Exploring the Fundamentals of Earth Science

Earth Science Folding The process by which crustal forces deform an area of crust so that layers of rock are pushed into folds. Faulting The process by which crustal forces cause a rock formation to break and slip along a fault. Fold A bent or warped stratum or sequence of strata that was originally horizontal, or neatly so. Fault A planar or gently curved fracture in the earths crust across which there has been relative displacement of the two blocks of rock parallel to the fracture.

Interpreting Field data Measuring Strike and Dip It takes 2 measurements to describe the orientation of a layer of rock exposed at a given location: Strike The direction of the intersection of a rock layer with a horizontal surface. Dip Measured at right angles to the strike, is simply the amount of tilting the angle at which the inclines from the horizontal. Constructing a Geological Map and Cross Section Geological maps include:

• Locations of outcrops
• The nature of the rocks
• The dips and strikes of inclined layers
• How Rocks Become Deformed 3 types of tectonic forces:
• Compressive forces squeeze and shorten a body
• Tensional forces stretch a body and tend to pull it apart
• Shearing forces push two sides of a body in opposite directions What Determines Whether a Rock Bends or Breaks?

If subject to tectonic forces near surface, where confining pressure is low it would tend to deform by fracturing and faulting. If deeper than few Kms it would change shape by gradually folding.(also if hot) Brittleness and Ductility Brittle rocks little change, then breaks Ductile rocks smooth and continuous plastic deformation. Igneous rocks are more brittle (stronger) than sedimentary rocks Basement rocks more brittle than overlaying sedimentary rocks. How Rocks Fold Types of Folds Anticlines Upfolds, or arches of layered rocks.

Synclines Downfolds, or troughs. Limbs two sides of a fold Axial plane an imaginary surface that divides a fold as symmetrically as possible, with one limb on either side Fold Axis The line made by the lengthwise intersection of the axial plane with the beds. Plunging Fold If the fold is not horizontal. Folds can be asymmetrical. Dome anticline structure, a broad circular or oval upward bulge of rock layers. Basin synclinal structure, a bowl shaped depression of rock layers in which the beds typically many kms in diameter. Fold Belt A linear region that has been subjected to folding and other deformation in a mountain building episode. Suggests that the region was compressed at one time by horizontal tectonic forces. How Rocks Fracture: Joints and Faults There are 2 types of fractures:

Joints Cracks along which there has been no appreciable movement. Can be caused by tectonic or non-tectonic forces Fault A fracture with relative movement of the rocks on both sides of it, parallel to the fracture. Can be caused by all forces, compressive, tensional, shearing.

Offset, or the relative movement of the 2 plates can be hundreds of kms. Fault plane the surface along which the formation fractures and slips. Dip-slip fault one in which there is relative movement of the fault plane. (comp. or tension) Strike-slip fault one in which the movement is horizontal, parallel to the strike of the fault plane. (shearing) Oblique-slip fault movement along the strike and simultaneously up or down the dip. (combo) Normal fault the rocks above the fault plane move down in relation to the rocks below, causing an extension of the section.

Reverse fault the rocks above the fault plane move upward in relation to the rock below, causing a shortening of the section. Thrust fault a reverse fault at which the dip of the fault is small, so that the overlaying block is pushed mainly horizontally. Rift Valley A depression where one block looks as though it had dropped between two flanking blocks that have been pulled apart.

The fault must be younger than the youngest rock it cuts and older than the oldest undisrupted formation that covers it.