Constructive Forces in Geology!
Volcanic Activity
•Divergent Boundary – 2 plates move apart and magma flows in to fill the gap
–Most on ocean floor, build oceanic ridges like the mid-Atlantic ridge
•Subuction – Plates move towards each other but one sinks under the other.
–The immediate result is DESTRUCTIVE.
–Over time magma forms and reaches the surface through eruptions, which is
CONSTRUCTIVE.
•Hot Spots – Plate moves across a ‘Hot Spot’ in the mantle causing magma to rise.
–Examples are Hawaii and Yellowstone
Mountain Building
•Collision – At a plate boundary or fault, crusts move towards each other and push upward
deforming into folded mountains.
–There is no magma involved, so these are not volcanic mountains
–Examples are the himalayas and the alps
•Uplift – of rock into mountains, generally happens along a fault.
–Example – Grand Tetons in Montana
Deposition
•Deposition – Layering of sediment over time, layer upon layer are deposited by rivers,
oceans, wind and ice then layers are compacted and cemented into rock
-Ex: Arches National Park, Utah
Weathering
Destructive Forces
Weathering is a process that breaks rocks into smaller pieces called sediment.
Weathering can be mechanical, in which rock is physically broken down, or chemical, in which
rock is broken down through chemical reactions.
Types of Mechanical Weathering
-Freezing and Thawing of Ice- When water freezes in a crack or pore in a
rock, it expands and makes the crack or pore bigger. This is called ice wedging.
With repeated freezing and thawing, the pieces of the rock will break off.
-Abrasion- Abrasion is the grinding away of rock by sediment that is carried by water, ice, wind, or
gravity. The sediment acts like a sand paper on the larger rock and breaks it into smaller pieces.
-Plants- Roots of trees and other plants enter cracks in rocks. As the roots grow, they force the
cracks farther apart. Eventually the small pieces of sediment will break off the rock.
-Animals-Animals that burrow in the ground, such as moles, gophers, prairie dogs, worms, and
even some insects, loosen and break apart rocks in the soil.
Types of Chemical Weathering
-Water- Water in rain, rivers and oceans is the most important agent of chemical weathering.
Water weathers rock by dissolving it into smaller pieces.
-Acid Rain-If you read the water section above, you know that water dissolves rock. All water is
somewhat acidic. However, certain pollutants cause very acidic rainwater called acid rain. Acid
rain causes chemical weathering in which rocks will dissolve more rapidly.
-Oxygen- Oxygen can break rocks that contain iron into smaller pieces. When iron combines with
oxygen and water, a process called oxidation takes place. The product of oxidation is rust. Rust
makes rock soft and crumbly, breaking it into pieces of sediment.
Are weathering and erosion the same thing?
Most people think weathering and erosion are the same thing but they are a little different.
Weathering is the process that breaks rock into sediment while erosion is the process that
moves the sediment.
So remember, weathering breaks it. Erosion moves it.
Erosion
Erosion is the movement of rock particles. Weathering and erosions work together to wear
down and carry away the rocks at Earth’s surface.
It’s amazing the effect that weathering and erosion havefrom the Grand Canyon to the Matterhorn in the Swiss Alpsweathering and erosion create unbelievable landscapes.
Types of Erosion
Water Erosion- One of the biggest forces that has shaped Earth’s
surface is water erosion. Rivers, ocean waves, and
rainwater runoff carry large amounts of sediment.
Gravity- Steep slopes, under the force of gravity, can move massive amounts of
sediment.
Mass movement, such as landslides and mudslides, are powered by gravity (although
water can also contribute to mass movement)
Glacial Erosion- Although glaciers work slowly, they are a major force
of erosion. As glaciers move over the land, they pick up rocks
and carry them away. The rocks remain on the bottom of the glacier,
carving the land beneath.
Wind Erosion- Have you ever been to the beach on a windy day? The sand
moves around pretty good. Think of the effect wind erosion has over
a long period of time, especially in areas where there are few plants
to hold the soil in place (such as the desert)