Types of Mountains

Introduction  Mountains have a variety of forms:  High, low  Steep, gentle slope  Volcanic, sedimentary  Crumpled layers, flat layers  Mountains are grouped based on how geologists think they formed.

Depositional Mountains  Formed by the accumulation of rocks on the earth’s surface  Volcanic or carried by wind or glaciers  1. Volcanoes  Build up in height when lava emerging from a central vent or break in the earth’s surface hardens into rock and cinders  Can form quickly (Paricutin in Mexico formed within a year!)  Can change or be destroyed quickly (Krakatoa in Indonesia destroyed two-thirds of the island when it erupted!)

Depositional Mountains  2. Sand dunes  Wind-deposited hills of sand  Those in the Sahara Desert in Algeria may be as high as 1400 feet and cover several square kilometers!

Depositional Mountains  3. Glacial Deposits  Produce several types of hills:  Moraines – a ridge of rock debris that was carried or pushed along by a glacier  Eskers – low, snaking, ridge-like deposits that remained after glaciers melted away or retreated

Depositional Mountains  Kames – formed when glacial debris washed into depressions in the ice; when glaciers melted, the rubble formed small, isolated, steep-sided hills  Drumlins – smooth elliptical hills that glaciers deposited as they moved along; feet in height and less than a mile long  (Famous drumlin of Revolutionary war = Bunker Hill in Boston)

Erosional Mountains  Mountains that were carved out by extensive erosion, usually from a plateau  Plateau = a region of flat rock structure having a high elevation; usually sedimentary rock  Mesas = broad, flat-topped hills remaining when erosion cut through a plateau  Buttes = smaller, flat-topped hills  Both have steep sides  Monadnock = an isolated mountain that was resistant to erosion while the area around it was eroded to a flat plain  Points to rapid erosion on a continental scale – the Flood!

Fold Mountains  Most major mountain ranges contain some folded rocks (Examples: The Rockies, the Appalachians, the Alps, and the Himalayas).  Fold Mountains = the folding of the rocks seems to have formed the mountains  Requires the proper temperature and pressure  Sedimentary rock generally begins as horizontal strata, so slanted or upturned strata indicate forces in the ground have bent the rock.  The rapid folding of deposited flood strata while rock was still soft explains this appearance better than uniformitarian ideas because there is no evidence of the rock fracturing in the tight folding.

Fold Mountains  5 Kinds of Fold Mountains:  Anticline – an arch of rock layers  forces acting toward each other  Syncline – a trough or downward fold  Monocline – folds in rock strata that occur when one part of a rock layer is lifted relative to another along a fault, forming a rounded step  Vertical forces  Dome – a structure in which the sedimentary strata arch upward like an inverted bowl  Upward vertical force in a single location  Basin – the opposite of a dome

Fault-Block Mountains  Formed when large sections of the crust moved along cracks in the rock  Joint – a large crack in a rock along which no movement (slippage) has occurred  Fault – a crack along which there has been slippage  Cause: a force applied against a rock caused stress the rock could not relieve by bending, so it cracked and moved to relieve the stress  Fault-block mountain = a mountain bounded by one or more normal faults  Examples: The Sierra Nevada in CA and The Great Basin and Range Province of Nevada and Utah

Fault-Block Mountains  3 Kinds of Faults: (describe the relative movement of the bodies of rock on either side of the fault)  1. Normal fault = if the body of rock under the fault rises in relation to the rock above the fault  2. Reverse fault = if the body of rock above the fault rises in relation to the other body  3. Strike-slip fault = if the only motion along the fault is horizontal