Types of Mountains: Block Mountains
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Block Mountains - Formed by Faulting
(Cracking):
- Unlike fold mountains (wrinkling), block
mountains are made when the Earth's crust cracks or fractures -
this is called faulting. (See Fig. 20 in your text).
- Faulting - Caused by Tension or
Compression:
- Tension Forces:
Forces that pull the crust apart or stretch it.
- Compression Forces:
Forces that push the crust together or squeeze it.
- Both tension and compression can cause
faulting.
- How Faulting Creates Block Mountains and
Rift Valleys (See Fig. 21 a & b in your text):
- Tension and Block Mountains (Horsts):
(Fig. 21a)
- Tension Forces Pull Apart:
Earth movements create tension, pulling the crust apart.
- Faults Develop:
Cracks (faults) form in the crust.
- Central Block Rises or Stays Put:
The block of land between the faults either stays at the same
level or is pushed upwards.
- Sides Subside:
The land on either side of the faults sinks down
(subsides).
- Upstanding Block = Horst (Block
Mountain): The block that is higher than the
sunken land becomes a horst or block mountain.
- Horst Features:
- Steep Edges (Scarp Slopes):
Faulted edges are very steep, like cliffs.
- Level Summit:
The top of the block mountain is often fairly flat or level.
- Examples of Horsts:
- Hunsruck Mountains
(Germany)
- Vosges Mountains
(France)
- Black Forest
(Germany) (Vosges and Black Forest are in the Rhineland region)
- Tension and Rift Valleys (Grabens):
(Fig. 21a)
- Tension Forces Pull Apart:
Same as above, tension pulls the crust.
- Faults Develop:
Cracks (faults) form.
- Central Block Sinks Down:
In this case, the block of land between the faults sinks down
(subsides).
- Sides Stay Up:
The land on either side of the faults remains higher.
- Sunken Block = Graben (Rift
Valley): The sunken block becomes a graben
or rift valley.
- Graben Features:
- Steep Walls:
Rift valleys have very steep sides (fault scarps).
- Example of Rift Valley:
- East African Rift Valley System:
Huge rift valley, 3,000 miles long, stretches from East Africa,
through the Red Sea, to Syria.
- Compression and Rift Valleys (Fig. 21b):
- Compressional Forces Push
Together: Earth movements can also create
compression, pushing the crust together.
- Thrust or Reverse Faults:
Compression often leads to a type of fault called a thrust or reverse
fault.
- Block Lowered:
A block of land can be pushed downwards relative to surrounding
areas.
- Rift Valley Formed:
This downward movement can also create a rift valley.
- Note:
Text says large block mountains and rift valleys are usually due
to tension, not compression, but compression can also form rift
valleys.
- Faults in Series & Complexity:
Faults can happen in groups (series), and can be made more complicated by
tilting and other movements.
- Denudation Modifies Block Mountains:
Over long periods, erosion (denudation) by water, wind, etc., will wear
down and change the shape of faulted landforms. (Fig. 21c shows how steep
sides can be worn back over time).
In Simple Words:
Block mountains are formed when the Earth's crust cracks (faults). Tension
forces pulling the crust apart can create block mountains (horsts) where a
central block rises, and rift valleys (grabens) where a central block sinks.
Block mountains have steep sides and flat tops. The East African Rift Valley is
a famous example of a huge rift valley. Erosion slowly changes the shape of
these faulted landscapes over time.