Geology 21 (Deserts)

Earth and Space Sciences X

29 Oct 201546:01

EducationalLearning

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TLDRThis video lecture discusses how desert landscapes are formed through processes like weathering, erosion, and deposition. It explains how climate, including low rainfall and high evaporation rates, leads to distinctive desert features like ephemeral streams, alluvial fans, playas, and dust storms. The lecturer describes how wind and water shape the landscape through weathering, transporting sediment that is deposited in dunes and loess layers. Imagery of famous deserts illustrates these processes at work, from the red, eroded hills to shifting sand dunes, providing a compelling overview of the geology behind desert scenery.

Takeaways

😎 Deserts cover 30% of the Earth's land surface
🌵 Deserts form between the tropics of Cancer & Capricorn due to air circulation patterns
🚰 Most desert streams are ephemeral - only flowing after rainfall
🏜 Interior drainage shapes desert landscapes over time
💨 Wind erodes deserts by saltation, suspension and deflation
🔥 Flash floods can reshape desert channels rapidly
🐪 Vegetation influences dune shape and location
😲 Entire meters of topsoil were lost in the US Dust Bowl
🌀 Different dune shapes depend on sand supply and wind direction
🏠 Ancient peoples used stable deposits like desert pavement and loess to build shelters

Q & A

What causes the red color in desert rocks?
-The red color comes from the chemical weathering of iron-rich sediments, which produces iron oxide or rust over long periods of time.
Why don't deserts have much vegetation?
-Deserts lack vegetation because they receive very little rainfall per year, making it difficult for plants to grow and thrive.
How are dunes formed in deserts?
-Dunes are formed when wind is obstructed, causing it to drop its bed load of sand. The deposited sand accumulates into mounds and ridges known as dunes.
What are ephemeral streams?
-Ephemeral streams are dry streambeds that only carry water in response to rainfall. They are common in deserts.
How does deflation occur?
-Deflation is the lifting and removal of loose surface material by wind. It can lower the land surface significantly over time.
What are alluvial fans?
-Alluvial fans are deposits of sediment that form at the mouth of canyons when episodic flooding occurs. They are common features in arid environments.
What causes the formation of rain shadows?
-Rain shadows form when moisture-laden air is forced upwards by mountain ranges. This causes the air to cool, condense, and precipitate on the windward side, leaving the leeward side dry.
How do desert pavements form?
-Desert pavement forms as wind blows away finer grains, leaving behind a dense layer of pebbles and cobbles. These become concentrated at the surface over time.
What are loess deposits?
-Loess deposits consist of silt-sized particles carried long distances by wind in suspension. They are blankets of silt deposited far from their source.
How does cross-bedding form in sand dunes?
-Cross-bedding forms as sand avalanches down the leeward sides of dunes at the angle of repose. This creates inclined layers as the dune migrates.

Outlines

00:00

😊 Intro to deserts and wind erosion forming landscapes

The first paragraph is an introduction to desert landscapes depicted in movies/TV, with red rock formations amongst sand. This scenery is explained by science - wind erosion carves the rocks. There will be a discussion on deserts and wind.

05:00

🌞 Hadley circulation cell causes dry zones and rainforests

The Hadley circulation cell is explained - hot air rises at equator, moves poleward aloft, cools and sinks around 30 degrees latitude, then flows back towards equator near surface. This causes dry descending air and deserts from ~30 degrees, contrasted with wet equatorial rainforests. Trade winds also result.

10:02

⛰️ Mountains and distance from oceans also cause deserts

Other factors also create deserts - interior continental locations distant from ocean moisture like the Gobi desert, and mountain rain shadows where air descends the leeward side dry, like the Atacama desert next to the Amazon rainforest on the windward Andes side.

15:03

😿 Desert streams are ephemeral, with dramatic flash floods

Deserts are defined by low rainfall vs high evaporation potential. Streams are normally dry ephemeral washes, flowing only during sporadic rainfall then disappearing into the ground. Flash floods can be dramatic events, with sediment-laden flows.

20:04

🏜 Internal drainage and tectonics form desert basins over time

Deserts often have internal drainage without reaching the ocean. Uplift and mountain building supply sediment to adjacent subsiding basins, with alluvial fans coalescing to surround playa lakes, sometimes completely filling basins with erosion remnants as inselbergs.

25:05

🧗 Sediment transport differs between wind and water

Wind and water erode and transport sediment differently in deserts. Water weathers and concentrates coarse material in washes better than wind. Unconfined wind suspends fine material that travels farther, with bed load saltation and creeping transporting sand.

30:05

🏖 Arid wind deposits: dunes, loess blankets, and desert pavement

Significant arid wind deposits include dunes (mounds of sand, often with slip faces and cross beds), loess (blankets of fine previously suspended sediment), and desert pavement (concentrated cobble veneers when fines are removed).

Mindmap

Keywords

💡desert

A desert is defined as a dry climate where yearly precipitation is less than the potential for evaporation. Deserts cover 30% of the Earth's surface and are found between the tropics of Cancer and Capricorn. Deserts lack vegetation and water sources due to the dry climate.

💡ephemeral stream

Ephemeral streams only carry water in response to rainfall. They are dry most of the time. When it rains, the water flows as runoff into the stream bed rather than soaking into the ground because desert soils are poor at absorbing water.

💡alluvial fan

Sediment laden rivers deposit debris at the mouth of a canyon, forming a fan shape. This is an alluvial fan. Coarse materials are deposited near the mouth while finer grains go to the edge of the fan.

💡playa

A playa is a dry, flat lake bed that is left after water evaporates from an ephemeral lake. Playas form in deserts when streams flow across a bajada during heavy rainfall but then evaporate.

💡bajada

A bajada is a coalescing of alluvial fans along the flanks of mountains. The fine sediments go to the outer edges of the bajada and the coarse stuff stays near the mouths of the channels.

💡deflation

Deflation is the lifting and removal of loose surficial material by wind. It lowered soil levels by multiple feet during the Dust Bowl event in the early 1900s US.

💡dune

Dunes are mounds or ridges of sand deposited by wind. They often form around obstructions like rocks or vegetation that block wind flow.

💡saltation

Saltation is the hopping and skipping motion of sand grains along the surface driven by wind. It is the main process of bedload transport in deserts.

💡loess

Loess consists of blankets of silt deposited by wind. It was originally picked up into suspension from deserts or glacial deposits before being dropped somewhere downwind.

💡angle of repose

The angle of repose is 34 degrees for sand. This is the maximum angle sand can stay stable, and is the typical angle for the slip face of dunes.

Highlights

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Transcripts

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