Author: The Geo Models

Bringing down Mountains: More Landslides

Model by Phil Prince

Words by Lisa Whalen

 

Last time we looked at a model replicating a translational landslide. Continuing with our landslide theme, this time Phil has built a model comparing translational slides with rotational or slump landslides. These are characterized by a more curved plane of weakness that allows the slide to move. The curvature leads to the moving material becoming deformed, first through extension or tensional forces as it initially pulls away to compression as the sliding mass piles up on top of itself.

 

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Going with the Flow

Words by Lisa Whalen

Model by Phillip Prince

Images credit: British Geological Survey

 

Sandbox models aren’t just for mountain building. In this video Phillip Prince replicates landslides.

A translational landslide is when rock or soil moves down-slope along a plane of weakness like a joint, fault or bedding surface. Best illustrated with chocolate and a weaker layer of caramel:

translationalExample

debrisSlide

A rotational landslide on the other hand has a curved surface of weakness and moves large blocks of material that tend to rotate backwards as they move.

multipleRotationalSlide

 

Here in southwest Virginia are some of the largest known landslides in the world, with one slide reaching almost 3 miles long. You can learn more about “The Mountain that Moved” on the USGS site about Sinking Creek Mountain. The landslides occurred in the Pleistocene, but weren’t discovered until the 1980’s because the enormous size of the landslides obscured their presence. Today they can be identified partly through different vegetation and the presence of springs and pare visible from many of my favorite hiking areas. They may have been caused by erosion or even by earthquakes.

As an aspiring mountaineer another way to think about translational landslides is to think of slab avalanches. In this case you have layers of snow separated by a weaker layer. Weather, more snow, or people (such as in the video below) can tip the scales of the forces that are keeping the layers of snow coherent and trigger an avalanche.

Two Steps to Triton

Words by L. Whalen
Model by P. Prince

Nothing stays the same very long – and tectonic environments are no exception. Structures from a later event can overprint a previous event. Sorting this all out is both fun and hard work. In this video, Phil recreates two stages of the geologic history of Triton Bay, West Papua (northwest New Guinea).

First, we see convergence forming parallel anticlines. Subsequent extension creates a characteristic rectangular pattern, that when filled in with water matches the local structure.

Ouachita Mountains: Rock Strength in Action

Words by Lisa Whalen

Video/Model: by Phil Prince

Ouachita_Mountains_in_Arkansas
image: wikipedia.org

 

The Ouachita orogeny occurred ~300 million years ago when part of South American collided with the southern part of North America.

Ouachita_Orogeny_geologic_map.png
image: By R.Q. Foote, L.M. Massingill, and R.H. Wells – Petroleum geology and the distribution of conventional crude oil, natural gas, and natural gas liquids, East Texas basin

In this model Phil Prince takes us to the Ouachita Mountains of Arkansas and Oklahoma. Here you will see how the strength of different rock layers coupled with deep erosion can produced the characteristic shape of this range.  Sinuous ridges formed from chert snake around valleys scooped out of shale abutting a broad sandstone plateau.

Valleys and Ridges: Understanding the Geologic Structures in Central Virginia Pt.1

Words by Lisa Whalen

Video by Phil Prince

In this first part of a four part series, Dr. Phil Prince explains why we get the valleys and ridges that are the namesake of the Valley and Ridge province of Virginia.

Valleys and ridges can result from the erosion of anticlines and synclines. Knowing the ages of the rock layers can help determine, which you’re looking at when the topographic profiles have been worn away through time.

When rock strata are folded and produce an anticline, or “positive topography,” if then worn down from the top, older rocks will be exposed in the center. The opposite is true for a syncline.

anticline_and_syncline__structural_geology_intro_by_zesst-d4t1o04
Illustration by Vidimus78

Notice in the cartoon above that the original anticline and syncline are not expressed topographically. Instead two parallel ridges are present where the yellow, and presumably less-erodible strata intersect with the surface.

In this first video we get to see how geologic information overlaid in Google Earth can help illustrate this concept.

Part 2 Coming Soon!

 

Also see the Seneca Rocks field trip series for more information