EROSION, DEPOSITION & LANDSCAPES

Tying it together…

You now have had significant experience learning about the variety of ways rocks form, and some unique formations associated with the study of plate tectonics. You are most familiar with what are known as uplifting forces. For example, the contribution of plate convergence to volcanic mountain ranges. New crust is formed, old crust is recycled back into the Earth. The focus of this unit seeks to broaden your understanding of leveling forces, those which contribute to wearing away of materials on the Earth’s surface.

Our weathering unit taught us that rocks can be broken down by both physical and chemical means based on climate and bedrock type. Understanding how Earth materials erode, becomes the next step in truly understanding the dynamic nature of landscapes.

What is erosion?

Erosion is the removal of rock particles and soil from an area

Erosion requires energy to take place

Erosion is driven by gravity

There are 5 distinct agents of erosion:

Rivers/streams Glaciers Wind Wave Action Mass Movement

Erosion in moving water varies greatly within a river, and between rivers. Lighter, less dense sediment is first to be transported by moving water.

Transport of Sediments in Streams

Sediment transport is dependent on 2 variables:

Velocity (speed) of water

Particle size and shapehttp://www.youtube.com/watch?v=jpexS4-9IF0

Describing Rock Particle Size

Rock Particles are categorized into 6 types:

Boulders Cobbles Pebbles Sand Silt Clay

*Dissolved minerals are also transported, although they are not visible

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Quantified Sediment Sizes

Sediment Transport Chart

In general, the chart shows that larger particles require an increased water velocity (speed) to be transported

If water velocity falls below that level, sediment is deposited on the bottom of the riverhttp://www.youtube.com/watch?v=rNgjRy4r5-4

Using the Chart

The chart shows average particle diameter (particle classification) and the velocity required to maintain movement of that particle in moving water

Water Velocity and Stream Morphology

Water velocity is not a constant along a river nor is it constant across a transect

Water Velocity and Stream Morphology

Water, for the most part, flows in a straight line and will do so until redirected by the stream channel. Water also over time dictates how the channel changes.

Because this is true, rivers have different channel shapes at different locations

In a straight line path, rivers are fastest in the middle, leaving a channel deepest in the center

As river water enters a curve, the faster water is on the outside causing more erosion and a greater depth

Water slows on the inside of a curve, causing deposition of particles

Stream Meanders

Over time, erosion at the outside of the bend causes the stream channel to drift. When the old channel is cut off, an oxbow lake is formed.

Sorting of Sediments

Faster moving water transports smaller sediments

Therefore, a stream bottom will have larger particles on the bottom in faster moving water and smaller particles on the bottom in slower moving water

Streams deposit rounded, sorted sediments based on water velocity

Rivers Entering Lakes and Oceans

The deposition of sediments at the mouth of a river results in the formation of a delta

The stream flow slows resulting in deposition of largest particles first

Particles such as silt and clay are carried away from the mouth until there is no longer sufficient velocity to transport them

http://www.youtube.com/watch?v=B6p5jXDhDvI

Stream Valley Shape

As streams downcut through soil and bedrock, they create a V-shaped valley

Drainage Patterns

Looking at maps and examining rivers often tells us something about the rock strata beneath. Stream drainage patterns are dictated by:

Differences in elevation

Characteristics of rock layers (resistance to weathering)

Orientation of rock layers (how the rocks are tilted or folded)

Differences in Elevation

Higher elevations with steep slopes result in deep ‘V’ shaped valleys

Level lowlands result in more meanders in the stream

Drainage Topography in Arid Regions

Weathering in dry regions is generally a very slow process. The majority of weathering and erosion takes place in the river channel.

Canyons are common because the canyon walls weather very little over time

Drainage Topography in Humid Regions

Gentler slopes are a result of more rapid weathering of rock

Common occurrence in NYS

Drainage Topography

Drainage patterns match the surrounding landscape and are flowing from high to low elevations

Mate these with an overhead pattern

Glaciers

New York State owes its many unique landforms to the presence of glaciers in the past. Evidence of repeated glacial events surrounds us in western New York. Glaciers are responsible for both erosional and depositional features.

Glaciers form when repeated snowfall accumulates in an area and forms into an ice pack that is able to flow

Types of Glaciers

For our purposes, there are 2 types of glaciers that we are concerned with:

Continental (ice sheet): Spans all or some of a continent

Valley (Alpine): Restricted to a valley

Glacial Presence in NYS

A massive glacier covered New York (most recently around 10,000 years ago), and it is important to understand how it shaped landforms in the area

Glacial Weathering

Abrasion is the dominant form of weathering underneath a glacier

Rocks and other sediment are ground against bedrock as they are dragged overhead

Evidence: We find parallel grooves, called striations, in bedrock that indicate the direction of movement

Glacial Erosion

As glaciers erode a valley, they leave evidence of having been there

Valleys take on a “U” shape

Finger Lakes

Glacial erosion can leave behind a variety of observable features

Finger Lakes are carved out of bedrock in the direction that the glacier moved

Drumlins

Elongated hills called drumlins are also found in areas that have undergone glaciation

The drumlins align with the direction of glacial movement

Erratics

Large boulders that do not match local bedrock type or are found in awkward elevations or places were likely transported there by a glacier

Eskers

Long, winding hills of sediment deposited by streams beneath a glacier

Sediment will be sorted and rounded because it was transported by running water

Kames

Rocky debris that is deposited in crevasses within a glacier to form a hill

Kettle Lakes

When large chunks of the glacier break off and are partially buried, they melt to form a kettle lake

Moraines

Moraines are long hills that are composed of material that a glacier has pushed or pulled along

Rocks in moraines are unsorted and angular (no abrasion in running water)

Outwash Plain

An outwash plain is where many streams (or just a few) flow out of the glacier as it melts. The plain is relatively flat with sorted and rounded sediments due to abrasion in the streams

Cirques, Aretes, Horns

All features found in mountainous areas once covered by glaciers

Horn: Sharp point at mountain peak

Cirque: Bowl-shaped depression caused by abrasion

Arete: Sharp ridge carved out of the mountainside

Wind Erosion and Weathering

Abrasion is the dominant form of weathering in dry climates

Rocks become “sand-blasted” and the particles are blown away

Wind Deposition

Sand and other particles are deposited in dunes

Wave Erosion and Weathering

Abrasion is the dominant form of weathering along beaches

Particles are washed on shore, swept back out, and gradually move in the direction of the current (long shore drift)

Long Shore Current

Long shore currents flow parallel to the beach. This flow steadily drags sediments along the shore in what is called long shore drift.

Mass Movement

The final, though less substantial type of erosion is called mass movement.

Mass movement is the sliding of any rock material or soil downhill as a result of gravity

Some mass movement is slow, and some is sudden

Landslide

Landslide is a commonly used term for the movement of a mass of bedrock or loose soil and rock down the slope of a hill, mountain or cliff.

http://www.youtube.com/watch?v=23NZTzpw6cY

Creep

Slow, imperceptible movement of soil down a slope

Slump

Blocks of land that have tilted and moved downhill along a surface that curves into the slope

Mudslides/Flows

Rapid movement of Earth saturated with water

Weathering, erosion, deposition, plate tectonics, folding and faulting all contribute to the beautiful landscapes around us, many of which can be found in New York.

Landscapes Features of NYS

Mountains Plateaus Plains Valleys Ridges Escarpments

Mountains

Mountains are classified as locations that have a much higher elevation than surrounding areas

NYS Example: Mount Marcy (Adirondack Mountains)

The Adirondack Mountains are part of an area which has been uplifted and domed upward and continues to do so. They are not volcanic in origin and are comprised mostly of metamorphic and intrusive igneous rocks.

Other Types of Mountains

Folded Mountains form when compression occurs typically during a tectonic collision

Fault Block Mountains occur when faults (lines of breakage) allow rock on either side to move up or down relative to the opposite side

Plateau

A region characterized by a high, flat area

Some may appear to look like mountains after rivers have cut through them

NYS Example: Allegheny Plateau

Plains

Generally low-lying, flat area

Example: Erie-Ontario Lowlands

Valleys

Valleys can be formed from erosion by rivers, glaciers, in between folds in bedrock, or where a fault block has fallen down.

Valleys are conspicuously lower areas of elevation compared to the surrounding landscape

Ridges

A chain of mountains or hills that form a crest for some distance

Escarpments

Escarpments are long cliffs formed by erosion or faulting

Land is relatively flat on either side

Niagara Escarpment is caused by differential weathering of bedrock

NYS Landscape Map

Use this map to identify major landscape regions in NYS

For example: Which landscape region is Rochester located in?

Answer:__________________________