Volcanoes, Weathering, and Soil Formation: A Comprehensive Guide
Volcanic Eruptions
Nonexplosive eruptions are relatively calm outpourings of lava that can release a huge amount of molten rock. Explosive eruptions release clouds of hot debris and gases, often at supersonic speeds.
Volcano Formation
Volcanoes form around vents that release magma onto Earth’s surface. The composition of magma determines if a volcanic eruption is explosive, nonexplosive, or somewhere in between. High water and silica content in magma increases the likelihood of an explosive eruption.
Lava Types
- Blocky lava: Thick consistency, barely creeps along the ground.
- Pahoehoe/Pillow lava: Thin consistency, flows like drippy wax.
Pyroclastic Material
- Volcanic bombs: Large blobs of magma that harden in the air.
- Volcanic blocks: Solid rock blasted from a volcano.
- Lapilli: Little stones, bits of magma that solidify before hitting the ground.
- Volcanic ash: Fine particles of rock and volcanic glass.
Types of Volcanoes
- Shield volcanoes: Built out of layers of lava from repeated non-explosive eruptions, spread out, not steep slopes.
- Cinder cone volcanoes: Made of pyroclastic material from moderately explosive eruptions, steep slopes with a narrow base.
- Composite volcanoes: Formed by explosive eruptions of pyroclastic material followed by quieter outpourings of lava, broad base.
Volcanic Features
- Craters: Funnel-shaped pits at the top of the central vent in most volcanoes.
- Calderas: Form when a magma chamber that supplies material to a volcano empties and its roof collapses.
- Lava plateaus: Runny lava pours from a series of fissures and may spread evenly over thousands of square kilometers.
Magma Formation and Volcano Locations
Volcanoes begin when magma collects in deeper regions of Earth’s crust and in the uppermost layers of the mantle. An increase in pressure raises the melting point of mantle rock, making it harder to melt, and vice versa. Magma rises towards the Earth’s surface because it is less dense than the solid rocks surrounding it.
Some volcanoes lie on tectonic plate boundaries. The Ring of Fire, a zone of plate boundaries surrounding the Pacific Ocean, has many volcanoes.
Magma Formation at Divergent Boundaries
Mantle material rises to fill the space opened by separating tectonic plates. As pressure decreases, the mantle melts.
Magma Formation at Convergent Boundaries
As an oceanic plate moves downward, some of the rock melts and forms magma.
Hot Spots
Places on Earth’s surface that are directly above columns of rising magma called magma plumes.
Volcanic Activity
- Extinct volcanoes: Volcanoes that haven’t erupted in recorded history and never will again.
- Dormant volcanoes: Volcanoes that aren’t currently erupting but have erupted sometime in recorded history.
- Active volcanoes: Those that are in the process of erupting or that show signs of erupting in the very near future.
Predicting Eruptions
Scientists monitor several factors to predict volcanic eruptions, including small earthquakes, changes in the slope of volcanoes, volcanic gases, and temperature changes detected from orbit.
Weathering and Erosion
Mechanical Weathering
- Ice wedging: When water expands after freezing, it pushes against the sides of cracks in rocks.
- Water, wind, gravity abrasion: The action of rocks and sediment grinding against each other and wearing away exposed surfaces.
- Plants: The force of expanding roots can make existing cracks in rocks larger.
- Animals: Earthworms, for example, burrow through the soil and move soil particles around.
Chemical Weathering
- Water: Dissolves minerals in rocks.
- Weak acids: React with minerals in rocks.
- Air: Contains oxygen, which can react with minerals in rocks (oxidation).
- Soil: Contains acids and microorganisms that can break down rocks.
Oxidation
A chemical reaction in which a substance combines with oxygen to form an oxide.
Differential Weathering
A process by which softer, less weather-resistant rocks wear away, leaving harder, more weather-resistant rocks behind.
Factors Affecting Weathering
- Shape of weathering surface: More surface area exposed to weathering leads to faster wearing down of rock.
- Climate: Oxidation occurs quicker in warm and humid climates. Temperature extremes also accelerate weathering rates.
- Elevation: Rocks at high elevations are exposed to more wind, rain, and ice than rocks at lower elevations.
Soil Formation
Bedrock
The layer of rock beneath soil.
Sources of Soil
- Parent rock: The rock that is the source of soil.
- Residual soil: Soil remaining above the bedrock it’s formed from.
- Transported soil: Soil deposited after it’s blown or washed away from its parent rock.
- Humus: The organic part of soils, containing very small particles of decayed plant and animal material in addition to bits of rocks. Humus contains nutrients necessary for plant growth.
Leaching
The process by which rainwater dissolves and carries away minerals and nutrients in the topsoil.
Soil Layers (Horizons)
- Horizon A (Topsoil): Contains the most humus.
- Horizon B (Subsoil): Contains less humus than topsoil.
- Horizon C: Consists of partially weathered bedrock, which is usually the parent rock.
Soil and Climate
- Tropical: Humid climates with lots of rain have thin topsoil due to leaching.
- Desert: Low amounts of rain result in low rates of chemical weathering and thin, poorly developed soils.
- Temperate: Abundant mechanical and chemical weathering create fertile soils.
- Arctic: Little rain and cold temperatures result in slow soil formation and poorly developed soils.
Stream Erosion and Transport
Stream Load
The geologic term used to describe the solid matter carried by a stream. Erosion moves the material from the bed and banks of a stream.
Types of Stream Load
- Bed load: Heavier material on the bottom of the stream that rolls, hops, and slides.
- Suspended load: Clay or silt particles that float, carried downstream by the water’s flow.
- Dissolved load: Invisible chemical ions in solution, caused by chemical weathering of rocks.