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How limestone formed.

Extracts from this essay...

Introduction

How limestone formed? Most limestones in our part of the world formed in shallow seas that covered our area hundreds of millions of years ago. Other names for limestone We use the scientific name "limestone" for this rock, but it is also known by many other names: When limestone is crushed into gravel and used for building roads or making concrete, it's often called an aggregate. Here are some ways to classify limestone (by grouping it with similar types of rocks): Geologists have a catch-all term that includes both limestone and dolostone. They call them carbonate rocks. Limestone is a sedimentary rock. How to recognize limestone Limestone is mostly light to dark gray in color. Its grains can be mud-sized, so that the rock looks dull, or they can be sand-sized or larger bits of broken shell or other limey material. You can scratch limestone into white powder with a nail or knife. (See more about the scratch test.) Limestone is made mostly of the mineral calcite, so it bubbles gently when you put a drop of white vinegar on it.

Middle

Using the sharp point of the nail, try to scratch the surface of your rock. If the nail digs in and scratches off a line of powdered rock, then the nail is harder than the rock. In this case, the nail scratched off a line of powdered white limestone. If the nail does not scratch off any powdered rock, then the rock is harder than the nail. In this case, some of the nail rubbed off on the rock! This left a line of silvery metal on the white chert. (This happens pretty often when you try to scratch hard, light-colored rocks.) James Hutton (1727-1797), the eminent 18th century gentleman farmer and founder of modern geoscience, authored the concept of the rock cycle, which depicts the interrelationships between igneous, sedimentary, and metamorphic rocks. The upper part of the earth (mantle, crust and surface) can be envisioned as a giant recycling machine; matter that makes up rocks is neither created nor destroyed, but is redistributed and transformed from one rock type to another.

Conclusion

igneous rocks form at sea floor spreading ridges. Fluid intrusion of these rocks, both during and after formation, results in some low grade metamorphism. As the rocks cool, and more magma is introduced from below, the plate is forced away from the spreading ridge, and acquires a sediment cover. As shown in the figure, in this case, the plate is eventually subducted under a continental plate. In the trench of the subduction zone, at relatively shallow depths, high pressure - low-high temperature metamorphism of the plate and its sediment cover occur. As the plate travels deeper, high temperature conditions cause partial melting of the crustal slab. Fluid intrusion plays a key role in partial melting. As the partial melt rises, and intrudes into the continental plate, the surrounding country rock is contact metamorphosed at high temperature conditions. This melt is either driven to the surface as volcanic eruptions, or crystallizes at depth to form plutonic igneous rocks. Sedimentary rocks form from the weathering, erosion, transport and deposition of arc material onto the continental platform and shelf.

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