The incident occurred as the peloton approached the first of two ascents of the third-category Alto de la Cresta del Gallo, the penultimate mountain prime 36km from the finish in Murcia.
According to reports, Kris Boeckmans (Lotto Soudal) was taking a drink when he hit a pothole on the road, which, in the ensuing mayhem, also brought down Martin, van Garderen and Nacer Bouhanni (Cofidis).
Boeckmans was the most seriously hurt with severe facial trauma including several fractures, concussion, three broken ribs and bleeding in his lung. At the behest of doctors, the Belgian is in a medically-induced coma at a hospital in Murcia.
It has been revealed van Garderen has a broken right shoulder and other injuries that will prevent him from helping his BMC Racing team defend its world team time trial championship in Richmond, Virginia next month.
"I am really disappointed because I think I was prepared to do well in this Vuelta - and above all for my worlds preparation," he said. "But I am also thinking it could be worse. So now my thinking is to recover well and come back mentally stronger."
Team-mate Peter Velits said the crash was unavoidable. "We were all around Tejay because we were close to the climb and the final circuit. We were trying to move up and suddenly it happened right in front of us. We did not have a chance to avoid it. That is cycling, but it is never nice to see crashes like this."
A career first for the 27-year-old American, Van Garderen was riding a second Grand Tour in a season. At the Tour de France last month, a respiratory problem forced his withdrawal during Stage 17.
"It is a pity. Our team has lost a fundamental man," said BMC Racing Team sport director Yvon Ledanois, who directed van Garderen at both Grand Tours. "This Vuelta was an opportunity to do well for him and for the BMC Racing Team. I am sure he will come back stronger physically, and above all, mentally."
BMC Racing chief medical officer Max Testa said van Garderen fractured the head of his right humerus, and also has a contusion on the right side of his chest and a bruised right lung.
"He did not break a rib, but in the impact, he had a high speed blow to his lung," Testa said. "The hospital will keep him overnight and Sunday morning, they will re-check if everything is good. As soon as he is able to travel, he will come back to the United States where we can make a plan for his recovery."
"A Shimano auxiliary motorbike recklessly and dangerously drove into the peloton at high speed hitting Peter Sagan in his rear wheel." - Tinkoff-Saxo team statement
Race leader Esteban Chaves (Orica-GreenEDGE) was also involved the pile-up but was not seriously hurt, and finished safely in the first group of 48 riders, led home by stage winner Jasper Stuyven (Trek Factory Racing).
The Colombian retained his 10-second lead over Tom Dumoulin (Giant-Alpecin), as status quo on the leaderboard's top 10 remained with the exception of Cannondale's Martin, who was lying third overall at the start of the day.
"It was very stressful, everyone wanted to be at the front," Chaves said. "Then one Lotto Soudal rider (Boeckmans) fell in front of me after a roundabout. I think this rider is really bad and I want to say to him that I hope you recover really well.
"After this, the guys stayed with me and helped me get back to the first group. I started the climbs in the first group, the downhill was really dangerous, really narrow, but in the end we kept the red jersey."
Said Orica-GreenEDGE sport director Neil Stephens: "The crash made a tricky finish even harder. Firstly, we lost riders out of our designated helpers and we also had to spend a lot of our energy getting (Chaves) back.
"Rightly so, the bunch was riding for the stage win. They didn't back off the pace when he crashed but they weren't trying to attack Esteban and that is no problem at all.
"The team reassessed the situation and a number of decisions were made on the road that were all right ones.
"I go back to the team hotel and I read I was fined 300 francs for insults and threats as well as behaviour that damages the image of cycling. Obviously, I will pay it, but I consider it unjust." - Peter Sagan, Tinkoff-Saxo
"Firstly Mat Hayman gave Esteban his wheel; the decision to ride full gas and leave poor Simon Gerrans still on the side of the road was the right call; then Cameron Meyer radioed back from the front bunch to ask if he should wait, which I said no; and finally Daryl (Impey) made the call on the last climb to give it everything to put Esteban in a good position.
"A lot of the calls come from the car but I'll say it again, a lot of important calls come from the bike and every call and question made from the bike today was a good call so the boys - physically and tactically - rode very well."
The team of Peter Sagan, who was hit by a neutral service motorbike in the closing stages of the race, says they are considering "legal actions against the person or persons responsible for crashing into Peter Sagan with 8.2km to go on the stage."
"A Shimano auxiliary motorbike recklessly and dangerously drove into the peloton at high speed hitting Peter Sagan in his rear wheel," read the statement from Tinkoff-Saxo.
"The unacceptable collision caused Peter Sagan to crash and left the rider with extensive superficial wounds on the left buttocks and leg. Sagan was fortunately able to finish the stage but a final medical examination is yet to be made.
"Tinkoff-Saxo believes that such accidents caused by reckless human error, regardless of whether they affect Tinkoff-Saxo's riders or riders of other teams, are unacceptable at the top level of the sport of cycling. Tinkoff-Saxo finds it intolerable that an incident like today's can occur and potentially send the ambitions of a rider and a full season of planning astray."
Said Sagan: "I didn't even hear the motorbike coming. I find it unacceptable that a motorbike tries to weave its way into the group at such high speed. They accelerate and try to sneak in when they see an empty space. However, they don't take into consideration that a rider might fall in front of them or change direction. They go extremely fast and the difference of speed compared to the riders is enormous.
"Unfortunately, it isn't the first time such an incident happens. Even if motorbikes are forced to go through a group of riders, they should do it very carefully and not recklessly.
"In my opinion, motorbike drivers don't take the safety of the riders in consideration seriously. Fortunately, my injuries aren't very serious but can you imagine what would have happened if he had ran over me?
"Last but not least, I go back to the team hotel and I read I was fined 300 francs for insults and threats as well as behaviour that damages the image of cycling. Obviously, I will pay it, but I consider it unjust."
Update: Stage winner Stuyven abandons
Just a few hours after young Belgian talent Jasper Stuyven won the eighth stage of the Vuelta, his Trek Factory Racing team announced his withdrawal from the race due to a broken left scaphoid.
Stuyven was one of the many involved in the large crash that saw the abandons of Boeckmans, Bouhanni, Martin and van Garderen.
"After the finish, I felt my wrist and it was getting more and more painful, so I asked to go to the hospital to make a check," he said.
"I was already scared that it could be the scaphoid because I know from friends who are cyclists what the pain feels like and where it’s located. And, unfortunately, the X-rays confirmed that it is broken and that my Vuelta finishes today.”
Nevertheless, Stuyven remains positive. “At first I was of course really disappointed, but then I realised that I really have to enjoy this moment, because now is the time to enjoy my first professional win. For what’s next, I will see when I get home and have the surgery.”
Said team doctor Nino Daniele, who accompanied Stuyven to the hospital: “The fracture is not dislocated, so in fact it won’t be difficult to resolve it, but it goes without saying that Jasper cannot ride his bike for now.
"First of all, it would be very painful, but then it’s also possible that the broken bones move and we absolutely want to avoid that.”
Jasper Stuyven will fly back to Belgium on Sunday in order to have surgery as soon as possible in his hometown of Leuven.
“If anything goes well, as I hope, in a couple of weeks, he should be back on the bike and probably sooner on the rollers with his cast,” Daniele said.