Fold­ing

Jamaica Gleaner - - YL: FEATURE - JU­DITH HENRY Con­trib­u­tor

FO­CUS QUES­TIONS

1. What are folds and fold­ing? 2. What the dif­fer­ent types of fold­ing? 3. How does fold­ing oc­cur?

Over a long pe­riod of time, stress and strain (pres­sure) causes rocks to buckle and frac­ture or crum­ple into folds. When rocks de­formed in such a way, it bends in­stead of break­ing. A fold can be de­fined as a bend in rock that is the re­sponse to com­pres­sional forces. This is when the rock is be­ing pushed in­wards from both sides.

Fold­ing is the process by which rocks bend in re­sponse to the com­pres­sional force.

When large-scale fold­ing oc­curs and mas­sive lay­ers of the earth’s crust are up­lifted due to com­pres­sional force, this pushes to­wards each other in op­po­site di­rec­tions. The up­lifted crust form fold moun­tains, ex­am­ple, The Hi­malayas.

There are dif­fer­ent types of folds cre­ated by com­pres­sional stress, de­pend­ing on which way the rock bends.

An an­ti­cline is a fold that arches up as both sides of the rock are pushed in­wards. The arch looks like an ‘A’. It con­vex up and has its old­est beds at its core.

A syn­cline is a fold that sinks down as both sides of the rock are pushed in­wards. The fold ‘sinks’ down­wards, with the younger lay­ers closer to the cen­ter of the struc­ture.

A mon­o­cline is a fold where the rock lay­ers form an Sshape, as the sides of the rock are com­pressed from ver­ti­cal move­ment. All the lay­ers of rock are hor­i­zon­tal, go­ing in one di­rec­tion. Mon­o­clines are lay­ers in only ‘one di­rec­tion.’ Rock beds ly­ing at two lev­els are sep­a­rated by steep in­clined limbs.

We also have domes, which are like an­ti­clines but in­stead of an arch, the fold is in a dome shape, like an in­verted bowl.

Sim­i­larly, there are also basins, which are like syn­clines but again, in­stead of a sink­ing arch, the fold is in the shape of a bowl sink­ing down into the ground.

TYPES OF FOLDS

Sym­met­ric fold: Both limbs equally bent, im­clined at al­most equal an­gles to the hor­i­zon­tal.

Asy­met­ri­cal fold: One limb pushed too far, one side grad­ual slop, other side steeper. Mon­o­cli­nal fold: One fold is ver­ti­cal. Iso­cli­nal fold: Both sides are in­clined in the same di­rec­tion. Re­cum­bent fold: One limb is puhed over in the ame di­rec­tion.

Over­thrust fold: Frac­ture oc­curs, one limb slides for­ward over the other limb.

Sym­met­ri­cal folds are folds with the same an­gle. This type of fold has two limbs of equal steep­ness. It re­sults when the two op­pos­ing forces of equal strength move to­wards each other.

Asym­met­ri­cal folds are folds with dif­fer­ent an­gles. This type has one limb steeper than the other. It re­sults when the op­pos­ing force is greater than the other.

Iso­cli­nal folds are sim­i­lar to sym­met­ri­cal folds. These folds both have the same an­gle and are par­al­lel to each other.

Over­turned folds oc­cur when the fold­ing is so in­tense that the fold ap­pears to have turned over on it­self. One limb pushes over the other limb, un­til the axis is al­most hor­i­zon­tal and al­most par­al­lel. This oc­curs when op­pos­ing force is much greater than the other.

Re­cum­bent folds are folds that are nearly hor­i­zon­tal. These folds are noted to be ly­ing down side­ways.

Chevron folds are folds in a zigzag pat­tern. The an­gles of the folds are shaped like chevrons, and in­stead of slowly curv­ing, the edges are sharp and straight.

Folds are most vis­i­ble in rocks that con­tain lay­er­ing.

Let’s look the dif­fer­ence be­tween fold­ing and fault­ing

FOLD­ING AND FAULT­ING

Forces in earth’s sur­face caused by the move­ment of eath’s plates squeezes and bends the sur­face of earth. Fold­ing oc­curs when the crust is bent up­wards or down­wards. Fault­ing oc­curs when the crust is ac­tu­ally bro­ken. Ju­dith Henry teaches at Ar­denne High School. Send ques­tions and com­ments to kerry-ann.hep­burn@glean­erjm.com

Folds forms at dif­fer­ent an­gles.

The part of a fold

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