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- In your opinion is this rock cut out anticline or syncline? What stress has caused this; tension, compression or shear?
- What type rock in your opinion is at the location? Igneous, Sedimentary or Metamorphic.
- What different colors do you see in the rock face?
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< What are the 3 basic types of rocks?
Igneous Rocks
Igneous rocks are crystalline solids which form directly from the cooling of magma. This is an exothermic process (it loses heat) and involves a phase change from the liquid to the solid state. The earth is made of igneous rock - at least at the surface where our planet is exposed to the coldness of space. Igneous rocks are given names based upon two things: composition (what they are made of) and texture (how big the crystals are).
Sedimentary Rocks
In most places on the surface, the igneous rocks which make up the majority of the crust are covered by a thin veneer of loose sediment, and the rock which is made as layers of this debris get compacted and cemented together. Sedimentary rocks are called secondary, because they are often the result of the accumulation of small pieces broken off of pre-existing rocks. There are three main types of sedimentary rocks:
Clastic: your basic sedimentary rock. Clastic sedimentary rocks are accumulations of clasts: little pieces of broken up rock which have piled up and been "lithified" by compaction and cementation.
Chemical: many of these form when standing water evaporates, leaving dissolved minerals behind. These are very common in arid lands, where seasonal "playa lakes" occur in closed depressions. Thick deposits of salt and gypsum can form due to repeated flooding and evaporation over long periods of time.
Organic: any accumulation of sedimentary debris caused by organic processes. Many animals use calcium for shells, bones, and teeth. These bits of calcium can pile up on the seafloor and accumulate into a thick enough layer to form an "organic" sedimentary rock.
Metamorphic Rocks
The metamorphic rocks get their name from "meta" (change) and "morph" (form). Any rock can become a metamorphic rock. All that is required is for the rock to be moved into an environment in which the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions. In most cases, this involves burial which leads to a rise in temperature and pressure. The metamorphic changes in the minerals always move in a direction designed to restore equilibrium. Common metamorphic rocks include slate, schist, gneiss, and marble.
Crustal Deformation
Rocks, whether igneous, sedimentary, or metamorphic, are subject to powerful stress by tectonic forces, gravity, and the weight of overlying rocks. The three types of stress are tension (stretching), compression (shortening), and shear (tearing and twisting). Strain is how rocks respond to these stresses, as expressed in folding (bending) and faulting (breaking). Whether a rock bends or breaks depends on several factors, including composition and how much pressure is on the rock. An important quality is whether the rock is brittle or ductile. The patterns created by these processes are evident in the landforms we see today.
Folding and Warping
Syncline and anticline are terms used to describe folds based on the relative ages of folded rock layers. A syncline is a fold in which the youngest rocks occur in the core of a fold (i.e. closest to the fold axis), whereas the oldest rocks occur in the core of an anticline.
Syncline
It is important to note that syncline and anticline do not necessarily relate to the shape or orientation of folded layers, although the origin of the words implies this. The term originates from the Greek word sun xun, meaning together, and the Greek word klei, meaning to lean, so syncline implies leaning together or leaning towards. Ant is the Greek prefix meaning opposite or opposing, so the word anticline implies oppositely leaning. Beds dip towards the fold axis in a syncline and away from the fold axis in an anticline only when the folded layers were upright before folding (i.e., where younger layers overlaid older layers). Before describing folds, it is therefore necessary to establish the primary order in which layers were deposited. To do this, facing, younging, or way-up criteria are used. These are synonymous terms for primary sedimentary structures (e.g. graded or cross-bedding) or igneous structures (e.g. vesicles, pillows) preserved in the folded layers. Where the relative ages of rocks are not known (as is often the case in metamorphic rocks), the term synform and not syncline should be used to describe folds where layers are bent downwards so that they dip towards the fold axis, and antiform and not anticline should be used where beds are arched upwards so that layers dip away from the fold axis.
Anticline
Where rock layers have been inverted prior to folding, such as by folding about a larger fold with a shallowly inclined axial surface, the oldest rocks now occur in the core of folds where layers dip towards the fold axis. Such folds are called synformal anticlines; synformal because of their shape and anticline because of the relative ages of folded layers. The youngest layers in an overturned sequence occur in the core of folds called antiformal synclines where layers dip away from the fold axis.
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