1. Intrusion Formation:
* Magma Upwelling: Molten rock (magma) rises from deep within the Earth's crust, seeking areas of lower pressure.
* Forced Intrusion: The magma forces its way through existing rock layers, often pushing them upwards. This creates a variety of intrusive igneous structures like batholiths, sills, dikes, and laccoliths.
2. Erosion and Differential Weathering:
* Erosion: Over time, the surrounding rock (which may be less resistant to weathering) is eroded away by wind, rain, and other natural forces.
* Differential Weathering: Igneous rocks are generally harder and more resistant to weathering than sedimentary rocks. This means the intrusive igneous rock will weather and erode at a slower rate than the surrounding rock.
3. Hill Formation:
* Exposed Intrusion: As the surrounding rock erodes, the more resistant igneous intrusion is left standing high above the landscape. This creates a hill or mountain.
* Shape of the Intrusion: The shape of the hill will depend on the shape of the original intrusion. For example, a laccolith (a dome-shaped intrusion) will create a dome-shaped hill.
Examples:
* Stone Mountain, Georgia: A large, exposed granite intrusion that forms a prominent hill.
* Half Dome, Yosemite National Park: A large, exposed granite batholith that has been sculpted by glacial erosion.
Exceptions:
* Buried Intrusions: Some igneous intrusions never reach the surface and remain buried beneath the surrounding rock. These won't form hills.
* Low-Lying Landscapes: Even if an intrusion is exposed, it might not form a hill in a flat or low-lying area.
In summary, igneous intrusions can end up as hills due to the combination of their resistance to weathering and the erosion of surrounding rocks. This process takes a very long time, often millions of years.
