CSULA - Geology: Glaciers and Deserts

Glaciers - Introduction

Glaciers are a part of both the hydrologic cycle and rock cycle
A thick mass of ice that originates on land from the accumulation, compaction, and recrystallization of snow
Location
- Occupy 10% of Earth’s surface
- Primarily located in polar regions (Antarctica & Greenland)
- But found on every continent
- Form above the snow line
Formation
- New layers form each year
- Weigh of overlying layers compresses buried layers
  - Snow recrystallizes – looks like sugar
  - Snow begins to grow, air pockets decrease
  - compacts & becomes very dense
  - After 2 winters => FIRN
- Firn
  - Generally 16x the size of a snow crystal
  - ½ as dense as water
  - Increase in size as the overburden increases
  - Over time, grows to form even larger crystals
  - Forms glacial ice
Movement
- When ice sheet thickness > 18 meters, the ice sheet:
  - Deforms
  - Flows
  - Movement slower at base than at top
  - Advance and retreat
  - Surge
- Types of glacial movements
  - Plastic flow
  - Basal slip
- Rates of movement
  - Average velocities vary considerably
  - Rates of up to several meters per day
  - Some glaciers exhibit extremely rapid movements called surges
- Budget of a glacier
  - Accumulation + loss = glacial budget

Glaciers - Types

Features
- Crevasses
- Moraines
Types
- Ice
  - Ice Sheets,
  - Ice Shelves,
  - Ice Caps,
  - Ice Streams,
  - and Ice fields
- Glaciers
  - Mountain Glaciers,
  - Valley Glaciers,
  - Piedmont Glaciers,
  - Cirque Glaciers,
  - Hanging Glaciers,
  - Tidewater Glaciers.

Glaciers - Erosion and Deposition

Glaciers erode by
- Plucking – lifting of rock blocks
- Abrasion
  - Rock flour (pulverized rock)
  - Striations (grooves in the bedrock) Glaciers
Landforms created by glacial erosion
- Glacial Valleys
- Fjords
- Pater noster lakes
- Cirques
- Tarns
- Arêtes
- Horns
Glacial deposits
- Glacial drift
  - All sediments of glacial origin
  - Types of glacial drift
    - Till – material that is deposited directly by ice
    - Stratified drift - sediment deposited by meltwater
- Depositional features
  - Moraines – layers or ridges of till
    - Types of moraines
      - Lateral
      - Medial
      - End
      - Ground
  - Outwash plain, or valley train
  - Kettles
  - Drumlins
  - Eskers
  - Kames

Glaciers - Ice Ages

Have occurred throughout Earth’s history
- Last one began 2 to 3 million years ago
- Division of geological time is called the Pleistocene epoch
- Ice covered 30% of Earth's land area
Indirect effects of Ice Age glaciers
- Migration of animals and plants
- Rebounding upward of the crust
- Worldwide change in sea level
- Climatic changes
Causes of glaciation
- Successful theory must account for
  - Cooling of Earth, as well as
  - Short-term climatic changes
Proposed possible causes
- Plate tectonics
  - Continents were arranged differently
  - Changes in oceanic circulation (Thermohaline Current)
- Variations in Earth's orbit
  - The Milankovitch hypothesis
    - Shape (eccentricity) of Earth’s orbit varies
    - Angle of Earth’s axis (obliquity) changes
    - Earth’s axis wobbles (precession)

Deserts - Introduction

Definition :
- A region so arid that it contains no permanent streams except for those that bring water in from elsewhere, and has very sparse vegetation cover.
- NOT related to temperature!
- Deserts can be
  - Hot (>35 °C)
  - Cold (< 20 °C)
Location
- Dry regions cover 30% of Earth’s land surface
Types of deserts
- Two climatic types are commonly recognized
  - Desert or arid
  - Steppe or semiarid
- Classified by environment in which they are formed
  - subtropical: in the hot dry latitudes between 20 and 30°, both north and south
  - rain shadow: on the landward side of coastal mountain ranges
  - coastal: along coasts bordering cold ocean currents
  - continental interior: deep within continents, far from major water sources
  - polar: in the cold dry polar regions, both north and south
- Earth’s dry regions coincide with the subtropical high pressure belts & solar heating

Deserts - Weathering and Erosion

Not as effective as in humid regions
Mechanical weathering forms unaltered rock and mineral fragments
Some chemical weathering does occur
- Clay forms
- Thin soil forms
Water Erosion
- Desert rainfall
  - Rain often occurs as heavy showers
  - Causes flash floods
  - Poorly integrated drainage
  - Most erosional work in a desert is done by running water
- Streams are dry most of the time
  - Desert streams are said to be ephemeral
  - Flow only during periods of rainfall
  - Different names are used for desert streams including wash, arroyo, wadi, donga, and nullah
  - A dry stream channel in the desert
  - The same stream channel following heavy rainfall
Wind erosion
- Differs from that of running water in two ways
  - Wind is less capable of picking up and transporting coarse materials
  - Wind is not confined to channels and can spread sediment over large areas
- Mechanisms of transport
  - Bedload
    - Saltation – skipping and bouncing along the surface
  - Suspended load
    - In the air as duststorms
- Deflation
  - Lifting of loose material
  - Produces Blowouts & Desert pavement
- Abrasion
  - Produces ventifacts (stones with flat faces) and yardangs (wind sculpted ridges)
  - Limited in vertical extent
Depositional Environments
- Water Deposits
  - Talus Aprons
  - Alluvial Fans
  - Bajada
  - Playas and Salt Lakes
- Wind deposits
  - Dunes
    - Mounds or ridges of sand
    - Often asymmetrically shaped
    - Characteristic features
      - Slip face
      - Cross beds
    - Types of Sand Dunes
  - Loess
    - Deposits of windblown silt
    - Extensive blanket deposits
    - Primary sources are deserts and glacial stratified drift

Deserts - Basin and Range

The evolution of a desert landscape
- Uplifted crustal blocks
- Interior drainage into basins produces
  - Alluvial fans and bajadas
  - Playas and playa lakes
Erosion of mountain mass causes local relief to continually diminish
Eventually mountains are reduced to a few large bedrock knobs called inselbergs projecting above a sediment filled basin
Landscape evolution in a mountainous desert – early stage
Landscape evolution in a mountainous desert – middle stage
Landscape evolution in a mountainous desert – late stage
Inselbergs in Southern California

LINKS

Glaciers

Deserts