A look at Yosemite's massive rock falls this week

Tonight, there are still road and trail closures in effect following Wednesday and Thursday's huge rock falls onto people in the valley below. There is no early warning system for huge mountain rock falls. That leaves geologists using the aftermath in an effort to set up such a system in the future.    

Wednesday a large piece of Yosemite's El Capitan broke away and fell onto a couple from Great Britain who were on a first anniversary vacation. Their internet blog, called Cam and Bear, shows many pictures of their many mountain adventurers. The husband Andrew Foster, 32, was killed and wife Lucy, 28, is in critical condition.

Yesterday, a second massive slide, off Yosemite's El Capitan, injured a man in a car when rock fell through the sunroof hitting Jim Evans. "And so we were trying to outrun it. It was like, go! Let's go!  And at the same time my husband reached up and he was like 'My head! My head!' because it was bleeding profusely and hurting," said Rachel Evans, his wife.

The Park Geologist says, because it wasn't raining, freezing, super-hot or shaken by an earthquake, the cause of both slides remains undetermined. "You know, that's still a mystery really and we will definitely be looking into that," said Greg Stock, Yosemite Park Geologist. Finding telltale signs of a potential rock fall is often impossible.

"The key things that are happening within the rocks. You know, they're usually not visible to us and after the rock fall occurs, we can see a lot better what's going on in their after the fact," says Geologist Stock.

With 150 deaths from 2006 to 2016, Yosemite was the nation's second most deadly national park; second only to Lake Mead's 254. People falling or natural cause deaths, such as heart attacks while hiking, are the most common reasons. Rock falls are very few and far between.

"You have a rock fall of the size we saw yesterday every few years," says Mr. Stock.

In fact, in the past 150 years, there have been a thousand major rock falls -- about 7 a year. Few have ever claimed lives. Even counting smaller falls, there are about only about 80 Yosemite rock falls a year large enough to report; thousands too small spread out over the park's 1169 square miles.