ rocks at the Earth's surface.  Weathering can be one of three flavours: 1. physical; or 2. chemical; or 3. biological.  One or more of the three types of weathering process breaks down the rock into smaller fragments and soluble compounds.

Erosional processes then carry the weathered product away from the site, wearing (denudating) the landscape.  The fragments and soluble compounds are moved from one place to another by transportation vectors such as air (wind), water and ice all subject to the force of gravity.

Rock fragments (class) are often smoothed or broken down into smaller particles during transportation.

Sooner or later the vectors of transportation lose energy eg the wind may change in velocity; or the nature of a water course may change eg. it may flow into a deeper body of water such as a lake or shallow estuary.  When the energy reduces in the vector the carry capacity of the agent reduces and eventually deposition takes place.

SUB-SURFACE PROCESSES

Burial & Diagenesis

Deposition of a sediment usually (but not always) happens under water: on the bed of a lake, estuary, sea or ocean. Layers of the sediment are eventually covered by new material deposited on top - geoscientists refer to this as burial.  As the weight of overlying material increases, the sediments are squeezed so that gaps between individual grains reduces and the grains are pressed closer to each other.

Over time ground water may deposit minerals as it flows through the pore spaces between the grains.  The  compaction of grains and the growth of minerals in the pore spaces are part of the process known as diagenesis.  The result of diagenesis is a brand new suite of sedimentary rocks.

If burial continues,  the sedimentary rock will be subject to greater temperature and pressure regimes as the relative depth to the surface increases.

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SEDIMENTARY
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METAMORPHIC
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IGNEOUS