Geology Exam 3

Erosion by rivers is affected by water velocity which is influenced by

Gradient (slope) of the land, discharge (volume of water over time), shape of the river channel, amount of debris in the channel

Gradient (slope) of the land

Velocity tends to be higher on steeper slopes.

Discharge (volume of water over time)

Increasing discharge increases velocity.

Shape of the river channel

Semicircular-shaped channels have less friction with water than narrow or shallow channels.

Amount of debris in the channel

Water flows faster in the absence of any type of impediment - fallen trees, rocks, etc.

This includes rapids.

Water will move faster between the rocks in rapids, but the overall velocity of the river decreases.

Velocity of a straight channel

in the middle, about ¼ of the way to the bottom.

Velocity of a curving channel

deflected to the outside of the bends

Symmetric stream channel

Erosion force potential greatest in the middle of the stream

Asymmetric stream channel

Erosional force potential greatest on the right side of the stream.

Longitudinal Profile

As a river flows from its source to its mouth: Gradient decreases, discharge increases, the number of rapids decrease, the river becomes wider and shallower

Base Level

A river cannot erode its channel much deeper than the elevation of the body of water that it flows into.

Ultimate Base Level

Sea level for most major rivers, but there are exceptions.

The Jordan River’s base level is the Dead Sea.

Local Base Level

Lakes, waterfalls, man-made reservoirs behind dams, other rivers, etc.

When a river reaches base level

velocity falls close to zero

If base level falls

The river will resume downcutting and erode a deeper channel, possibly forming a canyon.

If base level rises

The gradient is reduced, the river slows and begins to deposit sediment. The channel will build higher to reach the new base level.

Base level decreases due to

Lower sea level and Rising land elevations

Base level increases due to

Higher sea level Lower land elevations

Dissolved Load (smallest sizes)

chemically weathered materials

Suspended Load

Fine sand, silt, and mud that are held in the water

Bed Load (largest sizes)

Larger material that moves by rolling or sliding

Competence

A measure of the largest particle size that is in motion. Increases during floods.

Capacity

The total amount of sediment that a river is able to transport

Deltas

tend to form during periods of rising sea levels.

Alluvial Fans

the largest tend to form in deserts. (delta on land)

Downcutting

deepens valleys

Lateral Erosion

widens valleys

Headward Erosion

lengthens valleys

Mass Wasting

widens valleys

High Gradient

Often far above base level (mountains) Downcutting is dominant The channel is often narrow, straight, and may contain rapids and waterfalls. V-shaped canyons are common in these areas.

Moderate Gradient

Downcutting is no longer dominant. Lateral and headward erosion and mass wasting are all significant. The channel is wider and shallower. Meanders, floodplains, and levees form

Low Gradient

Elevations are often very close to base level. Downcutting is not significant, lateral erosion is dominant. The channel is often very wide and shallow, with many meanders, oxbow lakes, large floodplains, and broad levees.

Incised meanders

meandering channels with steep-sided canyons and no floodplain

Terraces

abandoned former floodplains

groundwater can be found in

soil, sediment, and many different rock types.

porosity

The percentage of void space in sediment and rock

porosity is influenced by

particle shape, particle size, sorting, packing, cementation

Particle Shape

Spherical or rounded particles generally have the greatest degree of empty spaces between them. Flat particles often stack more closely together.

Particle Size

if all else is equal, size alone won’t affect porosity. However, larger grains often lead to lower porosity due to sorting, packing, and cementation

Sorting

well sorted material has much higher porosity than poorly sorted material.

Packing

A greater degree of packing will lower porosity. Seen mainly in sediment.

Cementation

lowers porosity substantially. Graywacke has less porosity than quartz sandstone.

Permeability

The ability to transmit a fluid (water)

permeability and porosity in rocks

Pumice has porosity, but little permeability. Clay and shale have porosity, but the pores are very small and poorly connected. Most igneous and metamorphic rocks have little permeability.

aquifers

Materials with both high porosity and high permeability (most sediment and sedimentary rocks)

aquitards

Materials with low permeability (shale and most igneous/metamorphic rocks).

Effluent stream

found in humid climates

Influent stream

found in dry climates

unconfined aquifer

have a direct connection to the surface

recharged directly by rainfall and/or snowmelt

confined aquifer

Aquifers located below an aquitard layer

They recharge with water slowly, but groundwater is often cleaner due to filtration through the aquitard (often clay or shale).

Springs

places where groundwater returns to the surface

Geysers

exist where groundwater is superheated by magma. When pressure builds, the groundwater erupts.

Hot Springs

heated to a smaller degree and don’t erupt.

Groundwater Erosion

Based not on velocity, but on the effectiveness of chemical weathering

affect rocks such as limestone, dolostone, and rock gypsum much more than sandstone, shale, etc.

Karst

Areas where the land surface has been greatly modified by the collapse of caves

sinkholes, disappearing streams, and springs are common features.

usually appears in limestone regions where cave development is already in an advanced state.

Glaciers

form when snow accumulates at a greater rate than it melts, converts to the granular forms of névé and firn, then compacts due to pressure into glacial ice.

Types of Glaciers

Alpine (Valley) and Continental (Ice Sheet)

Alpine (Valley)

Form in, and are restricted to, high gradient river valleys in mountainous areas. Most common type in existence today.

Continental (Ice Sheet)

Form in high latitudes and are not constrained by topography. Much thicker than valley type. Most common during Ice Ages.

Glacial Ice Movement

basal sliding and plastic flow

glacial ice movement by basal sliding

The glacier, as a single mass, slides over the underlying rock on a layer of meltwater.

glacial ice movement by plastic flow

The individual ice grains slide over each other. They also deform and recrystallize.

Rates of Movement

Movement varies over the course of the year, but averages between 0.1 and 10 meters per day.

Plastic flow is fastest near the center.

Glaciers may temporarily move much faster, over 100 meters per day, by surging. This is often caused by a build-up of meltwater beneath the ice which may float the glacier.

Ablation

the loss of glacial ice by melting sublimation or calving (pieces falling off the end).

terminus

a glacier will advance if accumulation of new snow and ice exceeds ablation.

will retreat if ablation exceeds accumulation - but the glacial ice will still flow forward.

Plucking

erosion combined with mechanical weathering. Ice fractures the underlying bedrock, then freezes it to the glacier.

Abrasion

moving ice and imbedded rock grind down the bedrock into rock flour (fine silt and clay-sized material).

Erosional Landforms

Cirques, Horns, Arêtes, U-Shaped Valleys, Fjords, Striations and Grooves, Roches Moutonnées

Cirques

Amphitheater-shaped recesses carved into a mountain at the head of a glacial valley. They usually have very steep slopes on three sides.

often later occupied by tarns

tarns

bedrock-floored lakes

Horns

Eroded peaks formed when three or more cirques cut back into a mountain.

Arêtes

Eroded ridges of bedrock between glacial valleys.

Fjords

flooded glacial valleys that form from post-glacial sea level rise or sub-base level erosion.

Striations and Grooves

scratches in bedrock from mm to meters in width and depth. Used to interpret the direction of former ice movement. Kelleys Island has well-preserved grooves.

Roches Moutonnées

Eroded hills of bedrock that were resistant to glacial erosion. They are usually elongated and streamlined. The gentle side of the hill faces into the direction from which the ice came, the steeper side faces in the direction of ice movement.

Depositional Landforms

till and outwash

Till

Material deposited directly by glacial ice. It is unsorted and unstratified mud, sand, gravel, and boulders.

Outwash

Material laid down or reworked by meltwater. Sorted and stratified sand and gravel.

Landforms Made of Till

Glacial Erratics, Moraines, Drumlins

Glacial Erratics

boulders derived from non-local bedrock

Moraines - End Moraine

a ridge piled up along the front end of a glacier.

Terminal moraine

the last moraine, marks the greatest extent of the glacier.

Recessional moraine

forms as a glacier retreats then temporarlly stops.

Drumlins

streamlined hill sculpted beneath glacial ice. The steep side points toward the glacier.

Hill made of till

Other Moraines

Lateral moraine, Medial moraine, Ground moraine

Lateral moraine

forms along the sides of many valley glaciers.

Medial moraine

a dark streak marking the union of two lateral moraines

Ground moraine

relatively thin blanket of till left behind as a glacier retreats.

Landforms Made of Outwash

Outwash Plain, Eskers, Kames Kettles - Shallow depressions marking the location of old, buried ice blocks that melted. These are often later filled with water and become kettle lakes. Kettle lakes are floored by sediment. (tarns are floored by bedrock) As kettle lakes begin to fill with sediment, they become kettle bogs (wetlands).

Outwash Plain

Thick blanket of sand and gravel.

Eskers

Long, sinuous ridges that form in subglacial stream channels.

Kames

irregular, mound-like hills

Kettles

Shallow depressions marking the location of old, buried ice blocks that melted.

kettle lakes

Kettles that are later filled with water

floored by sediment

kettle bogs (wetlands)

As kettle lakes begin to fill with sediment

Deserts

areas that receive very little rainfall (<25 cm/year) or the equivalent in snowfall

Mechanical and chemical weathering are reduced, so soil formation is limited. Mass wasting and erosion are slowed due to low amounts of surface water, but they still operate.

What Creates Deserts?

The Rainshadow Effect (topographic deserts), Mid-continental Effect, Effect of cold ocean water (coastal deserts), Geographic Effect (trade wind/polar deserts)

The Rainshadow Effect (topographic deserts)

a consistently high elevation mountain range may block significant rainfall from reaching downwind locations.

The Rainshadow Effect examples

The Patagonian and Atacama Deserts (Andes Mts.), and the Gobi desert (Himalaya Mts.)

Mid-continental Effect

regions far from water receive less rain