Deserts

What causes a desert – Is there a global pattern? The word “desert” – means deserted or literally uninhabited Features of a typical desert absence of soil and abundant plant life barren rocky outcrops steep angular slopes common rock colors of tinted red and oranges deserts definitely represent dry-dry-dry environments What causes a desert – Is there a global pattern?

Two ways to produce deserts: Global winds high/low pressure systems Blocking precipitation Rain shadows

What global patterns show the distribution of deserts? Air sinks (H) Air rises (L) Air sinks (H) 30O North Latitude Equator 30O South Latitude

Desert created by a rain shadow Dry Air Precipitation Rain Shadow Moving air Dry Air Sierra Nevada Mountains Precipitation Rain Shadow Coast Ranges Evaporation Delano Santa Barbara

High Pressure (air sinks) Equator Rain Shadow deserts 30O N . High Pressure (air sinks) Equator Low Pressure (air rises) High Pressure (air sinks) 30O S

US RAIN SHADOWS 30O North

I Deserts Describe a least 3 common characteristics found in deserts 2. Describe the 2-ways deserts are formed on the earth’s surface 3. Explain why Bakersfield is considered a semi-desert region– what influences Bako’s amount of precipitation? Discuss with a friend I will get an A on my exams and quizzes

Differences between topography and weathering Coast Ranges The Desert Ppt > Evaporation Ppt < Evaporation Rounded hills with thick soil profiles supporting dominate chemical weathering Angular cliffs, jagged mountains unaltered rocks supporting dominate mechanical weathering Abundance of ephemeral streams carry water only when there is rain Common year around streams

Common Desert features produced by transportation of sediment by water Alluvial fans Bajada Inselbergs Playa Basins

Arial view of a “typical” desert Water transported features Alluvial fans: a fan shaped deposit of sediment protruding from the mouth of a canyon Alluvial fans Mountains Bajada: a collection of alluvial fans that have coalesced together Inselberg large bedrock “knobs” that are raised above sediment Playa Playa Playa: a flat basin area that shows the remnant of a “desert” lake – the lake has evaporated

Continued erosion Cross sectional view of a desert landscape proximal Alluvial fans Playa basin proximal distal Continued erosion Inselberg Bedrock Alluvial fans – coarse thin deposits proximal part of fan finer deposits at distal part of fan Playa basin – fine lake bed deposits (water drains the alluvial fan)

I Deserts Describe the following common desert features: alluvial fan playa basin inselberg bajada Discuss with a friend I will get an A on my exams and quizzes

Transportation of sediment by Wind Bed load – saltation processes Suspended loads Types of desert features from wind erosion Deflation, blowouts, desert pavement Wind abrasion Various types of wind deposits vantifacts sand dunes

Moving sand grains by wind – the bed load Saltation – wind blown sand “jumps” in the wind Wind must be strong enough to overcome inertia Saltation ends when wind is weak Grains are not moved very far “sandblasting”

The suspended wind blown sediment Finer sediments (silt-size and smaller) is lifted and suspended within the atmosphere Wind storms – sand blast the windshield Dust storm- suspended load Saltation

Wind Erosion – results of the power of wind Deflation – the lifting or removal of material by winds Dependent on wind velocity Dependent on grain-size to be picked up Deflation begins Deflation continues to remove finer particles Heavier particles left on desert floor – deflation Ends – Desert Pavement

Examples of desert pavement produced by deflation processes

Wind abrasion – the act of sand-blasting Ventifacts- rocks that have been shaped by sandblasting The rock sits there for many years Abrasion of wind-blown sand constantly shapes the rock

Ventifacts

I Deserts What is the primary way sediment is transported in desert areas? Describe the following processes saltation deflation 3. How is desert pavement created 4. What is a ventifact? – how are these created? Discuss with a friend I will get an A on my exams and quizzes

Wind Deposits – Sand Dunes Sand dunes – an accumulation of sand grains that reflect the consistent wind velocity and wind directions Typical formation of a sand dune dependant on wind direction and wind velocity quantity of sand About 28 degrees

Types of Sand dunes Barchan dune – crescent like shapes with horns pointed towards the wind Longitudinal Dunes – sand grains are blown parallel to wind direction- supplies are moderate Transverse dunes – sand ridges are oriented at right angles to wind – sand is plentiful and winds are consistent

Longitudinal Dunes Barchan Dunes Transverse Dunes Wind direction