Rock Life Cycle Help
Rock Life Cycle
During weathering, rock is either worn away or shifted from one spot to another. In the course of geological time, rock goes through an entire life cycle or rock cycle . One rock’s lifetime might include being blasted out of a volcanic vent to the surface, settling back on the earth as a layer of volcanic ash, being lithified with other sediments into a sedimentary rock layer, then being pushed down at a subduction zone to be transformed by pressure to metamorphic rock. Should the metamorphosed rock come in contact with a magma chamber or hot spot, it might melt and be sent back to the surface. The cycle would be complete or it would start all over again!
A rock can go through many changes and forms over billions of years. Figure 15-1 gives you an idea of a rock’s life cycle.
Fig. 15-1. The rock cycle shows the many different paths a rock can take over time.
Like people, a rock is affected by its environment. We have learned how different rocks form, now let’s look closer at the factors that play a part in a rock’s aging.
The activities of physical and mechanical weathering create cracks in rock that act as channels for air and water to get deeper into a rock’s interior. During weathering, rock is constantly being broken into smaller pieces and the surface area that is exposed to air and water gets larger. Both of these actions add to the overall chemical weathering.
Physical weathering happens when rock gets broken (cracked, crumbled, or smashed) into smaller pieces without any change to its chemical composition.
Physical or mechanical weathering is the breakdown of large rocks into smaller bits that have the same chemical and mineralogical make up as the original rock. Everyone is familiar with the breakdown of rock into smaller and smaller portions:
boulders pebbles sand silt dust
This size-graded breakdown takes place in different ways.
Many rocks are not solid all the way through. They have a lot of different size cracks and fractures caused by stress, called joints .
Most jointing takes place in rock as a result of the activities of internal or external forces. These include:
(a) Cooling and shrinking of molten matter;
(b) Flattening and tightening of drying sedimentary strata; and
(c) Plate tectonics.
Joints are often arranged in sets of vertical parallel lines. The largest, most visible fractures are called master joints . Sometimes, multiple sets of joints intersect at nearly right angles and create a joint system made up of crossing vertical and horizontal joints.
In sedimentary rocks, various sets of joints often match up with planes separating strata. You see the flaking away between layers of mica as stratification planes allow. Along with joints, rocks can also fracture between individual crystals or grains. Weathering factors, like water, enter tiny gaps between the grains, leading to grain-by-grain disintegration of rocks.
The two most important forms of physical rock breakdown are joint block separation and granular disintegration . Break up of rock happens in several different ways. Some of these include:
- Salt-crystal growth
- Unloading (weight)
- Expansion and contraction due to change of temperature and wetting/drying cycles
- Biological weathering, and
- Chemical weathering.
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