Glaciers and Deserts

Why study glaciers, ice, wind, and deserts in the same chapter?

• All related to solar heat
  • It drives evaporation $\Rightarrow$ precipitation and glaciers
  • Differential heating between land and water $\Rightarrow$ winds
  • Seasonal temperatures, wind and climate $\Rightarrow$ deserts
• Water, ice, and wind are all agents of change on Earth’s surface

Ice and Glaciers

Formation of Glaciers

• Formation of a glacier occurs in a region where snow will remain year round

  • Snowfall >> Melting
  • High latitudes or high elevations
  • North-facing slopes in Northern Hemisphere and South-facing in Southern are best

• From snow to glacial ice depends on compaction of snow over multiple seasons, reduction in air volume, and increase in density

• Firn: snow that survives at least one winter; recrystallized into a material denser than snow because it has less air

• Glacier: mass of ice moving under its own weight due to gravity

• Heat from the sun is generally constant (but not distributed uniformly across the earth over time) $\Rightarrow$ locations of glacirs
• Climatic factors may influence the global radiation budget, which impacts extents of glaciers and whether they can exist at all
  • Global cooling: ice will accumulate and build ice sheets and glaciers
  • Global warming: ice sheet retreat and glaciers get smaller
  • Factors that change climate: composition of the atmosphere ($C{O}_2$, $C{H}_4$, water vapor), pollution or particles suspended in the atmosphere, abnormal heat retention (or loss) from the oceans

Types of Glaciers

Two types based on size and occurrence

• Alpine Glaciers
  • Also known as mountain or valley glaciers
  • Occur at high altitude (cooler temperatures)
  • Most numerous globally (70,000 - 200,000)
• Continental Glaciers
  • Also known as ice caps or ice sheets
  • Occur near the poles (over land)
  • Larger, thicker (up to 1 km of ice), rarer
  • Contain far more ice than all Alpine glaciers
  • Greenland and Antarctic ice sheets

Ice Sheets

• Antartica: an ice sheet averaging 2,160 m thick and reaching a maximum thickness of about 4km nearly covers almost 14,000,000 $k{m}^2$ of Antartica
  • $61\%$ of all freshwater held here
  • $0.4\%$ of land not covered in ice (basically ice then bruh)
• Arctic: the continental glacier in Greenland covers about 1,700,000 $k{m}^2$ with a maximum thickness of about 3km
  • $20\%$ of land not covered in ice

Movement of Glaciers

• Glaciers flow as plastic ice masses and at different rates; overall movement is down slope
• Movement is slow when in contact with land and scarping the valley walls, either at the base or on sides
• Movement is faster near surface and in middle
• Net flow is downslope relative to underlying bedrock or the glacier surface when underlain by mountains
• Meltwater helps it flow often

Glacier movement has a terminus (it ends)

• Glaciers that encounter water will experience calving $\Rightarrow$ icebergs
• If temperatures at terminus are warm, then melting or evaporation occurs
• Ablation: processes by which glacial ice is lost
• Equilibrium line: no gain or loss; Above snow accumulates, below ablation
• Movement is slow and steady (10s m/y); surges are possible (10s of m/d).

• Advance: glacier becomes thicker and longer (accumulation > ablation)
• Retreat: ablation > accumulation

• Some glaciers are advancing but most Alpine glaciers retreating
• Greenland getting thicker in middle and melting more on edges $\Rightarrow$ Net loss of ice

Glacial Erosion

• Glaciers leave evidence of their former presence
  • U-shaped valley Rocky Mountains in Colorado
  • Finger Lakes, New York, USA
• Striations: rocks frozen into ice grinding along bedrock below
• Abrasion: general erosion by scraping of ice or sediment
• Plucking: Water helps the base of glacier freeze onto rock below and when glacier continues it will pluck/tear off bits of rock its frozen to
• Cirque: bowl-shaped depression as a result of plucking at the head of the glacier

• Arête: sharp edge between adjacent glaciers

• Horn: single peak resulting from erosion of several glaciers

Glacial Deposition

• All the sediment and rock debris eroded by the glacier tends to be carried with the glacier and deposited when the glacier melts/evaporates

• Till: sediment deposited directly from the ice

  • Till is angular and poorly sorted

• Outwash: till that is transported by meltwater and deposited elsewhere

• Glacial drift: ‘A general term applied to all rock material (clay, silt, sand, gravel, boulders) transported by a glacier and deposited directly by or from the ice, or by running water emanating from a glacier’. Till and outwash are varieties of glacial drift.

• Moraine: landform made of till; different types of moraines exist

  • End moraine: ridge of sediment at the nose of the glacier added to repeatedly by the annual advances and retreats of a glacier
    • Terminal moraine: no longer active end moraine that marks furthest advance of a retreating glacier

When hiking in Alpine regions you will see many different types of moraines not covered in vegetation

Some moraines are still active, and others are not.

A Famous Terminal Moraine?:

How the Ice Age Shaped New York

Last Glacial Maximum

• ~12,000 years ago North America was covered by ice
• Glaciers responsible for most of area’s topography
• Carving evident in western mountain ranges
• Great Lakes carved by glaciers and filled with meltwater
• Hudson Bay is a depression formed by weight of ice
• Mississippi River and groundwater reserves formed by glacial meltwater
• Glacial sediments formed fertile lands of midwest

Ice Ages and Possible Causes

• Several periods of extensive continental glacial coverage (i.e., ice ages) have occurred even in the recent geologic past (from 2 million to 10000 years ago, Pleistocene)
• Interglacial periods: when ice sheets retreat; currently, we are in an interglacial period
• Possible causes can be from events on Earth or externally
  • Solar energy output or sunspot activity (not likely, hard to prove)
  • Breakup of Pangaea and isolation of poles (but only for Pleistocene ice ages)
  • Evolution of land plants and reduction in atmospheric $C{O}_2$
  • Intense volcanic activity with dust and sulfuric-acid emissions (data needed)
• Best evidence available is for the Milankovitch Cycles of the Earth’s orbit around the sun
  • Shape of orbit varies every 100,000 years
  • Earth’s axial tilt varies every 41,000 years
  • Earth’s axis wobbles (precession cycle) every 26,000 years
• All cycles would affect solar heat distribution globally
• Climate records of temperature and sea-level variations suggest MC can account for advance and retreat but not initiation of an ice age

Wind and Deserts

Wind in Geologic Context

• Air moves along the Earth’s surface in response to air pressure differences, which are often related to surface temperature differences (i.e., solar heating)
• Movement complicated by the fact that land and water absorb heat differently (land varies more than water)
• Air can erode, transport, and deposit material
• Wind accounts for a minor amount of sediment erosion and transport, but regionally it is very important.

Wind Erosion

• Wind erosion most effective on exposed sediments
• Wind transport may be major or only means in deserts
• Abrasion: wearing away of a solid object by impact of wind-blown particles, forming ventifacts, which are ‘wind-made’ rocks

• Deflation: removal of loose (fine-grained) sediment by wind; leaves coarse ones behind
• Vegetation blocks wind
  • Keeps fine-grained sediments
• Desert pavement: larger rocks protect sand below; creates a stable surface
• Dune: ridge of particle (sand commonly); principal feature of wind deposition
  • Sediment size deposition based on velocity
  • Once a dune starts to form, it acts as an obstacle to wind and collects more
  • Dunes can be 3 to 100+m high
  • Dunes can migrate with the wind direction
  • Particles move up shallower windwards side by saltation, then fall down the steep slip face

• Loess: deposit of windblown silt (very fine grained, easy to carry)
  • Silt from glacial meltwater or deserts

• Right side is South Island of New Zealand

Deserts

Deserts

Region with so little vegetation that limited life is supported

• Sand or rock
• Hot or cold
• Dry or wet

Distribution of arid regions

• Location of natural arid deserts mostly due to warmer temperatures and higher air pressures, both which allow the atmosphere to hold more water
• Mostly occur near $30^o$ N&S where the air pressure is high, so it sinks and warms causing evaporation (i.e., drying the land the below)

• Location of natural deserts may be due to topography
• Air moving from the oceans inland carries water and is forced to rise and get colder $\to$ rain
• Most of the water is released on the ocean side and the back side of the mountain is left with dry air $\to$ a rain shadow

• Interior continental deserts result of distance from ocean
• Coastal deserts occur where cool ocean air that holds more water abuts hot continents
• West coast of South America and Africa
• Polar deserts occur because air reaching poles is relatively dry and becomes even colder, holding more water
• Limited evaporation from nearby oceans contributes to the little snow that falls there
• Almost all of Antartica is a desert

Desertification

Rapid development or expansion of deserts caused or accelerated by humans, mostly land use practices

• Arid and semi-arid lands have limited rainfall and vegetation, which helps keep soils somewhat moist and protected from wind erosion

• Vegetation may be cleared for food, shelter, energy livestock grazing, or to grow crops

• Crops may not be resilient enough to survive a drought, which leads to desertification that can be irreversible