Acid Test
This simple test helps you tell rocks that are
made of calcite from similar rocks made of other
minerals.
All you need is some white vinegar. (If your
parents or teacher can get it, some weak
hydrochloric acid will give you more dramatic
results.)
Put a large drop of vinegar (or a small drop of acid)
on your rock and watch what happens for a minute
or so.
If the rock is made of calcite, small bubbles will start to form inside the drop of vinegar.
Or, if you use weak acid (like in this picture), the acid will start to fizz like crazy!
If your rock is made of dolomite, the bare rock
won't make bubbles. However, if you scratch a
dolomite rock into powder, the powder will make
bubbles in vinegar or acid.
Scratch Test
This simple scratch test helps you tell hard rocks
(like granite, obsidian, and quartzite) from softer
rocks (like marble, coal, and limestone).
All you need is a shiny new nail (or a knife blade,
if you are older).
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. PETROLOGY, the study of rocks and their origins, is essentially the formal process by which we resolve the interrelationships expressed in the rock cycle.
Liquid (molten) rock material solidifies at depth or at the earth's surface to form . Uplift and exposure of rocks at the Earth's surface destabilizes these mineral structures (c.f. Bowen's Reaction Series). The minerals break down into smaller grains which are transported and deposited (either from solution or by lowering the hydraulic energy regime) as sediments. The sediments are lithified (compacted and cemented), and are formed. Changes in temperature, pressure, and/or rock or fluid chemistry can allow igneous and sedimentary rocks to change physically or chemically to form . At higher temperatures, metamorphic (or any other rock type) rocks may be partially melted, and crystallization of this melt will create igneous rocks. Uplift and erosion can expose all rock types at the surface, re-initiating the cycle.
THE ROCK CYCLE
This tectonic rock cycle is a clickable map (19/07/95 -map still under construction) that offers more detailed descriptions of the rock types found within their tectonic settings, with links to more detailed information about each subject
If we examine the rock cycle in terms of plate tectonics, as depicted in the figure above, we see that mafic (tholeiitic) 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.
