Running shoes are made shock absorptive by the use of ethyl vinyl acetate (EVA). Outsole EVA and midsole EVA can range from firm to soft. This shoe came it today where the firm EVA outsole wore through to the softer EVA outsole underneath.
Firm EVA on the outsole is a great way to prolong the life of shoe, however if applied thinly, it won’t withstand wear for very long.
Once you wear through into the soft EVA in the outsole, the structure of the heel will quickly deteriorate.
If you ever wear running shoes, be sure to monitor your shoes outsole wear.
Dr. Jenny Sanders Shoe Blog 
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Kirsten at BarkingDogShoes.com